mirror of https://github.com/zcash/zips.git
8628 lines
359 KiB
TeX
8628 lines
359 KiB
TeX
\documentclass{article}
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\RequirePackage[utf8]{inputenc}
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\RequirePackage[T1]{fontenc}
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||
\RequirePackage{amsmath}
|
||
\RequirePackage{amsthm}
|
||
\RequirePackage{bytefield}
|
||
\RequirePackage{graphicx}
|
||
\RequirePackage{newtxmath}
|
||
\RequirePackage{mathtools}
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||
\RequirePackage{xspace}
|
||
\RequirePackage{url}
|
||
\RequirePackage{changepage}
|
||
\RequirePackage{enumitem}
|
||
\RequirePackage{tabularx}
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||
\RequirePackage{hhline}
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||
\RequirePackage[usestackEOL]{stackengine}
|
||
\RequirePackage{comment}
|
||
\RequirePackage{needspace}
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||
\RequirePackage[nobottomtitles]{titlesec}
|
||
\RequirePackage[hang]{footmisc}
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||
\RequirePackage{xstring}
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||
\RequirePackage[unicode,bookmarksnumbered,bookmarksopen]{hyperref}
|
||
\RequirePackage{cleveref}
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||
\RequirePackage{nameref}
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||
\RequirePackage{etoolbox}
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||
\RequirePackage{subdepth}
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||
\RequirePackage{fix-cm}
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||
\RequirePackage{hyphenat}
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||
|
||
\RequirePackage[style=alphabetic,maxbibnames=99,dateabbrev=false,urldate=iso8601,backref=true,backrefstyle=none,backend=biber]{biblatex}
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||
\addbibresource{zcash.bib}
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||
|
||
% Fonts
|
||
\RequirePackage{lmodern}
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||
\RequirePackage{quattrocento}
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||
\RequirePackage[bb=ams]{mathalfa}
|
||
\RequirePackage[scr]{rsfso}
|
||
%\RequirePackage{txfonts}
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||
|
||
% Quattrocento is beautiful but doesn't have an italic face. So we scale
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||
% New Century Schoolbook italic to fit in with slanted Quattrocento and
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||
% match its x height.
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||
\renewcommand{\emph}[1]{\hspace{0.15em}{\fontfamily{pnc}\selectfont\scalebox{1.02}[0.999]{\textit{#1}}}\hspace{0.02em}}
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||
|
||
% While we're at it, let's match the tt x height to Quattrocento as well.
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||
\let\oldtexttt\texttt
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||
\let\oldmathtt\mathtt
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||
\renewcommand{\texttt}[1]{\scalebox{1.02}[1.07]{\oldtexttt{#1}}}
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||
\renewcommand{\mathtt}[1]{\scalebox{1.02}[1.07]{$\oldmathtt{#1}$}}
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|
||
% bold but not extended
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\newcommand{\textbnx}[1]{{\fontseries{b}\selectfont #1}}
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\crefformat{footnote}{#2\footnotemark[#1]#3}
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|
||
\DeclareLabelalphaTemplate{
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\labelelement{\field{citekey}}
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||
}
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||
|
||
\DefineBibliographyStrings{english}{
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page = {page},
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||
pages = {pages},
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backrefpage = {\mbox{$\uparrow$ p\!}},
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||
backrefpages = {\mbox{$\uparrow$ p\!}}
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||
}
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||
|
||
\setlength{\oddsidemargin}{-0.25in}
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||
\setlength{\textwidth}{7in}
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||
\setlength{\topmargin}{-0.75in}
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||
\setlength{\textheight}{9.2in}
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||
\setlength{\parindent}{0ex}
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||
\renewcommand{\arraystretch}{1.4}
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||
%\overfullrule=2cm
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||
|
||
\setlength{\footnotemargin}{0.6em}
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||
\setlength{\footnotesep}{2ex}
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||
\addtolength{\skip\footins}{3ex}
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|
||
\renewcommand{\bottomtitlespace}{8ex}
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|
||
% Use rubber lengths between paragraphs to improve default pagination.
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% <https://tex.stackexchange.com/questions/17178/vertical-spacing-pagination-and-ideal-results>
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\setlength{\parskip}{1.5ex plus 1pt minus 1pt}
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||
\setlist[enumerate]{before=\vspace{-1ex}}
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\setlist[itemize]{itemsep=0.5ex,topsep=0.2ex,before=\vspace{-1ex},after=\vspace{1.5ex}}
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\newlist{formulae}{itemize}{3}
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\setlist[formulae]{itemsep=0.2ex,topsep=0ex,leftmargin=1.5em,label=,after=\vspace{1.5ex}}
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|
||
\newlist{lines}{itemize}{3}
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||
\setlist[lines]{itemsep=-0.5ex,topsep=0ex,before=\vspace{1ex},leftmargin=0.6em,label=,after=\vspace{1ex}}
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|
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\newcommand{\docversion}{Version unavailable (check protocol.ver)}
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||
\newcommand{\SaplingSpec}{Overwinter+Sapling}
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\newtoggle{issapling}
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\togglefalse{issapling}
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\InputIfFileExists{protocol.ver}{}{}
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\newcommand{\doctitle}{Zcash Protocol Specification}
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\newcommand{\leadauthor}{Daira Hopwood}
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||
\newcommand{\coauthora}{Sean Bowe}
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||
\newcommand{\coauthorb}{Taylor Hornby}
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||
\newcommand{\coauthorc}{Nathan Wilcox}
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||
|
||
\newcommand{\keywords}{anonymity, applications, cryptographic protocols,\
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electronic commerce and payment, financial privacy, proof of work, zero knowledge}
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|
||
\hypersetup{
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||
pdfborderstyle={/S/U/W 0.7},
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||
pdfinfo={
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||
Title={\doctitle, \docversion},
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||
Author={\leadauthor, \coauthora, \coauthorb, \coauthorc},
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||
Keywords={\keywords}
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||
}
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||
}
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||
|
||
\makeatletter
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||
\renewcommand*{\@fnsymbol}[1]{\ensuremath{\ifcase#1\or \dagger\or \ddagger\or
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\mathsection\or \mathparagraph\else\@ctrerr\fi}}
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||
\newcommand\slightlylarge{\fontsize{10.5}{10.5}\selectfont}
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||
\newcommand\notsolarge{\fontsize{11}{11}\selectfont}
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||
\newcommand\largeish{\fontsize{12}{12}\selectfont}
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||
\newcommand\larger{\fontsize{13}{13}\selectfont}
|
||
\newcommand\Larger{\fontsize{16}{16}\selectfont}
|
||
\makeatother
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||
|
||
\titleformat*{\subsection}{\larger\bfseries}
|
||
\titleformat*{\subsubsection}{\largeish\bfseries}
|
||
\titleformat*{\paragraph}{\notsolarge\bfseries}
|
||
\titleformat*{\subparagraph}{\slightlylarge\bfseries}
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||
|
||
\renewcommand{\sectionautorefname}{\S\!}
|
||
\renewcommand{\subsectionautorefname}{\S\!}
|
||
\renewcommand{\subsubsectionautorefname}{\S\!}
|
||
\renewcommand{\paragraphautorefname}{\S\!}
|
||
\renewcommand{\subparagraphautorefname}{\S\!}
|
||
\newcommand{\crossref}[1]{\autoref{#1}\, \emph{`\nameref*{#1}\kern -0.05em'} on p.\,\pageref*{#1}}
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||
\newcommand{\theoremref}[1]{\autoref{#1} on p.\,\pageref*{#1}}
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||
|
||
% <https://tex.stackexchange.com/a/60212/78411>
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\newcommand{\subsubsubsection}[1]{\paragraph{#1}\mbox{}\\}
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||
\newcommand{\subsubsubsubsection}[1]{\subparagraph{#1}\mbox{}\\}
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||
\setcounter{secnumdepth}{4}
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||
\setcounter{tocdepth}{4}
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||
|
||
\newcommand{\nstrut}[1]{\texorpdfstring{#1\rule[-.2\baselineskip]{0pt}{\baselineskip}}{#1}}
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\newcommand{\nsection}[1]{\section{\nstrut{#1}}}
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||
\newcommand{\nsubsection}[1]{\subsection{\nstrut{#1}}}
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\newcommand{\nsubsubsection}[1]{\subsubsection{\nstrut{#1}}}
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||
\newcommand{\nsubsubsubsection}[1]{\subsubsubsection{\nstrut{#1}}}
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||
\newcommand{\nsubsubsubsubsection}[1]{\subsubsubsubsection{\nstrut{#1}}}
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||
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||
\newcommand{\introlist}{\needspace{15ex}}
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||
\newcommand{\introsection}{\needspace{35ex}}
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||
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||
\mathchardef\mhyphen="2D
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||
\newcommand{\lrarrow}{\texorpdfstring{$\leftrightarrow$}{↔}}
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% Using the astral plane character 𝕊 works, but triggers bugs in PDF readers 😛
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\newcommand{\rS}{\texorpdfstring{$\ParamS{r}$}{rS}}
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% <https://tex.stackexchange.com/a/309445/78411>
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||
\DeclareFontFamily{U}{FdSymbolA}{}
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||
\DeclareFontShape{U}{FdSymbolA}{m}{n}{
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<-> s*[.4] FdSymbolA-Regular
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}{}
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||
\DeclareSymbolFont{fdsymbol}{U}{FdSymbolA}{m}{n}
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||
\DeclareMathSymbol{\smallcirc}{\mathord}{fdsymbol}{"60}
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||
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||
\makeatletter
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||
\newcommand{\hollowcolon}{\mathpalette\hollow@colon\relax}
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||
\newcommand{\hollow@colon}[2]{
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\mspace{0.7mu}
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\vbox{\hbox{$\m@th#1\smallcirc$}\nointerlineskip\kern.45ex \hbox{$\m@th#1\smallcirc$}\kern-.06ex}
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||
\mspace{1mu}
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}
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\makeatother
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||
\newcommand{\typecolon}{\;\hollowcolon\;}
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||
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||
% <https://tex.stackexchange.com/a/235120/78411>
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||
\makeatletter
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||
\newcommand*\bigcdot{\mathpalette\bigcdot@{.5}}
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||
\newcommand*\bigcdot@[2]{\mathbin{\vcenter{\hbox{\scalebox{#2}{$\m@th#1\bullet$}}}}}
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||
\makeatother
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||
% We just want one ampersand symbol from boisik.
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\DeclareSymbolFont{bskadd}{U}{bskma}{m}{n}
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\DeclareFontFamily{U}{bskma}{\skewchar\font130 }
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||
\DeclareFontShape{U}{bskma}{m}{n}{<->bskma10}{}
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\DeclareMathSymbol{\binampersand}{\mathbin}{bskadd}{"EE}
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% $v$ is too close to $u$.
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% <https://tex.stackexchange.com/questions/130569/sharp-or-angled-v-in-math-mode-varv>
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\DeclareSymbolFont{matha}{OML}{txmi}{m}{it}
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||
\DeclareMathSymbol{\varv}{\mathord}{matha}{118}
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||
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||
\newcommand{\hairspace}{~\!}
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\newcommand{\hparen}{\hphantom{(}}
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\newcommand{\mhspace}[1]{\mbox{\hspace{#1}}}
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\newcommand{\tab}{\hspace{1.5em}}
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||
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\newcommand{\plus}{\hairspace +\hairspace}
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\newcommand{\vv}{\hspace{0.071em}\varv\hspace{0.064em}}
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||
\newcommand{\varvv}{\varv\kern 0.02em\varv}
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||
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||
\newcommand{\hfrac}[2]{\scalebox{0.8}{$\genfrac{}{}{0.5pt}{0}{#1}{#2}$}}
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||
\newcommand{\ssqrt}[1]{\rlap{\scalebox{0.64}[1]{$\sqrt{\scalebox{1.5625}[1]{${#1}\vphantom{b}$}}$}} %
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||
\hspace{0.005em}\scalebox{0.64}[1]{$\sqrt{\scalebox{1.5625}[1]{$\phantom{#1}\vphantom{b}$}}$}}
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||
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||
\RequirePackage[usenames,dvipsnames]{xcolor}
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||
% <https://en.wikibooks.org/wiki/LaTeX/Colors#The_68_standard_colors_known_to_dvips>
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||
\newcommand{\todo}[1]{{\color{Sepia}\sf{TODO: #1}}}
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||
\definecolor{green}{RGB}{0,100,10}
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||
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||
\newcommand{\changedcolor}{magenta}
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||
\newcommand{\setchanged}{\color{\changedcolor}}
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||
\newcommand{\changed}[1]{\texorpdfstring{{\setchanged{#1}}}{#1}}
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||
\newcommand{\saplingcolor}{green}
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||
\newcommand{\nuzerocolor}{blue}
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||
|
||
\iftoggle{issapling}{
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||
\newcommand{\sprout}[1]{}
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||
\newcommand{\notsprout}[1]{#1}
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||
\newcommand{\setsapling}{\color{\saplingcolor}}
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||
\newcommand{\sapling}[1]{\texorpdfstring{{\setsapling{#1}}}{#1}}
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||
\newcommand{\setnuzero}{\color{\nuzerocolor}}
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||
\newcommand{\nuzero}[1]{\texorpdfstring{{\setnuzero{#1}}}{#1}}
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||
\newcommand{\optSprout}[1]{{#1}^\mathsf{Sprout}}
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||
\pagecolor{yellow!3}
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||
} {
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||
\newcommand{\sprout}[1]{#1}
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||
\newcommand{\notsprout}[1]{}
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||
\newcommand{\setsapling}{}
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||
\newcommand{\sapling}[1]{}
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||
\newcommand{\setnuzero}{}
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||
\newcommand{\nuzero}[1]{}
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||
\newcommand{\optSprout}[1]{#1}
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||
}
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||
|
||
\newtheorem{theorem}{Theorem}
|
||
\numberwithin{theorem}{subsection}
|
||
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||
\newtheorem*{lemma*}{Lemma}
|
||
|
||
|
||
% Terminology
|
||
|
||
\newcommand{\term}[1]{\textsl{#1}\kern 0.05em\xspace}
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||
\newcommand{\titleterm}[1]{#1}
|
||
\newcommand{\termbf}[1]{\textbf{#1}\xspace}
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||
\newcommand{\quotedterm}[1]{``~\!\!\term{#1}''}
|
||
\newcommand{\conformance}[1]{\textbnx{#1}\xspace}
|
||
|
||
\newcommand{\Zcash}{\termbf{Zcash}}
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||
\newcommand{\Zerocash}{\termbf{Zerocash}}
|
||
\newcommand{\Sprout}{\termbf{Sprout}}
|
||
\newcommand{\SproutOrZcash}{\notsprout{\Sprout}\sprout{\Zcash}}
|
||
\newcommand{\SproutOrNothing}{\notsprout{\Sprout}}
|
||
\newcommand{\Sapling}{\termbf{Sapling}}
|
||
\newcommand{\NUZero}{\termbf{Overwinter}}
|
||
\newcommand{\Bitcoin}{\termbf{Bitcoin}}
|
||
\newcommand{\CryptoNote}{\termbf{CryptoNote}}
|
||
\newcommand{\ZEC}{\termbf{ZEC}}
|
||
\newcommand{\zatoshi}{\term{zatoshi}}
|
||
\newcommand{\zcashd}{\textsf{zcashd}\,}
|
||
|
||
\newcommand{\MUST}{\conformance{MUST}}
|
||
\newcommand{\MUSTNOT}{\conformance{MUST NOT}}
|
||
\newcommand{\SHOULD}{\conformance{SHOULD}}
|
||
\newcommand{\SHOULDNOT}{\conformance{SHOULD NOT}}
|
||
\newcommand{\RECOMMENDED}{\conformance{RECOMMENDED}}
|
||
\newcommand{\MAY}{\conformance{MAY}}
|
||
\newcommand{\ALLCAPS}{\conformance{ALL CAPS}}
|
||
|
||
\newcommand{\note}{\term{note}}
|
||
\newcommand{\notes}{\term{notes}}
|
||
\newcommand{\Note}{\titleterm{Note}}
|
||
\newcommand{\Notes}{\titleterm{Notes}}
|
||
\newcommand{\dummy}{\term{dummy}}
|
||
\newcommand{\dummyNotes}{\term{dummy notes}}
|
||
\newcommand{\DummyNotes}{\titleterm{Dummy Notes}}
|
||
\newcommand{\commitmentScheme}{\term{commitment scheme}}
|
||
\newcommand{\commitmentSchemes}{\term{commitment schemes}}
|
||
\newcommand{\commitmentTrapdoor}{\term{commitment trapdoor}}
|
||
\newcommand{\commitmentTrapdoors}{\term{commitment trapdoors}}
|
||
\newcommand{\trapdoor}{\term{trapdoor}}
|
||
\newcommand{\noteCommitment}{\term{note commitment}}
|
||
\newcommand{\noteCommitments}{\term{note commitments}}
|
||
\newcommand{\xNoteCommitments}{\term{Note commitments}}
|
||
\newcommand{\NoteCommitment}{\titleterm{Note Commitment}}
|
||
\newcommand{\NoteCommitments}{\titleterm{Note Commitments}}
|
||
\newcommand{\noteCommitmentTree}{\term{note commitment tree}}
|
||
\newcommand{\noteCommitmentTrees}{\term{note commitment trees}}
|
||
\newcommand{\NoteCommitmentTrees}{\titleterm{Note Commitment Trees}}
|
||
\newcommand{\notePosition}{\term{note position}}
|
||
\newcommand{\notePositions}{\term{note positions}}
|
||
\newcommand{\positionedNote}{\term{positioned note}}
|
||
\newcommand{\positionedNotes}{\term{positioned notes}}
|
||
\newcommand{\noteTraceabilitySet}{\term{note traceability set}}
|
||
\newcommand{\noteTraceabilitySets}{\term{note traceability sets}}
|
||
\newcommand{\KeyComponents}{\titleterm{Key Components}}
|
||
\newcommand{\valueCommitment}{\term{value commitment}}
|
||
\newcommand{\valueCommitments}{\term{value commitments}}
|
||
\newcommand{\joinSplitDescription}{\term{JoinSplit description}}
|
||
\newcommand{\joinSplitDescriptions}{\term{JoinSplit descriptions}}
|
||
\newcommand{\JoinSplitDescriptions}{\titleterm{JoinSplit Descriptions}}
|
||
\newcommand{\sequenceOfJoinSplitDescriptions}{\changed{sequence of} \joinSplitDescription\changed{\term{s}}\xspace}
|
||
\newcommand{\joinSplitTransfer}{\term{JoinSplit transfer}}
|
||
\newcommand{\joinSplitTransfers}{\term{JoinSplit transfers}}
|
||
\newcommand{\JoinSplitTransfer}{\titleterm{JoinSplit Transfer}}
|
||
\newcommand{\JoinSplitTransfers}{\titleterm{JoinSplit Transfers}}
|
||
\newcommand{\joinSplitSignature}{\term{JoinSplit signature}}
|
||
\newcommand{\joinSplitSignatures}{\term{JoinSplit signatures}}
|
||
\newcommand{\JoinSplitSignature}{\titleterm{JoinSplit Signature}}
|
||
\newcommand{\joinSplitSigningKey}{\term{JoinSplit signing key}}
|
||
\newcommand{\joinSplitVerifyingKey}{\term{JoinSplit verifying key}}
|
||
\newcommand{\joinSplitStatement}{\term{JoinSplit statement}}
|
||
\newcommand{\joinSplitStatements}{\term{JoinSplit statements}}
|
||
\newcommand{\JoinSplitStatement}{\titleterm{JoinSplit Statement}}
|
||
\newcommand{\joinSplitProof}{\term{JoinSplit proof}}
|
||
\newcommand{\shieldedTransfer}{\term{shielded transfer}}
|
||
\newcommand{\shieldedTransfers}{\term{shielded transfers}}
|
||
\newcommand{\shieldedSpend}{\term{shielded spend}}
|
||
\newcommand{\shieldedSpends}{\term{shielded spends}}
|
||
\newcommand{\shieldedInput}{\term{shielded input}}
|
||
\newcommand{\shieldedInputs}{\term{shielded inputs}}
|
||
\newcommand{\spendDescription}{\term{Spend description}}
|
||
\newcommand{\spendDescriptions}{\term{Spend descriptions}}
|
||
\newcommand{\SpendDescriptions}{\titleterm{Spend Descriptions}}
|
||
\newcommand{\spendTransfer}{\term{Spend transfer}}
|
||
\newcommand{\spendTransfers}{\term{Spend transfers}}
|
||
\newcommand{\SpendTransfers}{\titleterm{Spend Transfers}}
|
||
\newcommand{\spendCircuit}{\term{Spend circuit}}
|
||
\newcommand{\spendStatement}{\term{Spend statement}}
|
||
\newcommand{\spendStatements}{\term{Spend statements}}
|
||
\newcommand{\SpendStatement}{\titleterm{Spend Statement}}
|
||
\newcommand{\spendProof}{\term{Spend proof}}
|
||
\newcommand{\spendAuthSignature}{\term{spend authorization signature}}
|
||
\newcommand{\spendAuthSignatures}{\term{spend authorization signatures}}
|
||
\newcommand{\SpendAuthSignature}{\titleterm{Spend Authorization Signature}}
|
||
\newcommand{\outputDescription}{\term{Output description}}
|
||
\newcommand{\outputDescriptions}{\term{Output descriptions}}
|
||
\newcommand{\OutputDescriptions}{\titleterm{Output Descriptions}}
|
||
\newcommand{\outputTransfer}{\term{Output transfer}}
|
||
\newcommand{\outputTransfers}{\term{Output transfers}}
|
||
\newcommand{\OutputTransfers}{\titleterm{Output Transfers}}
|
||
\newcommand{\outputCircuit}{\term{Output circuit}}
|
||
\newcommand{\outputStatement}{\term{Output statement}}
|
||
\newcommand{\outputStatements}{\term{Output statements}}
|
||
\newcommand{\OutputStatement}{\titleterm{Output Statement}}
|
||
\newcommand{\outputProof}{\term{Output proof}}
|
||
\newcommand{\shieldedOutput}{\term{shielded output}}
|
||
\newcommand{\shieldedOutputs}{\term{shielded outputs}}
|
||
\newcommand{\statement}{\term{statement}}
|
||
\newcommand{\ZkSNARKStatements}{\titleterm{Zk-SNARK Statements}}
|
||
\newcommand{\zeroKnowledgeProof}{\term{zero-knowledge proof}}
|
||
\newcommand{\zeroKnowledgeProofs}{\term{zero-knowledge proofs}}
|
||
\newcommand{\provingSystem}{\term{proving system}}
|
||
\newcommand{\provingSystems}{\term{proving systems}}
|
||
\newcommand{\zeroKnowledgeProvingSystem}{\term{zero-knowledge proving system}}
|
||
\newcommand{\ZeroKnowledgeProvingSystem}{\titleterm{Zero-Knowledge Proving System}}
|
||
\newcommand{\ZeroKnowledgeProvingSystems}{\titleterm{Zero-Knowledge Proving Systems}}
|
||
\newcommand{\quadraticArithmeticProgram}{\term{quadratic arithmetic program}}
|
||
\newcommand{\quadraticArithmeticPrograms}{\term{quadratic arithmetic programs}}
|
||
\newcommand{\QuadraticArithmeticPrograms}{\titleterm{Quadratic Arithmetic Programs}}
|
||
\newcommand{\linearCombination}{\term{linear combination}}
|
||
\newcommand{\linearCombinations}{\term{linear combinations}}
|
||
\newcommand{\representedGroup}{\term{represented group}}
|
||
\newcommand{\representedGroups}{\term{represented groups}}
|
||
\newcommand{\RepresentedGroup}{\titleterm{Represented Group}}
|
||
\newcommand{\hashExtractor}{\term{hash extractor}}
|
||
\newcommand{\HashExtractor}{\titleterm{Hash Extractor}}
|
||
\newcommand{\groupHash}{\term{group hash}}
|
||
\newcommand{\groupHashes}{\term{group hashes}}
|
||
\newcommand{\GroupHash}{\titleterm{Group Hash}}
|
||
\newcommand{\representedPairing}{\term{represented pairing}}
|
||
\newcommand{\RepresentedPairing}{\titleterm{Represented Pairing}}
|
||
\newcommand{\RepresentedGroupsAndPairings}{\titleterm{Represented Groups and Pairings}}
|
||
\newcommand{\PHGR}{\mathsf{PHGR13}}
|
||
\newcommand{\Groth}{\mathsf{Groth16}}
|
||
\newcommand{\EncodingOfPHGRProofs}{\titleterm{Encoding of PHGR13 Proofs}}
|
||
\newcommand{\EncodingOfGrothProofs}{\titleterm{Encoding of Groth16 Proofs}}
|
||
\newcommand{\PHGRProvingSystem}{\titleterm{PHGR13}}
|
||
\newcommand{\GrothProvingSystem}{\titleterm{Groth16}}
|
||
\newcommand{\BNCurve}{\mathsf{BN\mhyphen{}254}}
|
||
\newcommand{\BLSCurve}{\mathsf{BLS12\mhyphen{}381}}
|
||
\newcommand{\JubjubCurve}{\mathsf{Jubjub}}
|
||
\newcommand{\Jubjub}{\titleterm{Jubjub}}
|
||
\newcommand{\EdJubjub}{\mathsf{EdJubjub}}
|
||
\newcommand{\commonRandomString}{\term{Common Random String}}
|
||
\newcommand{\BNRepresentedPairing}{\titleterm{BN-254}}
|
||
\newcommand{\BLSRepresentedPairing}{\titleterm{BLS12-381}}
|
||
\newcommand{\ppzkSNARK}{\term{preprocessing zk-SNARK}}
|
||
\newcommand{\provingKey}{\term{proving key}}
|
||
\newcommand{\provingKeys}{\term{proving keys}}
|
||
\newcommand{\zkProvingKeys}{\term{zero-knowledge proving keys}}
|
||
\newcommand{\verifyingKey}{\term{verifying key}}
|
||
\newcommand{\verifyingKeys}{\term{verifying keys}}
|
||
\newcommand{\zkVerifyingKeys}{\term{zero-knowledge verifying keys}}
|
||
\newcommand{\joinSplitParameters}{\term{JoinSplit parameters}}
|
||
\newcommand{\SproutZKParameters}{\titleterm{\notsprout{\Sprout }zk-SNARK Parameters}}
|
||
\newcommand{\SaplingZKParameters}{\titleterm{\Sapling zk-SNARK Parameters}}
|
||
\newcommand{\arithmeticCircuit}{\term{arithmetic circuit}}
|
||
\newcommand{\rankOneConstraintSystem}{\term{Rank 1 Constraint System}}
|
||
\newcommand{\rankOneConstraintSystems}{\term{Rank 1 Constraint Systems}}
|
||
\newcommand{\primary}{\term{primary}}
|
||
\newcommand{\primaryInput}{\term{primary input}}
|
||
\newcommand{\primaryInputs}{\term{primary inputs}}
|
||
\newcommand{\auxiliaryInput}{\term{auxiliary input}}
|
||
\newcommand{\auxiliaryInputs}{\term{auxiliary inputs}}
|
||
\newcommand{\fullValidator}{\term{full validator}}
|
||
\newcommand{\fullValidators}{\term{full validators}}
|
||
\newcommand{\anchor}{\term{anchor}}
|
||
\newcommand{\anchors}{\term{anchors}}
|
||
\newcommand{\block}{\term{block}}
|
||
\newcommand{\blocks}{\term{blocks}}
|
||
\newcommand{\header}{\term{header}}
|
||
\newcommand{\headers}{\term{headers}}
|
||
\newcommand{\blockHeader}{\term{block header}}
|
||
\newcommand{\blockHeaders}{\term{block headers}}
|
||
\newcommand{\Blockheader}{\term{Block header}}
|
||
\newcommand{\BlockHeader}{\titleterm{Block Header}}
|
||
\newcommand{\blockVersionNumber}{\term{block version number}}
|
||
\newcommand{\blockVersionNumbers}{\term{block version numbers}}
|
||
\newcommand{\Blockversions}{\term{Block versions}}
|
||
\newcommand{\blockTime}{\term{block time}}
|
||
\newcommand{\blockHeight}{\term{block height}}
|
||
\newcommand{\blockHeights}{\term{block heights}}
|
||
\newcommand{\activationHeight}{\term{activation block height}}
|
||
\newcommand{\activationHeights}{\term{activation block heights}}
|
||
\newcommand{\genesisBlock}{\term{genesis block}}
|
||
\newcommand{\transaction}{\term{transaction}}
|
||
\newcommand{\transactions}{\term{transactions}}
|
||
\newcommand{\Transactions}{\titleterm{Transactions}}
|
||
\newcommand{\transactionFee}{\term{transaction fee}}
|
||
\newcommand{\transactionFees}{\term{transaction fees}}
|
||
\newcommand{\transactionVersionNumber}{\term{transaction version number}}
|
||
\newcommand{\transactionVersionNumbers}{\term{transaction version numbers}}
|
||
\newcommand{\Transactionversion}{\term{Transaction version}}
|
||
\newcommand{\coinbaseTransaction}{\term{coinbase transaction}}
|
||
\newcommand{\coinbaseTransactions}{\term{coinbase transactions}}
|
||
\newcommand{\CoinbaseTransactions}{\titleterm{Coinbase Transactions}}
|
||
\newcommand{\transparent}{\term{transparent}}
|
||
\newcommand{\xTransparent}{\term{Transparent}}
|
||
\newcommand{\Transparent}{\titleterm{Transparent}}
|
||
\newcommand{\transparentValuePool}{\term{transparent value pool}}
|
||
\newcommand{\transparentAddress}{\term{transparent address}}
|
||
\newcommand{\transparentAddresses}{\term{transparent addresses}}
|
||
\newcommand{\xTransparentAddresses}{\term{Transparent addresses}}
|
||
\newcommand{\TransparentAddresses}{\titleterm{Transparent Addresses}}
|
||
\newcommand{\transparentTransfers}{\term{transparent transfers}}
|
||
\newcommand{\shielded}{\term{shielded}}
|
||
\newcommand{\shieldedNote}{\term{shielded note}}
|
||
\newcommand{\shieldedNotes}{\term{shielded notes}}
|
||
\newcommand{\xShielded}{\term{Shielded}}
|
||
\newcommand{\Shielded}{\titleterm{Shielded}}
|
||
\newcommand{\blockchain}{\term{block chain}}
|
||
\newcommand{\blockchains}{\term{block chains}}
|
||
\newcommand{\validBlockchain}{\term{valid block chain}}
|
||
\newcommand{\bestValidBlockchain}{\term{best valid block chain}}
|
||
\newcommand{\mempool}{\term{mempool}}
|
||
\newcommand{\treestate}{\term{treestate}}
|
||
\newcommand{\treestates}{\term{treestates}}
|
||
\newcommand{\nullifier}{\term{nullifier}}
|
||
\newcommand{\nullifiers}{\term{nullifiers}}
|
||
\newcommand{\xNullifier}{\term{Nullifier}}
|
||
\newcommand{\xNullifiers}{\term{Nullifiers}}
|
||
\newcommand{\Nullifier}{\titleterm{Nullifier}}
|
||
\newcommand{\Nullifiers}{\titleterm{Nullifiers}}
|
||
\newcommand{\nullifierSet}{\term{nullifier set}}
|
||
\newcommand{\nullifierSets}{\term{nullifier sets}}
|
||
\newcommand{\NullifierSets}{\titleterm{Nullifier Sets}}
|
||
\newcommand{\paymentAddress}{\term{shielded payment address}}
|
||
\newcommand{\paymentAddresses}{\term{shielded payment addresses}}
|
||
\newcommand{\PaymentAddresses}{\titleterm{Shielded Payment Addresses}}
|
||
\newcommand{\diversifiedPaymentAddress}{\term{diversified payment address}}
|
||
\newcommand{\diversifiedPaymentAddresses}{\term{diversified payment addresses}}
|
||
\newcommand{\diversifier}{\term{diversifier}}
|
||
\newcommand{\diversifiers}{\term{diversifiers}}
|
||
\newcommand{\incomingViewingKey}{\term{incoming viewing key}}
|
||
\newcommand{\incomingViewingKeys}{\term{incoming viewing keys}}
|
||
\newcommand{\IncomingViewingKeys}{\titleterm{Incoming Viewing Keys}}
|
||
\newcommand{\fullViewingKey}{\term{full viewing key}}
|
||
\newcommand{\fullViewingKeys}{\term{full viewing keys}}
|
||
\newcommand{\FullViewingKeys}{\titleterm{Full Viewing Keys}}
|
||
\newcommand{\receivingKey}{\term{receiving key}}
|
||
\newcommand{\receivingKeys}{\term{receiving keys}}
|
||
\newcommand{\spendingKey}{\term{spending key}}
|
||
\newcommand{\spendingKeys}{\term{spending keys}}
|
||
\newcommand{\SpendingKeys}{\titleterm{Spending Keys}}
|
||
\newcommand{\payingKey}{\term{paying key}}
|
||
\newcommand{\transmissionKey}{\term{transmission key}}
|
||
\newcommand{\transmissionKeys}{\term{transmission keys}}
|
||
\newcommand{\diversifiedTransmissionKey}{\term{diversified transmission key}}
|
||
\newcommand{\diversifiedTransmissionKeys}{\term{diversified transmission keys}}
|
||
\newcommand{\authSigningKey}{\term{spend authorizing key}}
|
||
\newcommand{\authSigningKeys}{\term{spend authorizing keys}}
|
||
\newcommand{\authProvingKey}{\term{proof authorizing key}}
|
||
\newcommand{\authProvingKeys}{\term{proof authorizing keys}}
|
||
\newcommand{\humanReadablePart}{\term{Human-Readable Part}}
|
||
\newcommand{\notePlaintext}{\term{note plaintext}}
|
||
\newcommand{\notePlaintexts}{\term{note plaintexts}}
|
||
\newcommand{\NotePlaintexts}{\titleterm{Note Plaintexts}}
|
||
\newcommand{\noteCiphertext}{\term{transmitted note ciphertext}}
|
||
\newcommand{\notesCiphertext}{\term{transmitted notes ciphertext}}
|
||
\newcommand{\incrementalMerkleTree}{\term{incremental Merkle tree}}
|
||
\newcommand{\MerkleTree}{\titleterm{Merkle Tree}}
|
||
\newcommand{\merkleRoot}{\term{root}}
|
||
\newcommand{\merkleNode}{\term{node}}
|
||
\newcommand{\merkleNodes}{\term{nodes}}
|
||
\newcommand{\merkleHash}{\term{hash value}}
|
||
\newcommand{\merkleHashes}{\term{hash values}}
|
||
\newcommand{\merkleLeafNode}{\term{leaf node}}
|
||
\newcommand{\merkleLeafNodes}{\term{leaf nodes}}
|
||
\newcommand{\merkleInternalNode}{\term{internal node}}
|
||
\newcommand{\merkleInternalNodes}{\term{internal nodes}}
|
||
\newcommand{\MerkleInternalNodes}{\term{Internal nodes}}
|
||
\newcommand{\merklePath}{\term{path}}
|
||
\newcommand{\merkleLayer}{\term{layer}}
|
||
\newcommand{\merkleLayers}{\term{layers}}
|
||
\newcommand{\merkleIndex}{\term{index}}
|
||
\newcommand{\merkleIndices}{\term{indices}}
|
||
\newcommand{\zkSNARK}{\term{zk-SNARK}}
|
||
\newcommand{\zkSNARKs}{\term{zk-SNARKs}}
|
||
\newcommand{\zkSNARKProof}{\term{zk-SNARK proof}}
|
||
\newcommand{\zkSNARKCircuit}{\term{zk-SNARK circuit}}
|
||
\newcommand{\zkSNARKCircuits}{\term{zk-SNARK circuits}}
|
||
\newcommand{\libsnark}{\term{libsnark}}
|
||
\newcommand{\bellman}{\term{bellman}}
|
||
\newcommand{\memo}{\term{memo field}}
|
||
\newcommand{\memos}{\term{memo fields}}
|
||
\newcommand{\Memos}{\titleterm{Memo Fields}}
|
||
\newcommand{\keyAgreementScheme}{\term{key agreement scheme}}
|
||
\newcommand{\keyAgreementSchemes}{\term{key agreement schemes}}
|
||
\newcommand{\keyDerivationFunction}{\term{Key Derivation Function}}
|
||
\newcommand{\keyDerivationFunctions}{\term{Key Derivation Functions}}
|
||
\newcommand{\KeyAgreement}{\titleterm{Key Agreement}}
|
||
\newcommand{\KeyDerivation}{\titleterm{Key Derivation}}
|
||
\newcommand{\KeyAgreementAndDerivation}{\titleterm{Key Agreement and Derivation}}
|
||
\newcommand{\hashFunction}{\term{hash function}}
|
||
\newcommand{\hashFunctions}{\term{hash functions}}
|
||
\newcommand{\HashFunction}{\titleterm{Hash Function}}
|
||
\newcommand{\HashFunctions}{\titleterm{Hash Functions}}
|
||
\newcommand{\encryptionScheme}{\term{encryption scheme}}
|
||
\newcommand{\symmetricEncryptionScheme}{\term{authenticated one-time symmetric encryption scheme}}
|
||
\newcommand{\SymmetricEncryption}{\titleterm{Authenticated One-Time Symmetric Encryption}}
|
||
\newcommand{\signatureScheme}{\term{signature scheme}}
|
||
\newcommand{\pseudoRandomFunction}{\term{Pseudo Random Function}}
|
||
\newcommand{\pseudoRandomFunctions}{\term{Pseudo Random Functions}}
|
||
\newcommand{\PseudoRandomFunctions}{\titleterm{Pseudo Random Functions}}
|
||
\newcommand{\pseudoRandomGenerator}{\term{Pseudo Random Generator}}
|
||
\newcommand{\pseudoRandomGenerators}{\term{Pseudo Random Generators}}
|
||
\newcommand{\PseudoRandomGenerators}{\titleterm{Pseudo Random Generators}}
|
||
\newcommand{\expandedSeed}{\term{expanded seed}}
|
||
\newcommand{\shaHashFunction}{\term{SHA-256 hash function}}
|
||
\newcommand{\shaCompress}{\term{SHA-256 compression}}
|
||
\newcommand{\shaCompressFunction}{\term{SHA-256 compression function}}
|
||
\newcommand{\BlakeTwo}{\titleterm{BLAKE2}}
|
||
\newcommand{\xPedersenHash}{\term{Pedersen hash}}
|
||
\newcommand{\xPedersenHashes}{\term{Pedersen hashes}}
|
||
\newcommand{\PedersenHashFunction}{\titleterm{Pedersen Hash Function}}
|
||
\newcommand{\xPedersenCommitment}{\term{Pedersen commitment}}
|
||
\newcommand{\xPedersenCommitments}{\term{Pedersen commitments}}
|
||
\newcommand{\xPedersenValueCommitment}{\term{Pedersen value commitment}}
|
||
\newcommand{\xPedersenValueCommitments}{\term{Pedersen value commitments}}
|
||
\newcommand{\windowedPedersenCommitment}{\term{windowed Pedersen commitment}}
|
||
\newcommand{\windowedPedersenCommitments}{\term{windowed Pedersen commitments}}
|
||
\newcommand{\WindowedPedersenCommitment}{\titleterm{Windowed Pedersen Commitment}}
|
||
\newcommand{\homomorphicPedersenCommitment}{\term{homomorphic Pedersen commitment}}
|
||
\newcommand{\homomorphicPedersenCommitments}{\term{homomorphic Pedersen commitments}}
|
||
\newcommand{\HomomorphicPedersenCommitment}{\titleterm{Homomorphic Pedersen Commitment}}
|
||
\newcommand{\distinctXCriterion}{\term{distinct-$x$ criterion}}
|
||
|
||
% Conventions
|
||
|
||
\newcommand{\bytes}[1]{\underline{\raisebox{-0.22ex}{}\smash{#1}}}
|
||
\newcommand{\zeros}[1]{[0]^{#1}}
|
||
\newcommand{\ones}[1]{[1]^{#1}}
|
||
\newcommand{\bit}{\mathbb{B}}
|
||
\newcommand{\overlap}[2]{\rlap{#2}\hspace{#1}{#2}}
|
||
\newcommand{\byte}{\mathbb{B}\kern -0.1em\raisebox{0.55ex}{\overlap{0.0001em}{\scalebox{0.7}{$\mathbb{Y}$}}}}
|
||
\newcommand{\Nat}{\mathbb{N}}
|
||
\newcommand{\PosInt}{\mathbb{N}^+}
|
||
\newcommand{\Rat}{\mathbb{Q}}
|
||
\newcommand{\GF}[1]{\mathbb{F}_{\!#1}}
|
||
\newcommand{\GFstar}[1]{\mathbb{F}^\ast_{#1}}
|
||
\newcommand{\typeexp}[2]{{#1}\vphantom{)}^{[{#2}]}}
|
||
\newcommand{\bitseq}[1]{\typeexp{\bit}{#1}}
|
||
\newcommand{\bitseqs}{\bitseq{\Nat}}
|
||
\newcommand{\byteseq}[1]{\typeexp{\byte}{#1}}
|
||
\newcommand{\byteseqs}{\byteseq{\Nat}}
|
||
\newcommand{\concatbits}{\mathsf{concat}_\bit}
|
||
\newcommand{\bconcat}{\,||\,}
|
||
\newcommand{\listcomp}[1]{[~{#1}~]}
|
||
\newcommand{\fun}[2]{{#1} \mapsto {#2}}
|
||
\newcommand{\first}{\mathsf{first}}
|
||
\newcommand{\for}{\text{ for }}
|
||
\newcommand{\from}{\text{ from }}
|
||
\newcommand{\upto}{\text{ up to }}
|
||
\newcommand{\downto}{\text{ down to }}
|
||
\newcommand{\tand}{\text{ \;and\, }}
|
||
\newcommand{\tor}{\text{ \;or\, }}
|
||
\newcommand{\squash}{\!\!\!}
|
||
\newcommand{\caseif}{\squash\text{if }}
|
||
\newcommand{\caseotherwise}{\squash\text{otherwise}}
|
||
\newcommand{\sidecondition}[1]{\hspace{3em}\left[{#1}\right]}
|
||
\newcommand{\sorted}{\mathsf{sorted}}
|
||
\newcommand{\length}{\mathsf{length}}
|
||
\newcommand{\mean}{\mathsf{mean}}
|
||
\newcommand{\median}{\mathsf{median}}
|
||
\newcommand{\bound}[2]{\mathsf{bound\,}_{#1}^{#2}}
|
||
\newcommand{\Lower}{\mathsf{lower}}
|
||
\newcommand{\Upper}{\mathsf{upper}}
|
||
\newcommand{\bitlength}{\mathsf{bitlength}}
|
||
\newcommand{\size}{\mathsf{size}}
|
||
\newcommand{\mantissa}{\mathsf{mantissa}}
|
||
\newcommand{\ToCompact}{\mathsf{ToCompact}}
|
||
\newcommand{\ToTarget}{\mathsf{ToTarget}}
|
||
\newcommand{\hexint}[1]{\mathtt{0x{#1}}}
|
||
\newcommand{\dontcare}{\kern -0.06em\raisebox{0.1ex}{\footnotesize{$\times$}}}
|
||
\newcommand{\ascii}[1]{\textbf{``\texttt{#1}''}}
|
||
\newcommand{\Justthebox}[2][-1.3ex]{\;\raisebox{#1}{\usebox{#2}}\;}
|
||
\newcommand{\hSigCRH}{\mathsf{hSigCRH}}
|
||
\newcommand{\hSigLength}{\mathsf{\ell_{hSig}}}
|
||
\newcommand{\hSigType}{\bitseq{\hSigLength}}
|
||
\newcommand{\Index}{\mathsf{Index}}
|
||
\newcommand{\EquihashGen}[1]{\mathsf{EquihashGen}_{#1}}
|
||
\newcommand{\CRH}{\mathsf{CRH}}
|
||
\newcommand{\SHACompress}{\mathsf{SHA256Compress}}
|
||
\newcommand{\SHAFull}{\mathsf{SHA\mhyphen256}}
|
||
\newcommand{\BlakeTwob}[1]{\mathsf{BLAKE2b\kern 0.05em\mhyphen{#1}}}
|
||
\newcommand{\BlakeTwos}[1]{\mathsf{BLAKE2s\kern 0.05em\mhyphen{#1}}}
|
||
\newcommand{\BlakeTwobGeneric}{\mathsf{BLAKE2b}}
|
||
\newcommand{\BlakeTwosGeneric}{\mathsf{BLAKE2s}}
|
||
\newcommand{\BlakeTwoGeneric}{\mathsf{BLAKE2}}
|
||
\newcommand{\SHACompressBox}[1]{\SHACompress\left(\Justthebox{#1}\right)}
|
||
\newcommand{\SHAFullBox}[1]{\SHAFull\left(\Justthebox{#1}\right)}
|
||
\newcommand{\CRHivkBox}[1]{\CRHivk\left(\Justthebox{#1}\right)}
|
||
\newcommand{\setof}[1]{\{{#1}\}}
|
||
\newcommand{\powerset}[1]{\mathscr{P}\!\left({#1}\right)}
|
||
\newcommand{\barerange}[2]{{{#1}\,..\,{#2}}}
|
||
\newcommand{\range}[2]{\setof{\barerange{#1}{#2}}}
|
||
\newcommand{\rangenozero}[2]{\range{#1}{#2} \difference \setof{0}}
|
||
\newcommand{\alln}{\barerange{1}{n}}
|
||
\newcommand{\minimum}{\mathsf{min}}
|
||
\newcommand{\maximum}{\mathsf{max}}
|
||
\newcommand{\floor}[1]{\mathsf{floor}\!\left({#1}\right)}
|
||
\newcommand{\trunc}[1]{\mathsf{trunc}\!\left({#1}\right)}
|
||
\newcommand{\ceiling}[1]{\mathsf{ceiling}\left({#1}\right)}
|
||
\newcommand{\vsum}[2]{\smashoperator[r]{\sum_{#1}^{#2}}}
|
||
\newcommand{\vproduct}[2]{\smashoperator[r]{\prod_{#1}^{#2}}}
|
||
\newcommand{\vxor}[2]{\smashoperator[r]{\bigoplus_{#1}^{#2}}}
|
||
\newcommand{\xor}{\oplus}
|
||
\newcommand{\band}{\binampersand}
|
||
\newcommand{\suband}{\raisebox{-0.6ex}{\kern-0.06em\scalebox{0.65}{$\binampersand$}}}
|
||
\newcommand{\bchoose}{\,?\,}
|
||
\newcommand{\mult}{\cdot}
|
||
\newcommand{\smult}{\!\cdot\!}
|
||
\newcommand{\scalarmult}[2]{\boldsymbol{[}{#1}\boldsymbol{]}\,{#2}}
|
||
\newcommand{\rightarrowR}{\buildrel{\scriptstyle\mathrm{R}}\over\rightarrow}
|
||
\newcommand{\leftarrowR}{\buildrel{\scriptstyle\mathrm{R}}\over\leftarrow}
|
||
\newcommand{\union}{\cup}
|
||
\newcommand{\intersection}{\cap}
|
||
\newcommand{\difference}{\setminus}
|
||
\newcommand{\suchthat}{\,\vert\;}
|
||
\newcommand{\paramdot}{\bigcdot}
|
||
\newcommand{\lincomb}[1]{\left(\vphantom{a^q_b}\kern-.025em{#1}\kern-0.04em\right)}
|
||
\newcommand{\constraint}[3]{\lincomb{#1}\hairspace \times\hairspace \lincomb{#2}\hairspace =\hairspace \lincomb{#3}}
|
||
|
||
% Key pairs
|
||
|
||
\newcommand{\PaymentAddress}{\mathsf{addr_{pk}}}
|
||
\newcommand{\DiversifiedPaymentAddress}{\mathsf{addr_{d}}}
|
||
\newcommand{\PaymentAddressLeadByte}{\hexint{16}}
|
||
\newcommand{\PaymentAddressSecondByte}{\hexint{9A}}
|
||
\newcommand{\InViewingKey}{\mathsf{ivk}}
|
||
\newcommand{\InViewingKeyLeadByte}{\hexint{A8}}
|
||
\newcommand{\InViewingKeySecondByte}{\hexint{AB}}
|
||
\newcommand{\InViewingKeyThirdByte}{\hexint{D3}}
|
||
\newcommand{\SpendingKeyLeadByte}{\hexint{AB}}
|
||
\newcommand{\SpendingKeySecondByte}{\hexint{36}}
|
||
\newcommand{\PtoSHAddressLeadByte}{\hexint{1C}}
|
||
\newcommand{\PtoSHAddressSecondByte}{\hexint{BD}}
|
||
\newcommand{\PtoPKHAddressLeadByte}{\hexint{1C}}
|
||
\newcommand{\PtoPKHAddressSecondByte}{\hexint{B8}}
|
||
\newcommand{\PaymentAddressTestnetLeadByte}{\hexint{16}}
|
||
\newcommand{\PaymentAddressTestnetSecondByte}{\hexint{B6}}
|
||
\newcommand{\InViewingKeyTestnetLeadByte}{\hexint{A8}}
|
||
\newcommand{\InViewingKeyTestnetSecondByte}{\hexint{AC}}
|
||
\newcommand{\InViewingKeyTestnetThirdByte}{\hexint{0C}}
|
||
\newcommand{\SpendingKeyTestnetLeadByte}{\hexint{AC}}
|
||
\newcommand{\SpendingKeyTestnetSecondByte}{\hexint{08}}
|
||
\newcommand{\PtoSHAddressTestnetLeadByte}{\hexint{1C}}
|
||
\newcommand{\PtoSHAddressTestnetSecondByte}{\hexint{BA}}
|
||
\newcommand{\PtoPKHAddressTestnetLeadByte}{\hexint{1D}}
|
||
\newcommand{\PtoPKHAddressTestnetSecondByte}{\hexint{25}}
|
||
\newcommand{\NotePlaintextLeadByteSprout}{\hexint{00}}
|
||
\newcommand{\NotePlaintextLeadByteSapling}{\hexint{01}}
|
||
\newcommand{\AuthPublic}{\mathsf{a_{pk}}}
|
||
\newcommand{\AuthPrivate}{\mathsf{a_{sk}}}
|
||
\newcommand{\AuthPrivateSup}[1]{\mathsf{a^\mathrm{#1}_{sk}}}
|
||
\newcommand{\AuthPrivateLength}{\mathsf{\ell_{\AuthPrivate}}}
|
||
\newcommand{\AuthPublicOld}[1]{\mathsf{a^{old}_{pk,\mathnormal{#1}}}}
|
||
\newcommand{\AuthPrivateOld}[1]{\mathsf{a^{old}_{sk,\mathnormal{#1}}}}
|
||
\newcommand{\AuthEmphPublicOld}[1]{\mathsf{a^{old}_{\textsf{\textbf{pk}},\mathnormal{#1}}}}
|
||
\newcommand{\AuthPublicOldX}[1]{\mathsf{a^{old}_{pk,\mathrm{#1}}}}
|
||
\newcommand{\AuthPrivateOldX}[1]{\mathsf{a^{old}_{sk,\mathrm{#1}}}}
|
||
\newcommand{\AuthPublicNew}[1]{\mathsf{a^{new}_{pk,\mathnormal{#1}}}}
|
||
\newcommand{\AuthPrivateNew}[1]{\mathsf{a^{new}_{sk,\mathnormal{#1}}}}
|
||
\newcommand{\AddressPublicNew}[1]{\mathsf{addr^{new}_{pk,\mathnormal{#1}}}}
|
||
\newcommand{\enc}{\mathsf{enc}}
|
||
\newcommand{\DHSecret}[1]{\mathsf{sharedSecret}_{#1}}
|
||
\newcommand{\EphemeralPublic}{\mathsf{epk}}
|
||
\newcommand{\EphemeralPrivate}{\mathsf{esk}}
|
||
\newcommand{\TransmitPublic}{\mathsf{pk_{enc}}}
|
||
\newcommand{\TransmitPublicSup}[1]{\mathsf{pk}^{#1}_\mathsf{enc}}
|
||
\newcommand{\TransmitPublicNew}[1]{\mathsf{pk^{new}_{\enc,\mathnormal{#1}}}}
|
||
\newcommand{\TransmitPrivate}{\mathsf{sk_{enc}}}
|
||
\newcommand{\TransmitPrivateSup}[1]{\mathsf{sk}^{#1}_\mathsf{enc}}
|
||
\newcommand{\TransmitBase}{\mathsf{g}}
|
||
|
||
% Sapling
|
||
|
||
\newcommand{\AuthPrivateSeed}{\mathsf{sk}}
|
||
\newcommand{\AuthPrivateSeedLength}{\mathsf{\ell_{\AuthPrivateSeed}}}
|
||
\newcommand{\PreAuthSignPrivate}{\mathsf{preask}}
|
||
\newcommand{\AuthSignPrivate}{\mathsf{ask}}
|
||
\newcommand{\AuthSignBase}{\mathcal{G}}
|
||
\newcommand{\AuthSignPublic}{\mathsf{ak}}
|
||
\newcommand{\PreAuthProvePrivate}{\mathsf{prersk}}
|
||
\newcommand{\AuthProvePrivate}{\mathsf{rsk}}
|
||
\newcommand{\AuthProveBase}{\mathcal{H}}
|
||
\newcommand{\AuthProvePublic}{\mathsf{rk}}
|
||
\newcommand{\NotePosition}{\mathsf{pos}}
|
||
\newcommand{\NotePositionBase}{\mathcal{J}}
|
||
\newcommand{\NotePositionTypeSprout}{\range{0}{2^{\MerkleDepthSprout}-1}}
|
||
\newcommand{\NotePositionTypeSapling}{\range{0}{2^{\MerkleDepthSapling}-1}}
|
||
\newcommand{\NullifierRand}{\mathsf{nr}}
|
||
\newcommand{\Hashnr}{H^{\NullifierRand}}
|
||
\newcommand{\Diversifier}{\mathsf{d}}
|
||
\newcommand{\DiversifierLength}{\mathsf{\ell_{\Diversifier}}}
|
||
\newcommand{\DiversifierType}{\byteseq{\DiversifierLength/8}}
|
||
\newcommand{\DiversifiedTransmitBase}{\mathsf{g_d}}
|
||
\newcommand{\DiversifiedTransmitPublic}{\mathsf{pk_d}}
|
||
\newcommand{\CRHivk}{\mathsf{CRH}^{\InViewingKey}}
|
||
\newcommand{\CRHivkText}{\texorpdfstring{$\CRHivk$}{CRHivk}}
|
||
\newcommand{\CRHivkOutput}{\CRHivk\mathsf{.Output}}
|
||
\newcommand{\CRHivkOutputLength}{\ell_{\InViewingKey}}
|
||
|
||
% PRFs
|
||
|
||
\newcommand{\PRF}[2]{\mathsf{{PRF}^{#2}_\mathnormal{#1}}}
|
||
\newcommand{\PRFaddr}[1]{\PRF{#1}{addr}}
|
||
\newcommand{\PRFnf}[1]{\PRF{#1}{\nf}}
|
||
\newcommand{\PRFsn}[1]{\PRF{#1}{sn}}
|
||
\newcommand{\PRFpk}[1]{\PRF{#1}{pk}}
|
||
\newcommand{\PRFrho}[1]{\PRF{#1}{\NoteAddressRand}}
|
||
\newcommand{\PRFnr}[1]{\PRF{#1}{\NullifierRand}}
|
||
\newcommand{\PRFOutputLength}{\mathsf{\ell_{PRF}}}
|
||
\newcommand{\PRFOutput}{\bitseq{\PRFOutputLength}}
|
||
|
||
% PRGs
|
||
|
||
\newcommand{\PRG}[2]{\mathsf{{PRG}^{#2}_\mathnormal{#1}}}
|
||
\newcommand{\PRGExpandSeed}[1]{\PRG{#1}{ExpandSeed}}
|
||
\newcommand{\PRGOutputLength}{\mathsf{\ell_{PRG}}}
|
||
\newcommand{\PRGOutput}{\bitseq{\PRGOutputLength}}
|
||
|
||
% Commitments
|
||
|
||
\newcommand{\UncommittedSprout}{\optSprout{\mathsf{Uncommitted}}}
|
||
\newcommand{\UncommittedSapling}{\mathsf{Uncommitted^{Sapling}}}
|
||
\newcommand{\NoteCommitmentSprout}{\optSprout{\mathsf{NoteCommitment}}}
|
||
\newcommand{\NoteCommitmentSapling}{\mathsf{NoteCommitment^{Sapling}}}
|
||
|
||
\newcommand{\CommitAlg}{\mathsf{COMM}}
|
||
\newcommand{\Commit}[1]{\CommitAlg_{#1}}
|
||
\newcommand{\CommitTrapdoor}{\CommitAlg\mathsf{.Trapdoor}}
|
||
\newcommand{\CommitInput}{\CommitAlg\mathsf{.Input}}
|
||
\newcommand{\CommitOutput}{\CommitAlg\mathsf{.Output}}
|
||
\newcommand{\NoteCommitSproutAlg}{\mathsf{\sprout{COMM}\notsprout{NoteCommit}}^{\mathsf{Sprout}}}
|
||
\newcommand{\NoteCommitSprout}[1]{\NoteCommitSproutAlg_{#1}}
|
||
\newcommand{\NoteCommitSproutTrapdoor}{\NoteCommitSproutAlg\mathsf{.Trapdoor}}
|
||
\newcommand{\NoteCommitSproutInput}{\NoteCommitSproutAlg\mathsf{.Input}}
|
||
\newcommand{\NoteCommitSproutOutput}{\NoteCommitSproutAlg\mathsf{.Output}}
|
||
\newcommand{\NoteCommitSaplingAlg}{\mathsf{NoteCommit}^{\mathsf{Sapling}}}
|
||
\newcommand{\NoteCommitSapling}[1]{\NoteCommitSaplingAlg_{#1}}
|
||
\newcommand{\NoteCommitSaplingTrapdoor}{\NoteCommitSaplingAlg\mathsf{.Trapdoor}}
|
||
\newcommand{\NoteCommitSaplingInput}{\NoteCommitSaplingAlg\mathsf{.Input}}
|
||
\newcommand{\NoteCommitSaplingOutput}{\NoteCommitSaplingAlg\mathsf{.Output}}
|
||
\newcommand{\ValueCommitAlg}{\mathsf{ValueCommit}}
|
||
\newcommand{\ValueCommit}[1]{\ValueCommitAlg_{#1}}
|
||
\newcommand{\ValueCommitTrapdoor}{\ValueCommitAlg\mathsf{.Trapdoor}}
|
||
\newcommand{\ValueCommitInput}{\ValueCommitAlg\mathsf{.Input}}
|
||
\newcommand{\ValueCommitOutput}{\ValueCommitAlg\mathsf{.Output}}
|
||
|
||
% Symmetric encryption
|
||
|
||
\newcommand{\Sym}{\mathsf{Sym}}
|
||
\newcommand{\SymEncrypt}[1]{\Sym\mathsf{.Encrypt}_{#1}}
|
||
\newcommand{\SymDecrypt}[1]{\Sym\mathsf{.Decrypt}_{#1}}
|
||
\newcommand{\SymSpecific}{\mathsf{AEAD\_CHACHA20\_POLY1305}}
|
||
\newcommand{\SymCipher}{\mathsf{ChaCha20}}
|
||
\newcommand{\SymAuth}{\mathsf{Poly1305}}
|
||
\newcommand{\Ptext}{\mathsf{P}}
|
||
\newcommand{\Plaintext}{\mathsf{Sym.}\mathbf{P}}
|
||
\newcommand{\Ctext}{\mathsf{C}}
|
||
\newcommand{\Ciphertext}{\mathsf{Sym.}\mathbf{C}}
|
||
\newcommand{\Key}{\mathsf{K}}
|
||
\newcommand{\Keyspace}{\mathsf{Sym.}\mathbf{K}}
|
||
\newcommand{\TransmitPlaintext}[1]{\Ptext^\enc_{#1}}
|
||
\newcommand{\TransmitCiphertext}[1]{\Ctext^\enc_{#1}}
|
||
\newcommand{\TransmitKey}[1]{\Key^\enc_{#1}}
|
||
\newcommand{\Adversary}{\mathcal{A}}
|
||
\newcommand{\Oracle}{\mathsf{O}}
|
||
\newcommand{\CryptoBoxSeal}{\mathsf{crypto\_box\_seal}}
|
||
|
||
% Key agreement
|
||
|
||
\newcommand{\KA}{\mathsf{KA}}
|
||
\newcommand{\KAPublic}{\KA\mathsf{.Public}}
|
||
\newcommand{\KAPrivate}{\KA\mathsf{.Private}}
|
||
\newcommand{\KASharedSecret}{\KA\mathsf{.SharedSecret}}
|
||
\newcommand{\KAFormatPrivate}{\KA\mathsf{.FormatPrivate}}
|
||
\newcommand{\KADerivePublic}{\KA\mathsf{.DerivePublic}}
|
||
\newcommand{\KAAgree}{\KA\mathsf{.Agree}}
|
||
\newcommand{\KABase}{\KA\mathsf{.Base}}
|
||
|
||
\newcommand{\KASprout}{\mathsf{\optSprout{KA}}}
|
||
\newcommand{\KASproutPublic}{\KASprout\mathsf{.Public}}
|
||
\newcommand{\KASproutPrivate}{\KASprout\mathsf{.Private}}
|
||
\newcommand{\KASproutSharedSecret}{\KASprout\mathsf{.SharedSecret}}
|
||
\newcommand{\KASproutFormatPrivate}{\KASprout\mathsf{.FormatPrivate}}
|
||
\newcommand{\KASproutDerivePublic}{\KASprout\mathsf{.DerivePublic}}
|
||
\newcommand{\KASproutAgree}{\KASprout\mathsf{.Agree}}
|
||
\newcommand{\KASproutBase}{\KASprout\mathsf{.Base}}
|
||
|
||
\newcommand{\KASapling}{\mathsf{KA^{Sapling}}}
|
||
\newcommand{\KASaplingPublic}{\KASapling\mathsf{.Public}}
|
||
\newcommand{\KASaplingPrivate}{\KASapling\mathsf{.Private}}
|
||
\newcommand{\KASaplingSharedSecret}{\KASapling\mathsf{.SharedSecret}}
|
||
\newcommand{\KASaplingFormatPrivate}{\KASapling\mathsf{.FormatPrivate}}
|
||
\newcommand{\KASaplingDerivePublic}{\KASapling\mathsf{.DerivePublic}}
|
||
\newcommand{\KASaplingAgree}{\KASapling\mathsf{.Agree}}
|
||
|
||
\newcommand{\CurveMultiply}{\mathsf{Curve25519}}
|
||
\newcommand{\CurveBase}{\bytes{9}}
|
||
\newcommand{\Clamp}{\mathsf{clamp_{Curve25519}}}
|
||
|
||
% KDF
|
||
|
||
\newcommand{\KDF}{\mathsf{KDF}}
|
||
\newcommand{\KDFSprout}{\optSprout{\KDF}}
|
||
\newcommand{\KDFSapling}{\mathsf{KDF^{Sapling}}}
|
||
\newcommand{\kdftag}{\mathsf{kdftag}}
|
||
\newcommand{\kdfinput}{\mathsf{kdfinput}}
|
||
|
||
% Notes
|
||
|
||
\newcommand{\Value}{\mathsf{v}}
|
||
\newcommand{\ValueNew}[1]{\Value^\mathsf{new}_{#1}}
|
||
\newcommand{\ValueOld}[1]{\Value^\mathsf{old}_{#1}}
|
||
\newcommand{\ValueCommitRand}{\mathsf{rcv}}
|
||
\newcommand{\ValueCommitRandLength}{\mathsf{\ell_{\ValueCommitRand}}}
|
||
\newcommand{\ValueCommitRandOld}{\ValueCommitRand^\mathsf{old}}
|
||
\newcommand{\ValueCommitRandNew}{\ValueCommitRand^\mathsf{new}}
|
||
\newcommand{\NoteTuple}[1]{\mathbf{n}_{#1}}
|
||
\newcommand{\NoteTypeSprout}{\optSprout{\mathsf{Note}}}
|
||
\newcommand{\NoteTypeSapling}{\mathsf{Note^{Sapling}}}
|
||
\newcommand{\NotePlaintext}[1]{\mathbf{np}_{#1}}
|
||
\newcommand{\NoteCommitRand}{\mathsf{\sprout{r}\notsprout{rcm}}}
|
||
\newcommand{\NoteCommitRandLength}{\mathsf{\ell_{\NoteCommitRand}}}
|
||
\newcommand{\NoteCommitRandOld}[1]{\NoteCommitRand^\mathsf{old}_{#1}}
|
||
\newcommand{\NoteCommitRandNew}[1]{\NoteCommitRand^\mathsf{new}_{#1}}
|
||
\newcommand{\NoteAddressRand}{\mathsf{\uprho}}
|
||
\newcommand{\NoteAddressRandOld}[1]{\NoteAddressRand^\mathsf{old}_{#1}}
|
||
\newcommand{\NoteAddressRandNew}[1]{\NoteAddressRand^\mathsf{new}_{#1}}
|
||
\newcommand{\NoteAddressPreRand}{\mathsf{\upvarphi}}
|
||
\newcommand{\NoteAddressPreRandLength}{\mathsf{\ell_{\NoteAddressPreRand}}}
|
||
\newcommand{\OutputIndex}{\mathsf{idx}}
|
||
\newcommand{\OutputIndexType}{\mathsf{OutputIndex}}
|
||
\newcommand{\NoteCommitS}{\mathsf{s}}
|
||
\newcommand{\cv}{\mathsf{cv}}
|
||
\newcommand{\cvOld}[1]{\cv^\mathsf{old}_{#1}}
|
||
\newcommand{\cvNew}[1]{\cv^\mathsf{new}_{#1}}
|
||
\newcommand{\cm}{\mathsf{cm}}
|
||
\newcommand{\cmOld}[1]{\cm^\mathsf{old}_{#1}}
|
||
\newcommand{\cmNew}[1]{\cm^\mathsf{new}_{#1}}
|
||
\newcommand{\snOld}[1]{\mathsf{sn}^\mathsf{old}_{#1}}
|
||
\newcommand{\nf}{\mathsf{nf}}
|
||
\newcommand{\nfOld}[1]{\nf^\mathsf{old}_{#1}}
|
||
\newcommand{\Memo}{\mathsf{memo}}
|
||
\newcommand{\DecryptNote}{\mathtt{DecryptNote}}
|
||
\newcommand{\ReplacementCharacter}{\textsf{U+FFFD}}
|
||
|
||
% Money supply
|
||
|
||
\newcommand{\MAXMONEY}{\mathsf{MAX\_MONEY}}
|
||
\newcommand{\BlockSubsidy}{\mathsf{BlockSubsidy}}
|
||
\newcommand{\MinerSubsidy}{\mathsf{MinerSubsidy}}
|
||
\newcommand{\FoundersReward}{\mathsf{FoundersReward}}
|
||
\newcommand{\SlowStartInterval}{\mathsf{SlowStartInterval}}
|
||
\newcommand{\SlowStartShift}{\mathsf{SlowStartShift}}
|
||
\newcommand{\SlowStartRate}{\mathsf{SlowStartRate}}
|
||
\newcommand{\HalvingInterval}{\mathsf{HalvingInterval}}
|
||
\newcommand{\MaxBlockSubsidy}{\mathsf{MaxBlockSubsidy}}
|
||
\newcommand{\NumFounderAddresses}{\mathsf{NumFounderAddresses}}
|
||
\newcommand{\FounderAddressChangeInterval}{\mathsf{FounderAddressChangeInterval}}
|
||
\newcommand{\FoundersFraction}{\mathsf{FoundersFraction}}
|
||
\newcommand{\BlockHeight}{\mathsf{height}}
|
||
\newcommand{\Halving}{\mathsf{Halving}}
|
||
\newcommand{\FounderAddress}{\mathsf{FounderAddress}}
|
||
\newcommand{\FounderAddressList}{\mathsf{FounderAddressList}}
|
||
\newcommand{\FounderAddressIndex}{\mathsf{FounderAddressIndex}}
|
||
\newcommand{\RedeemScriptHash}{\mathsf{RedeemScriptHash}}
|
||
|
||
\newcommand{\blockSubsidy}{\term{block subsidy}}
|
||
\newcommand{\minerSubsidy}{\term{miner subsidy}}
|
||
\newcommand{\foundersReward}{\term{Founders' Reward}}
|
||
\newcommand{\slowStartPeriod}{\term{slow-start period}}
|
||
\newcommand{\halvingInterval}{\term{halving interval}}
|
||
|
||
\newcommand{\PoWLimit}{\mathsf{PoWLimit}}
|
||
\newcommand{\PoWAveragingWindow}{\mathsf{PoWAveragingWindow}}
|
||
\newcommand{\PoWMedianBlockSpan}{\mathsf{PoWMedianBlockSpan}}
|
||
\newcommand{\PoWMaxAdjustDown}{\mathsf{PoWMaxAdjustDown}}
|
||
\newcommand{\PoWMaxAdjustUp}{\mathsf{PoWMaxAdjustUp}}
|
||
\newcommand{\PoWDampingFactor}{\mathsf{PoWDampingFactor}}
|
||
\newcommand{\PoWTargetSpacing}{\mathsf{PoWTargetSpacing}}
|
||
\newcommand{\MeanTarget}{\mathsf{MeanTarget}}
|
||
\newcommand{\MedianTime}{\mathsf{MedianTime}}
|
||
\newcommand{\AveragingWindowTimespan}{\mathsf{AveragingWindowTimespan}}
|
||
\newcommand{\MinActualTimespan}{\mathsf{MinActualTimespan}}
|
||
\newcommand{\MaxActualTimespan}{\mathsf{MaxActualTimespan}}
|
||
\newcommand{\ActualTimespan}{\mathsf{ActualTimespan}}
|
||
\newcommand{\ActualTimespanDamped}{\mathsf{ActualTimespanDamped}}
|
||
\newcommand{\ActualTimespanBounded}{\mathsf{ActualTimespanBounded}}
|
||
\newcommand{\Threshold}{\mathsf{Threshold}}
|
||
\newcommand{\ThresholdBits}{\mathsf{ThresholdBits}}
|
||
|
||
\newcommand{\targetThreshold}{\term{target threshold}}
|
||
\newcommand{\targetThresholds}{\term{target thresholds}}
|
||
|
||
% Signatures
|
||
|
||
\newcommand{\Sig}{\mathsf{Sig}}
|
||
\newcommand{\SigPublic}{\Sig\mathsf{.Public}}
|
||
\newcommand{\SigPrivate}{\Sig\mathsf{.Private}}
|
||
\newcommand{\SigMessage}{\Sig\mathsf{.Message}}
|
||
\newcommand{\SigSignature}{\Sig\mathsf{.Signature}}
|
||
\newcommand{\SigGen}{\Sig\mathsf{.Gen}}
|
||
\newcommand{\SigSign}[1]{\Sig\mathsf{.Sign}_{#1}}
|
||
\newcommand{\SigVerify}[1]{\Sig\mathsf{.Verify}_{#1}}
|
||
\newcommand{\SigRandom}{\Sig\mathsf{.Random}}
|
||
\newcommand{\SigRandomizePublic}{\Sig\mathsf{.RandomizePublic}}
|
||
\newcommand{\SigRandomizePrivate}{\Sig\mathsf{.RandomizePrivate}}
|
||
\newcommand{\SigRandomnessId}{\Sig\mathsf{.Id}}
|
||
\newcommand{\SigRandomness}{r}
|
||
|
||
\newcommand{\JoinSplitSig}{\mathsf{JoinSplitSig}}
|
||
\newcommand{\JoinSplitSigPublic}{\JoinSplitSig\mathsf{.Public}}
|
||
\newcommand{\JoinSplitSigPrivate}{\JoinSplitSig\mathsf{.Private}}
|
||
\newcommand{\JoinSplitSigMessage}{\JoinSplitSig\mathsf{.Message}}
|
||
\newcommand{\JoinSplitSigSignature}{\JoinSplitSig\mathsf{.Signature}}
|
||
\newcommand{\JoinSplitSigGen}{\JoinSplitSig\mathsf{.Gen}}
|
||
\newcommand{\JoinSplitSigSign}[1]{\JoinSplitSig\mathsf{.Sign}_{#1}}
|
||
\newcommand{\JoinSplitSigVerify}[1]{\JoinSplitSig\mathsf{.Verify}_{#1}}
|
||
\newcommand{\JoinSplitSigSpecific}{\mathsf{Ed25519}}
|
||
\newcommand{\JoinSplitSigHashName}{\mathsf{SHA\mhyphen512}}
|
||
|
||
\newcommand{\SpendAuthSig}{\mathsf{SpendAuthSig}}
|
||
\newcommand{\SpendAuthSigPublic}{\SpendAuthSig\mathsf{.Public}}
|
||
\newcommand{\SpendAuthSigPrivate}{\SpendAuthSig\mathsf{.Private}}
|
||
\newcommand{\SpendAuthSigMessage}{\SpendAuthSig\mathsf{.Message}}
|
||
\newcommand{\SpendAuthSigSignature}{\SpendAuthSig\mathsf{.Signature}}
|
||
\newcommand{\SpendAuthSigGen}{\SpendAuthSig\mathsf{.Gen}}
|
||
\newcommand{\SpendAuthSigSign}[1]{\SpendAuthSig\mathsf{.Sign}_{#1}}
|
||
\newcommand{\SpendAuthSigVerify}[1]{\SpendAuthSig\mathsf{.Verify}_{#1}}
|
||
\newcommand{\SpendAuthSigSpecific}{\mathsf{EdJubjub}}
|
||
\newcommand{\SpendAuthSigHashName}{\mathsf{BlakeTwob{512}}}
|
||
|
||
\newcommand{\EdDSA}{\mathsf{EdDSA}}
|
||
\newcommand{\EdDSAr}{R}
|
||
\newcommand{\EdDSAs}{S}
|
||
\newcommand{\EdDSAR}{\bytes{R}}
|
||
\newcommand{\EdDSAS}{\bytes{S}}
|
||
\newcommand{\RandomSeedLength}{\mathsf{\ell_{Seed}}}
|
||
\newcommand{\RandomSeedType}{\bitseq{\mathsf{\ell_{Seed}}}}
|
||
\newcommand{\pksig}{\mathsf{pk_{sig}}}
|
||
\newcommand{\sk}{\mathsf{sk}}
|
||
\newcommand{\hSigInput}{\mathsf{hSigInput}}
|
||
\newcommand{\crhInput}{\mathsf{crhInput}}
|
||
\newcommand{\dataToBeSigned}{\mathsf{dataToBeSigned}}
|
||
|
||
% Merkle tree
|
||
|
||
\newcommand{\MerkleDepth}{\mathsf{MerkleDepth}}
|
||
\newcommand{\MerkleDepthSprout}{\optSprout{\MerkleDepth}}
|
||
\newcommand{\MerkleDepthSapling}{\MerkleDepth^\mathsf{Sapling}}
|
||
\newcommand{\MerkleNode}[2]{\mathsf{M}^{#1}_{#2}}
|
||
\newcommand{\MerkleSibling}{\mathsf{sibling}}
|
||
\newcommand{\MerkleCRH}{\mathsf{MerkleCRH}}
|
||
\newcommand{\MerkleCRHSprout}{\optSprout{\MerkleCRH}}
|
||
\newcommand{\MerkleCRHSapling}{\MerkleCRH^\mathsf{Sapling}}
|
||
\newcommand{\MerkleHashLength}{\mathsf{\ell_{Merkle}}}
|
||
\newcommand{\MerkleHashLengthSprout}{\mathsf{\ell_{\sprout{Merkle}\notsprout{MerkleSprout}}}}
|
||
\newcommand{\MerkleHashLengthSapling}{\mathsf{\ell_{MerkleSapling}}}
|
||
\newcommand{\MerkleHash}{\bitseq{\MerkleHashLength}}
|
||
\newcommand{\MerkleHashSprout}{\bitseq{\MerkleHashLengthSprout}}
|
||
\newcommand{\MerkleHashSapling}{\bitseq{\MerkleHashLengthSapling}}
|
||
\newcommand{\MerkleLayer}{\range{0}{\MerkleDepth-1}}
|
||
\newcommand{\MerkleLayerSprout}{\range{0}{\MerkleDepthSprout-1}}
|
||
\newcommand{\MerkleLayerSapling}{\range{0}{\MerkleDepthSapling-1}}
|
||
|
||
% Transactions
|
||
|
||
\newcommand{\fOverwintered}{\mathtt{fOverwintered}}
|
||
\newcommand{\versionField}{\mathtt{version}}
|
||
\newcommand{\txInCount}{\mathtt{tx\_in\_count}}
|
||
\newcommand{\txIn}{\mathtt{tx\_in}}
|
||
\newcommand{\txOutCount}{\mathtt{tx\_out\_count}}
|
||
\newcommand{\txOut}{\mathtt{tx\_out}}
|
||
\newcommand{\lockTime}{\mathtt{lock\_time}}
|
||
\newcommand{\nJoinSplit}{\mathtt{nJoinSplit}}
|
||
\newcommand{\vJoinSplit}{\mathtt{vJoinSplit}}
|
||
\newcommand{\vpubOldField}{\mathtt{vpub\_old}}
|
||
\newcommand{\vpubNewField}{\mathtt{vpub\_new}}
|
||
\newcommand{\anchorField}{\mathtt{anchor}}
|
||
\newcommand{\joinSplitSig}{\mathtt{joinSplitSig}}
|
||
\newcommand{\joinSplitPrivKey}{\mathtt{joinSplitPrivKey}}
|
||
\newcommand{\joinSplitPubKey}{\mathtt{joinSplitPubKey}}
|
||
\newcommand{\nullifierField}{\mathtt{nullifier}}
|
||
\newcommand{\nullifiersField}{\mathtt{nullifiers}}
|
||
\newcommand{\cvField}{\mathtt{cv}}
|
||
\newcommand{\cmField}{\mathtt{cm}}
|
||
\newcommand{\commitment}{\mathtt{commitment}}
|
||
\newcommand{\commitments}{\mathtt{commitments}}
|
||
\newcommand{\ephemeralKey}{\mathtt{ephemeralKey}}
|
||
\newcommand{\encCiphertext}{\mathtt{encCiphertext}}
|
||
\newcommand{\encCiphertexts}{\mathtt{encCiphertexts}}
|
||
\newcommand{\randomSeed}{\mathtt{randomSeed}}
|
||
\newcommand{\spendAuthSig}{\mathtt{spendAuthSig}}
|
||
\newcommand{\Varies}{\textit{Varies}}
|
||
\newcommand{\heading}[1]{\multicolumn{1}{c|}{#1}}
|
||
\newcommand{\type}[1]{\texttt{#1}}
|
||
\newcommand{\compactSize}{\type{compactSize uint}}
|
||
|
||
|
||
\newcommand{\sighashTxHashes}{\term{SIGHASH transaction hashes}}
|
||
\newcommand{\sighashType}{\term{SIGHASH type}}
|
||
\newcommand{\sighashTypes}{\term{SIGHASH types}}
|
||
\newcommand{\SIGHASHALL}{\mathsf{SIGHASH\_ALL}}
|
||
\newcommand{\scriptSig}{\mathtt{scriptSig}}
|
||
\newcommand{\scriptPubKey}{\mathtt{scriptPubKey}}
|
||
\newcommand{\ScriptOP}[1]{\texttt{OP\_{#1}}}
|
||
|
||
% Equihash and block headers
|
||
|
||
\newcommand{\validEquihashSolution}{\term{valid Equihash solution}}
|
||
\newcommand{\powtag}{\mathsf{powtag}}
|
||
\newcommand{\powheader}{\mathsf{powheader}}
|
||
\newcommand{\powcount}{\mathsf{powcount}}
|
||
\newcommand{\nVersion}{\mathtt{nVersion}}
|
||
\newcommand{\hashPrevBlock}{\mathtt{hashPrevBlock}}
|
||
\newcommand{\hashMerkleRoot}{\mathtt{hashMerkleRoot}}
|
||
\newcommand{\hashReserved}{\mathtt{hashReserved}}
|
||
\newcommand{\hashFinalSaplingRoot}{\mathtt{hashFinalSaplingRoot}}
|
||
\newcommand{\nTimeField}{\mathtt{nTime}}
|
||
\newcommand{\nTime}{\mathsf{nTime}}
|
||
\newcommand{\nBitsField}{\mathtt{nBits}}
|
||
\newcommand{\nBits}{\mathsf{nBits}}
|
||
\newcommand{\nNonce}{\mathtt{nNonce}}
|
||
\newcommand{\solutionSize}{\mathtt{solutionSize}}
|
||
\newcommand{\solution}{\mathtt{solution}}
|
||
\newcommand{\SHAd}{\term{SHA-256d}}
|
||
|
||
% Proving system
|
||
|
||
\newcommand{\ZK}{\mathsf{ZK}}
|
||
\newcommand{\ZKProvingKey}{\mathsf{ZK.ProvingKey}}
|
||
\newcommand{\ZKVerifyingKey}{\mathsf{ZK.VerifyingKey}}
|
||
\newcommand{\pk}{\mathsf{pk}}
|
||
\newcommand{\vk}{\mathsf{vk}}
|
||
\newcommand{\ZKGen}{\mathsf{ZK.Gen}}
|
||
\newcommand{\ZKProof}{\mathsf{ZK.Proof}}
|
||
\newcommand{\ZKPrimary}{\mathsf{ZK.PrimaryInput}}
|
||
\newcommand{\ZKAuxiliary}{\mathsf{ZK.AuxiliaryInput}}
|
||
\newcommand{\ZKSatisfying}{\mathsf{ZK.SatisfyingInputs}}
|
||
\newcommand{\ZKProve}[1]{\mathsf{ZK.}\mathtt{Prove}_{#1}}
|
||
\newcommand{\ZKVerify}[1]{\mathsf{ZK.}\mathtt{Verify}_{#1}}
|
||
\newcommand{\Simulator}{\mathcal{S}}
|
||
\newcommand{\Distinguisher}{\mathcal{D}}
|
||
\newcommand{\JoinSplit}{\mathsf{ZKJoinSplit}}
|
||
\newcommand{\JoinSplitVerify}{\JoinSplit\mathsf{.Verify}}
|
||
\newcommand{\JoinSplitProve}{\JoinSplit\mathsf{.Prove}}
|
||
\newcommand{\JoinSplitProof}{\JoinSplit\mathsf{.Proof}}
|
||
\newcommand{\Spend}{\mathsf{ZKSpend}}
|
||
\newcommand{\SpendVerify}{\Spend\mathsf{.Verify}}
|
||
\newcommand{\SpendProve}{\Spend\mathsf{.Prove}}
|
||
\newcommand{\SpendProof}{\Spend\mathsf{.Proof}}
|
||
\newcommand{\Output}{\mathsf{ZKOutput}}
|
||
\newcommand{\OutputVerify}{\Output\mathsf{.Verify}}
|
||
\newcommand{\OutputProve}{\Output\mathsf{.Prove}}
|
||
\newcommand{\OutputProof}{\Output\mathsf{.Proof}}
|
||
\newcommand{\Proof}[1]{\pi_{\!{#1}}}
|
||
\newcommand{\ProofJoinSplit}{\pi_\JoinSplit}
|
||
\newcommand{\ProofSpend}{\pi_\Spend}
|
||
\newcommand{\ProofOutput}{\pi_\Output}
|
||
\newcommand{\zkproof}{\mathtt{zkproof}}
|
||
\newcommand{\POUR}{\texttt{POUR}}
|
||
\newcommand{\Prob}[2]{\mathrm{Pr}\scalebox{0.88}{\ensuremath{
|
||
\left[\!\!\begin{array}{c}#1\end{array} \middle| \begin{array}{l}#2\end{array}\!\!\right]
|
||
}}}
|
||
\newcommand{\BNImpl}{\mathtt{ALT\_BN128}}
|
||
|
||
% JoinSplit
|
||
|
||
\newcommand{\hSig}{\mathsf{h_{Sig}}}
|
||
\newcommand{\hSigText}{\texorpdfstring{$\hSig$}{hSig}}
|
||
\newcommand{\h}[1]{\mathsf{h_{\mathnormal{#1}}}}
|
||
\newcommand{\NOld}{\mathrm{N}^\mathsf{old}}
|
||
\newcommand{\NNew}{\mathrm{N}^\mathsf{new}}
|
||
\newcommand{\allN}[1]{\mathrm{1}..\mathrm{N}^\mathsf{#1}}
|
||
\newcommand{\allOld}{\allN{old}}
|
||
\newcommand{\allNew}{\allN{new}}
|
||
\newcommand{\setofOld}{\setof{\allOld}}
|
||
\newcommand{\setofNew}{\setof{\allNew}}
|
||
\newcommand{\vmacs}{\mathtt{vmacs}}
|
||
\newcommand{\vpubOld}{\mathsf{v_{pub}^{old}}}
|
||
\newcommand{\vpubNew}{\mathsf{v_{pub}^{new}}}
|
||
\newcommand{\nOld}[1]{\NoteTuple{#1}^\mathsf{old}}
|
||
\newcommand{\nNew}[1]{\NoteTuple{#1}^\mathsf{new}}
|
||
\newcommand{\vOld}[1]{\mathsf{v}_{#1}^\mathsf{old}}
|
||
\newcommand{\vNew}[1]{\mathsf{v}_{#1}^\mathsf{new}}
|
||
\newcommand{\RandomSeed}{\mathsf{randomSeed}}
|
||
\newcommand{\rt}{\mathsf{rt}}
|
||
\newcommand{\treepath}[1]{\mathsf{path}_{#1}}
|
||
\newcommand{\Receive}{\mathsf{Receive}}
|
||
\newcommand{\EnforceMerklePath}[1]{\mathsf{enforceMerklePath}_{~\!\!#1}}
|
||
|
||
% Elliptic curve stuff
|
||
|
||
\newcommand{\Curve}{E}
|
||
\newcommand{\Zero}{\mathcal{O}}
|
||
\newcommand{\Generator}{\mathcal{P}}
|
||
\newcommand{\Selectu}{\scalebox{1.52}{$u$}}
|
||
\newcommand{\SelectuOf}[1]{\Selectu\!\left({#1}\right)\!}
|
||
\newcommand{\Selectv}{\scalebox{1.52}{$\varv$}}
|
||
\newcommand{\SelectvOf}[1]{\Selectv\!\left({#1}\right)\!}
|
||
|
||
\newcommand{\ParamP}[1]{{{#1}_\mathbb{P}}}
|
||
\newcommand{\ParamPexp}[2]{{{#1}_\mathbb{P}\!}^{#2}}
|
||
\newcommand{\GroupP}[1]{\mathbb{P}_{#1}}
|
||
\newcommand{\GroupPstar}[1]{\mathbb{P}^\ast_{#1}}
|
||
\newcommand{\CurveP}[1]{\Curve_{\GroupP{#1}}}
|
||
\newcommand{\ZeroP}[1]{\Zero_{\GroupP{#1}}}
|
||
\newcommand{\GenP}[1]{\Generator_{\GroupP{#1}}}
|
||
\newcommand{\ellP}[1]{\ell_{\GroupP{#1}}}
|
||
\newcommand{\reprP}[1]{\repr_{\GroupP{#1}}}
|
||
\newcommand{\abstP}[1]{\abst_{\GroupP{#1}}}
|
||
\newcommand{\PairingP}{\ParamP{\hat{e}}}
|
||
|
||
\newcommand{\ParamG}[1]{{{#1}_\mathbb{G}}}
|
||
\newcommand{\ParamGexp}[2]{{{#1}_\mathbb{G}\!}^{#2}}
|
||
\newcommand{\GroupG}[1]{\mathbb{G}_{#1}}
|
||
\newcommand{\GroupGstar}[1]{\mathbb{G}^\ast_{#1}}
|
||
\newcommand{\GroupGHash}[1]{\mathsf{GroupHash}^\GroupG{#1}}
|
||
\newcommand{\CurveG}[1]{\Curve_{\GroupG{#1}}}
|
||
\newcommand{\ZeroG}[1]{\Zero_{\GroupG{#1}}}
|
||
\newcommand{\GenG}[1]{\Generator_{\GroupG{#1}}}
|
||
\newcommand{\ellG}[1]{\ell_{\GroupG{#1}}}
|
||
\newcommand{\reprG}[1]{\repr_{\GroupG{#1}}}
|
||
\newcommand{\abstG}[1]{\abst_{\GroupG{#1}}}
|
||
\newcommand{\PairingG}{\ParamG{\hat{e}}}
|
||
\newcommand{\ExtractG}{\ParamG{\mathsf{Extract}}}
|
||
|
||
\newcommand{\ParamS}[1]{{{#1}_\mathbb{\hskip 0.03em S}}}
|
||
\newcommand{\ParamSexp}[2]{{{#1}_\mathbb{\hskip 0.03em S}\!}^{#2}}
|
||
\newcommand{\GroupS}[1]{\mathbb{S}_{#1}}
|
||
\newcommand{\GroupSstar}[1]{\mathbb{S}^\ast_{#1}}
|
||
\newcommand{\CurveS}[1]{\Curve_{\GroupS{#1}}}
|
||
\newcommand{\ZeroS}[1]{\Zero_{\GroupS{#1}}}
|
||
\newcommand{\GenS}[1]{\Generator_{\GroupS{#1}}}
|
||
\newcommand{\ellS}[1]{\ell_{\GroupS{#1}}}
|
||
\newcommand{\reprS}[1]{\repr_{\GroupG{#1}}}
|
||
\newcommand{\abstS}[1]{\abst_{\GroupG{#1}}}
|
||
\newcommand{\PairingS}{\ParamS{\hat{e}}}
|
||
|
||
\newcommand{\ParamJ}[1]{{{#1}_\mathbb{\hskip 0.01em J}}}
|
||
\newcommand{\ParamJexp}[2]{{{#1}_\mathbb{\hskip 0.01em J}\!}^{#2}}
|
||
\newcommand{\GroupJ}{\mathbb{J}}
|
||
\newcommand{\GroupJHash}[1]{\mathsf{GroupHash}^\mathbb{J}_{#1}}
|
||
\newcommand{\CurveJ}{\Curve_{\GroupJ}}
|
||
\newcommand{\ZeroJ}{\Zero_{\GroupJ}}
|
||
\newcommand{\GenJ}{\Generator_{\GroupJ}}
|
||
\newcommand{\ellJ}{\ell_{\GroupJ}}
|
||
\newcommand{\reprJ}{\repr_{\GroupJ}}
|
||
\newcommand{\reprJOf}[1]{\reprJ\!\left({#1}\right)\!}
|
||
\newcommand{\abstJ}{\abst_{\GroupJ}}
|
||
\newcommand{\abstJOf}[1]{\abstJ\!\left({#1}\right)\!}
|
||
\newcommand{\ExtractJ}{\ParamJ{\mathsf{Extract}}}
|
||
\newcommand{\FindGroupJHash}{\mathsf{FindGroupHash}^\mathbb{J}}
|
||
\newcommand{\FindGroupJHashOf}[1]{\FindGroupJHash\!\left({#1}\right)\!}
|
||
|
||
\newcommand{\ParamM}[1]{{{#1}_\mathbb{\hskip 0.03em M}}}
|
||
\newcommand{\ParamMexp}[2]{{{#1}_\mathbb{\hskip 0.03em M}\!}^{#2}}
|
||
|
||
% TODO: should this be a named constant?
|
||
\newcommand{\JubjubScalarThreshold}{2^{251}}
|
||
|
||
\newcommand{\pack}{\mathsf{pack}}
|
||
|
||
\newcommand{\Acc}{\mathsf{Acc}}
|
||
\newcommand{\Base}{\mathsf{Base}}
|
||
\newcommand{\Addend}{\mathsf{Addend}}
|
||
\newcommand{\Sum}{\mathsf{Sum}}
|
||
\newcommand{\ainv}{a_{\mathsf{inv}}}
|
||
|
||
\newcommand{\repr}{\mathsf{repr}}
|
||
\newcommand{\abst}{\mathsf{abst}}
|
||
\newcommand{\xP}{{x_{\hspace{-0.12em}P}}}
|
||
\newcommand{\yP}{{y_{\hspace{-0.03em}P}}}
|
||
|
||
\newcommand{\CRS}{\mathsf{CRS}}
|
||
\newcommand{\CRSType}{\mathsf{CRSType}}
|
||
|
||
% Conversions
|
||
|
||
\newcommand{\ECtoOSP}{\mathsf{EC2OSP}}
|
||
\newcommand{\ECtoOSPXL}{\mathsf{EC2OSP\mhyphen{}XL}}
|
||
\newcommand{\ECtoOSPXS}{\mathsf{EC2OSP\mhyphen{}XS}}
|
||
\newcommand{\FEtoIP}{\mathsf{FE2IP}}
|
||
\newcommand{\FEtoIPP}{\mathsf{FE2IPP}}
|
||
\newcommand{\ItoLEBSP}[1]{\mathsf{I2LEBSP}_{#1}}
|
||
\newcommand{\ItoBEBSP}[1]{\mathsf{I2BEBSP}_{#1}}
|
||
\newcommand{\ItoLEOSPvar}{\mathsf{I2LEOSP_{var}}}
|
||
\newcommand{\LEOStoIP}[1]{\mathsf{LEOS2IP}_{#1}}
|
||
\newcommand{\LEBStoOSP}[1]{\mathsf{LEBS2OSP}_{#1}}
|
||
\newcommand{\LEBStoOSPOf}[2]{\LEBStoOSP{#1}\!\left({#2}\right)}
|
||
|
||
% Sapling circuits
|
||
|
||
\newcommand{\DecompressValidate}{\mathsf{DecompressValidate}}
|
||
\newcommand{\FixedScalarMult}{\mathsf{FixedScalarMult}}
|
||
\newcommand{\VariableScalarMult}{\mathsf{VariableScalarMult}}
|
||
\newcommand{\MontToEdwards}{\mathsf{MontToEdwards}}
|
||
\newcommand{\EdwardsToMont}{\mathsf{EdwardsToMont}}
|
||
\newcommand{\AffineEdwardsJubjub}{\mathsf{AffineEdwardsJubjub}}
|
||
\newcommand{\AffineMontJubjub}{\mathsf{AffineMontJubjub}}
|
||
\newcommand{\CompressedEdwardsJubjub}{\mathsf{CompressedEdwardsJubjub}}
|
||
\newcommand{\PedersenHash}{\mathsf{PedersenHash}}
|
||
\newcommand{\PedersenGenAlg}{\mathcal{I}}
|
||
\newcommand{\PedersenGen}[2]{\PedersenGenAlg^{\kern -0.05em{#1}}_{\kern 0.1em {#2}}}
|
||
\newcommand{\PedersenEncode}[1]{\langle{#1}\rangle}
|
||
\newcommand{\PedersenEncodeSub}[2]{\langle{#2}\rangle_{\kern -0.1em {#1}\vphantom{S'}}}
|
||
\newcommand{\PedersenEncodeNonneg}[1]{\langle{#1}\rangle^{\PedersenRangeOffset}}
|
||
\newcommand{\PedersenHashToPoint}{\mathsf{PedersenHashToPoint}}
|
||
\newcommand{\MixingPedersenHash}{\mathsf{MixingPedersenHash}}
|
||
\newcommand{\WindowedPedersenCommitAlg}{\mathsf{WindowedPedersenCommit}}
|
||
\newcommand{\WindowedPedersenCommit}[1]{\WindowedPedersenCommitAlg_{#1}}
|
||
\newcommand{\HomomorphicPedersenCommitAlg}{\mathsf{HomomorphicPedersenCommit}}
|
||
\newcommand{\HomomorphicPedersenCommit}[1]{\HomomorphicPedersenCommitAlg_{#1}}
|
||
\newcommand{\Digits}{\mathsf{Digits}}
|
||
\newcommand{\PedersenRangeOffset}{\Delta}
|
||
\newcommand{\Mask}{\mathsf{Mask}}
|
||
\newcommand{\abs}{\mathsf{abs}}
|
||
|
||
% Consensus rules
|
||
|
||
\newcommand{\consensusrule}[1]{\needspace{3ex}\subparagraph{Consensus rule:}{#1}}
|
||
\newenvironment{consensusrules}{\introlist\subparagraph{Consensus rules:}\begin{itemize}}{\end{itemize}}
|
||
\newcommand{\sproutspecificitem}[1]{\item \sproutspecific{#1}}
|
||
\newcommand{\sproutonlyitem}[1]{\item \sproutonly{#1}}
|
||
\newcommand{\saplingonwarditem}[1]{\sapling{\item {[\Sapling onward]}\, {#1}}}
|
||
\newcommand{\prenuzeroitem}[1]{\item \prenuzero{#1}}
|
||
\newcommand{\nuzeroonlyitem}[1]{\nuzero{\item {[\NUZero only, pre-\Sapling\!]}\, {#1}}}
|
||
\newcommand{\nuzeroonwarditem}[1]{\nuzero{\item {[\NUZero onward]}\, {#1}}}
|
||
\newcommand{\sproutspecific}[1]{\notsprout{[\Sprout\!]\,} {#1}}
|
||
\newcommand{\sproutonly}[1]{\notsprout{[\Sprout only]\,} {#1}}
|
||
\newcommand{\saplingonward}[1]{\sapling{[\Sapling onward]\, {#1}}}
|
||
\newcommand{\prenuzero}[1]{\notsprout{[Pre-\NUZero\!]\,} {#1}}
|
||
\newcommand{\nuzeroonly}[1]{\nuzero{[\NUZero only, pre-\Sapling\!]\, {#1}}}
|
||
\newcommand{\nuzeroonward}[1]{\nuzero{[\NUZero onward]\, {#1}}}
|
||
|
||
\newcommand{\securityrequirement}[1]{\needspace{3ex}\subparagraph{Security requirement:}{#1}}
|
||
\newenvironment{securityrequirements}{\introlist\subparagraph{Security requirements:}\begin{itemize}}{\end{itemize}}
|
||
\newcommand{\pnote}[1]{\subparagraph{Note:}{#1}}
|
||
\newenvironment{pnotes}{\introlist\subparagraph{Notes:}\begin{itemize}}{\end{itemize}}
|
||
\newcommand{\sproutspecificpnote}[1]{\notsprout{[\Sprout\!]\,\,} \textbf{Note:\,} {#1}}
|
||
\newcommand{\sproutonlypnote}[1]{\notsprout{[\Sprout only]\,\,} \textbf{Note:\,} {#1}}
|
||
\newcommand{\prenuzeropnote}[1]{\notsprout{[Pre-\NUZero\!]\,\,} \textbf{Note:\,} {#1}}
|
||
\newcommand{\nuzeroonlypnote}[1]{\nuzero{[\NUZero only, pre-\Sapling\!]\,\,} \textbf{Note:\,} {#1}}
|
||
\newcommand{\nuzeroonwardpnote}[1]{\nuzero{[\NUZero onward]\,\,} \textbf{Note:\,} {#1}}
|
||
\newcommand{\fact}[1]{\subparagraph{Fact:}{#1}}
|
||
\newcommand{\facts}[1]{\subparagraph{Facts:}{#1}}
|
||
|
||
\newcommand{\affiliation}{\hairspace$^\dagger$\;}
|
||
|
||
\begin{document}
|
||
|
||
\title{\textbf{\doctitle} \\
|
||
\Large \docversion}
|
||
\author{
|
||
\Large \leadauthor\hairspace\thanks{\;Zerocoin Electric Coin Company} \\
|
||
\Large \coauthora\affiliation — \coauthorb\affiliation — \coauthorc\affiliation}
|
||
\date{\today}
|
||
\maketitle
|
||
|
||
\renewcommand{\abstractname}{}
|
||
\vspace{-8ex}
|
||
\begin{abstract}
|
||
\normalsize \noindent \textbf{Abstract.}
|
||
\Zcash is an implementation of the \term{Decentralized Anonymous Payment}
|
||
scheme \Zerocash, with security fixes and adjustments
|
||
to terminology, functionality and performance. It bridges the existing
|
||
\emph{transparent} payment scheme used by \Bitcoin with a
|
||
\emph{shielded} payment scheme secured by zero-knowledge succinct
|
||
non-interactive arguments of knowledge (\zkSNARKs). It attempts to
|
||
address the problem of mining centralization by use of the Equihash
|
||
memory-hard proof-of-work algorithm.
|
||
|
||
\vspace{1.5ex}
|
||
\sprout{\noindent This specification defines the \Zcash consensus protocol and explains
|
||
its differences from \Zerocash and \Bitcoin.}
|
||
\sapling{\noindent This \em{draft} specification defines the next
|
||
upgrade of the \Zcash consensus protocol, codenamed \NUZero, and the
|
||
subsequent upgrade, codenamed \Sapling. It is a work in progress
|
||
and should not be used as a reference for the current protocol.}
|
||
|
||
\vspace{2.5ex}
|
||
\noindent \textbf{Keywords:}~ \StrSubstitute[0]{\keywords}{,}{, }.
|
||
\end{abstract}
|
||
|
||
\vspace{-10ex}
|
||
\phantomsection
|
||
\addcontentsline{toc}{section}{\larger\nstrut{Contents}}
|
||
|
||
\renewcommand{\contentsname}{}
|
||
% <https://tex.stackexchange.com/a/182744/78411>
|
||
\renewcommand{\baselinestretch}{0.85}\normalsize
|
||
\tableofcontents
|
||
\renewcommand{\baselinestretch}{1.0}\normalsize
|
||
\newpage
|
||
|
||
|
||
\nsection{Introduction}
|
||
|
||
\Zcash is an implementation of the \term{Decentralized Anonymous Payment}
|
||
scheme \Zerocash \cite{BCG+2014}, with some security fixes and adjustments
|
||
to terminology, functionality and performance. It bridges the existing
|
||
\emph{transparent} payment scheme used by \Bitcoin \cite{Naka2008} with a
|
||
\emph{shielded} payment scheme secured by zero-knowledge succinct
|
||
non-interactive arguments of knowledge (\zkSNARKs).
|
||
|
||
Changes from the original \Zerocash are explained in \crossref{differences},
|
||
and highlighted in \changed{\changedcolor} throughout the document.
|
||
\notsprout{Changes specific to the \NUZero upgrade (which are also changes from
|
||
\Zerocash) are highlighted in \nuzero{\nuzerocolor}.
|
||
Changes specific to the \Sapling upgrade following \NUZero (which are also
|
||
changes from \Zerocash) are highlighted in \sapling{\saplingcolor}.
|
||
The name \Sprout is used for the \Zcash protocol prior to \Sapling
|
||
(both before and after \NUZero).
|
||
}
|
||
|
||
Technical terms for concepts that play an important rôle in \Zcash are
|
||
written in \term{slanted text}. \emph{Italics} are used for emphasis and
|
||
for references between sections of the document.
|
||
|
||
The key words \MUST, \MUSTNOT, \SHOULD, and \SHOULDNOT in this
|
||
document are to be interpreted as described in \cite{RFC-2119} when they
|
||
appear in \ALLCAPS. These words may also appear in this document in
|
||
lower case as plain English words, absent their normative meanings.
|
||
|
||
\vspace{2ex}
|
||
\introlist
|
||
This specification is structured as follows:
|
||
|
||
\begin{itemize}
|
||
\item Notation — definitions of notation used throughout the document;
|
||
\item Concepts — the principal abstractions needed to understand the protocol;
|
||
\item Abstract Protocol — a high-level description of the protocol in terms
|
||
of ideal cryptographic components;
|
||
\item Concrete Protocol — how the functions and encodings of the abstract
|
||
protocol are instantiated;
|
||
\notsprout{
|
||
\item Upgrade Transitions — the strategy for upgrading from \Sprout to \NUZero
|
||
and then \Sapling;
|
||
}
|
||
\item Consensus Changes from \Bitcoin — how \Zcash differs from \Bitcoin at
|
||
the consensus layer, including the Proof of Work;
|
||
\item Differences from the \Zerocash protocol — a summary of changes from the
|
||
protocol in \cite{BCG+2014}.
|
||
\notsprout{
|
||
\item Appendix: Circuit Design — details of how the \Sapling circuit is defined
|
||
as a Quadratic Arithmetic Program.
|
||
}
|
||
\end{itemize}
|
||
|
||
|
||
\nsubsection{Caution}
|
||
|
||
\Zcash security depends on consensus. Should a program interacting with the
|
||
\Zcash network diverge from consensus, its security will be weakened or destroyed.
|
||
The cause of the divergence doesn't matter: it could be a bug in your program,
|
||
it could be an error in this documentation which you implemented as described,
|
||
or it could be that you do everything right but other software on the network
|
||
behaves unexpectedly. The specific cause will not matter to the users of your
|
||
software whose wealth is lost.
|
||
|
||
Having said that, a specification of \emph{intended} behaviour is essential
|
||
for security analysis, understanding of the protocol, and maintenance of
|
||
\Zcash and related software. If you find any mistake in this specification,
|
||
please contact \texttt{<security@z.cash>}.
|
||
|
||
\nsubsection{High-level Overview}
|
||
|
||
The following overview is intended to give a concise summary of the ideas
|
||
behind the protocol, for an audience already familiar with \blockchain-based
|
||
cryptocurrencies such as \Bitcoin. It is imprecise in some aspects and is not
|
||
part of the normative protocol specification. \notsprout{This overview applies
|
||
to both \Sprout and \Sapling, differences in the cryptographic constructions
|
||
used notwithstanding.}
|
||
|
||
\introsection
|
||
Value in \Zcash is either \transparent or \shielded. Transfers of \transparent
|
||
value work essentially as in \Bitcoin and have the same privacy properties.
|
||
\xShielded value is carried by \notes\hairspace\footnote{\label{notesandnullifiers}
|
||
In \Zerocash \cite{BCG+2014}, \notes were called \quotedterm{coins}, and \nullifiers
|
||
were called \quotedterm{serial numbers}.},
|
||
which specify an amount and \sprout{a \payingKey. The \payingKey is part of}
|
||
\notsprout{(indirectly)}
|
||
a \paymentAddress, which is a destination to which \notes can be sent.
|
||
As in \Bitcoin, this is associated with a private key that can be used to
|
||
spend \notes sent to the address; in \Zcash this is called a \spendingKey.
|
||
|
||
To each \note there is cryptographically associated a \noteCommitment, and
|
||
a \nullifier\hairspace\cref{notesandnullifiers} (so that there is a 1:1:1 relation
|
||
between \notes, \noteCommitments, and \nullifiers). Computing the \nullifier
|
||
requires the associated private \spendingKey\sapling{ (or the \fullViewingKey for \Sapling \notes)}.
|
||
It is infeasible to correlate the \noteCommitment with the corresponding
|
||
\nullifier without knowledge of at least this \sprout{\spendingKey}\notsprout{key}.
|
||
An unspent valid \note, at a given point on the \blockchain,
|
||
is one for which the \noteCommitment has been publically revealed on the
|
||
\blockchain prior to that point, but the \nullifier has not.
|
||
\notsprout{\todo{The ``1:1:1'' part isn't correct for \Sapling.}}
|
||
|
||
\introlist
|
||
A \transaction can contain \transparent inputs, outputs, and scripts, which all
|
||
work as in \Bitcoin \cite{Bitc-Protocol}.
|
||
\sprout{
|
||
It also contains a sequence of zero or more \joinSplitDescriptions.
|
||
Each of these describes a \joinSplitTransfer\hairspace\footnote{
|
||
\joinSplitTransfers in \Zcash generalize \quotedterm{Mint} and \quotedterm{Pour}
|
||
\transactions in \Zerocash; see \crossref{trstructure} for differences.}
|
||
which takes in a \transparent value and up to two input \notes, and produces a
|
||
\transparent value and up to two output \notes.
|
||
}
|
||
\notsprout{
|
||
It also includes \joinSplitDescriptions, \spendDescriptions, and \outputDescriptions.
|
||
Together these describe \shieldedTransfers which take in \shieldedInput \notes,
|
||
and/or produce \shieldedOutput \notes.
|
||
(For \Sprout, each \joinSplitDescription handles up to two \shieldedInputs and
|
||
up to two \shieldedOutputs. For \Sapling, each \shieldedInput or \shieldedOutput
|
||
has its own description.)
|
||
It is also possible for value to be transferred between the \transparent and
|
||
\shielded domains.
|
||
}
|
||
|
||
The \nullifiers of the input \notes are revealed (preventing them from being
|
||
spent again) and the commitments of the output \notes are revealed (allowing
|
||
them to be spent in future).
|
||
\sprout{
|
||
Each \joinSplitDescription also includes a computationally sound \zkSNARK proof,
|
||
which proves that all of the following hold except with negligable probability:
|
||
|
||
\begin{itemize}
|
||
\item The input and output values balance (individually for each \joinSplitTransfer).
|
||
\item For each input \note of non-zero value, some revealed \noteCommitment
|
||
exists for that \note.
|
||
\item The prover knew the private \spendingKeys of the input \notes.
|
||
\item The \nullifiers and \noteCommitments are computed correctly.
|
||
\item The private \spendingKeys of the input \notes are cryptographically
|
||
linked to a signature over the whole \transaction, in such a way that
|
||
the \transaction cannot be modified by a party who did not know these
|
||
private keys.
|
||
\item Each output \note is generated in such a way that it is infeasible to
|
||
cause its \nullifier to collide with the \nullifier of any other \note.
|
||
\end{itemize}
|
||
}
|
||
\notsprout{
|
||
A \transaction also includes computationally sound \zkSNARK proofs, which prove
|
||
that all of the following hold except with negligable probability:
|
||
|
||
For each \shieldedInput,
|
||
|
||
\begin{itemize}
|
||
\item \saplingonward{there is a revealed \valueCommitment to the same value as
|
||
the input \note;}
|
||
\item some revealed \noteCommitment exists for the input \note;
|
||
\item the prover knew the \authProvingKey of the input \note;
|
||
\item the \nullifier and \noteCommitment are computed correctly.
|
||
\end{itemize}
|
||
|
||
and for each \shieldedOutput,
|
||
|
||
\begin{itemize}
|
||
\item \saplingonward{there is a revealed \valueCommitment to the same value as
|
||
the output \note;}
|
||
\item the \noteCommitment is computed correctly;
|
||
\item the output \note is generated in such a way that it is infeasible to
|
||
cause its \nullifier to collide with the \nullifier of any other \note.
|
||
\end{itemize}
|
||
|
||
For \Sprout, the \joinSplitStatement also includes an explicit balance check.
|
||
For \Sapling, the \valueCommitments corresponding to the inputs and outputs are
|
||
checked to balance (together with any net \transparent input or output)
|
||
outside the \zkSNARK.
|
||
|
||
In addition, various measures (differing between \Sprout and \Sapling) are
|
||
used to ensure that the \transaction cannot be modified by a party not authorized
|
||
to do so.
|
||
}
|
||
|
||
Outside the \zkSNARK, it is \sprout{also} checked that the \nullifiers for the input
|
||
\notes had not already been revealed (i.e.\ they had not already been spent).
|
||
|
||
A \paymentAddress includes
|
||
\sprout{two public keys: a \payingKey matching that of \notes sent to the address, and}
|
||
a \transmissionKey for a key-private asymmetric encryption
|
||
scheme. \quotedterm{Key-private} means that ciphertexts do not reveal information
|
||
about which key they were encrypted to, except to a holder of the corresponding
|
||
private key, which in this context is called the \receivingKey. This facility is
|
||
used to communicate encrypted output \notes on the \blockchain to their
|
||
intended recipient, who can use the \receivingKey to scan the \blockchain for
|
||
\notes addressed to them and then decrypt those \notes.
|
||
|
||
The basis of the privacy properties of \Zcash is that when a \note is spent,
|
||
the spender only proves that some commitment for it had been revealed, without
|
||
revealing which one. This implies that a spent \note cannot be linked to the
|
||
\transaction in which it was created. That is, from an adversary's point of
|
||
view the set of possibilities for a given \note input to a \transaction
|
||
---its \noteTraceabilitySet--- includes \emph{all} previous notes that the
|
||
adversary does not control or know to have been spent. This contrasts with
|
||
other proposals for private payment systems, such as CoinJoin \cite{Bitc-CoinJoin}
|
||
or \CryptoNote \cite{vanS2014}, that are based on mixing of a limited number of
|
||
transactions and that therefore have smaller \noteTraceabilitySets.
|
||
|
||
The \nullifiers are necessary to prevent double-spending: each note only has
|
||
one valid \nullifier, and so attempting to spend a \note twice would reveal the
|
||
\nullifier twice, which would cause the second \transaction to be rejected.
|
||
|
||
|
||
\nsection{Notation}
|
||
|
||
$\bit$ means the type of bit values, i.e.\ $\setof{0, 1}$.
|
||
|
||
$\byte$ means the type of byte values, i.e.\ $\range{0}{255}$.
|
||
|
||
$\Nat$ means the type of nonnegative integers. $\PosInt$
|
||
means the type of positive integers. $\Rat$ means the type of rationals.
|
||
|
||
$x \typecolon T$ is used to specify that $x$ has type $T$.
|
||
A cartesian product type is denoted by $S \times T$, and a function type
|
||
by $S \rightarrow T$. An argument to a function can determine other argument
|
||
or result types.
|
||
|
||
The type of a randomized algorithm is denoted by $S \rightarrowR T$.
|
||
The domain of a randomized algorithm may be $()$, indicating that it requires
|
||
no arguments. Given $f \typecolon S \rightarrowR T$ and $s \typecolon S$,
|
||
sampling a variable $x \typecolon T$ from the output of $f$ applied to $s$
|
||
is denoted by $x \leftarrowR f(s)$.
|
||
|
||
Initial arguments to a function or randomized algorithm may be
|
||
written as subscripts, e.g.\ if $x \typecolon X$, $y \typecolon Y$, and
|
||
$f \typecolon X \times Y \rightarrow Z$, then an invocation of
|
||
$f(x, y)$ can also be written $f_x(y)$.
|
||
|
||
\notsprout{
|
||
$\fun{x \typecolon T}{e_x \typecolon U}$ means the function of type $T \rightarrow U$
|
||
mapping formal parameter $x$ to $e_x$ (an expression depending on $x$).
|
||
The types $T$ and $U$ are always explicit.
|
||
|
||
$\powerset{T}$ means the powerset of $T$.
|
||
}
|
||
|
||
$\typeexp{T}{\ell}$, where $T$ is a type and $\ell$ is an integer,
|
||
means the type of sequences of length $\ell$ with elements in $T$. For example,
|
||
$\bitseq{\ell}$ means the set of sequences of $\ell$ bits, and
|
||
$\byteseq{k}$ means the set of seqences of $k$ bytes.
|
||
|
||
$\byteseqs$ means the type of byte sequences of arbitrary length.
|
||
|
||
$\length(S)$ means the length of (number of elements in) $S$.
|
||
|
||
$T \subseteq U$ indicates that $T$ is an inclusive subset or subtype of $U$.
|
||
|
||
\notsprout{
|
||
$\setof{x \typecolon T \suchthat p(x)}$ means the subset of $x$ from $T$
|
||
for which $p(x)$ holds.
|
||
}
|
||
|
||
$S \union T$ means the set union of $S$ and $T$, or the type corresponding
|
||
to it.
|
||
|
||
$S \intersection T$ means the set intersection of $S$ and $T$.
|
||
|
||
\notsprout{
|
||
$S \difference T$ means the set difference obtained by removing elements
|
||
in $T$ from $S$, i.e. $\setof{x \typecolon S \suchthat x \neq T}$.
|
||
}
|
||
|
||
$\hexint{}$ followed by a string of $\mathtt{monospace}$ hexadecimal
|
||
digits means the corresponding integer converted from hexadecimal.
|
||
|
||
$\ascii{...}$ means the given string represented as a
|
||
sequence of bytes in US-ASCII. For example, $\ascii{abc}$ represents the
|
||
byte sequence $[\hexint{61}, \hexint{62}, \hexint{63}]$.
|
||
|
||
$\zeros{\ell}$ means the sequence of $\ell$ zero bits.
|
||
\notsprout{$\ones{\ell}$ means the sequence of $\ell$ one bits.}
|
||
|
||
$a..b$, used as a subscript, means the sequence of values
|
||
with indices $a$ through $b$ inclusive. For example,
|
||
$\AuthPublicNew{\allNew}$ means the sequence $[\AuthPublicNew{\mathrm{1}},
|
||
\AuthPublicNew{\mathrm{2}}, ...\,\AuthPublicNew{\NNew}]$.
|
||
(For consistency with the notation in \cite{BCG+2014} and in \cite{BK2016},
|
||
this specification uses 1-based indexing and inclusive ranges,
|
||
notwithstanding the compelling arguments to the contrary made in
|
||
\cite{EWD-831}.)
|
||
|
||
$\range{a}{b}$ means the set or type of integers from $a$ through
|
||
$b$ inclusive.
|
||
|
||
$\listcomp{f(x) \for x \from a \upto b}$ means the sequence
|
||
formed by evaluating $f$ on each integer from $a$ to $b$ inclusive, in
|
||
ascending order. Similarly, $\listcomp{f(x) \for x \from a \downto b}$ means
|
||
the sequence formed by evaluating $f$ on each integer from $a$ to $b$
|
||
inclusive, in descending order.
|
||
|
||
$a \bconcat b$ means the concatenation of sequences $a$ then $b$.
|
||
|
||
$\concatbits(S)$ means the sequence of bits obtained by
|
||
concatenating the elements of $S$ viewed as bit sequences. If the
|
||
elements of $S$ are byte sequences, they are converted to bit sequences
|
||
with the \emph{most significant} bit of each byte first.
|
||
|
||
$\sorted(S)$ means the sequence formed by sorting the elements
|
||
of $S$.
|
||
|
||
$\GF{n}$ means the finite field with $n$ elements, and
|
||
$\GFstar{n}$ means its group under multiplication.
|
||
$\GF{n}[z]$ means the ring of polynomials over $z$ with coefficients
|
||
in $\GF{n}$.
|
||
|
||
$a \mult b$ means the product of multiplying $a$ and $b$.
|
||
This may refer to multiplication of integers, rationals, or
|
||
finite field elements according to context.
|
||
|
||
$a^b$, for $a$ an integer or finite field element and
|
||
$b$ an integer, means the result of raising $a$ to the exponent $b$.
|
||
|
||
$a \bmod q$, for $a \typecolon \Nat$ and $q \typecolon \PosInt$,
|
||
means the remainder on dividing $a$ by $q$.
|
||
|
||
$a \xor b$ means the bitwise-exclusive-or of $a$ and $b$,
|
||
and $a \band b$ means the bitwise-and of $a$ and $b$. These are
|
||
defined on integers or bit sequences according to context.
|
||
|
||
$\vsum{i=1}{\mathrm{N}} a_i$ means the sum of $a_{\allN{}}$.\;
|
||
\notsprout{$\vproduct{i=1}{\mathrm{N}} a_i$ means the product
|
||
of $a_{\allN{}}$.\;}
|
||
$\vxor{i=1}{\mathrm{N}} a_i$ means the bitwise exclusive-or of $a_{\allN{}}$.
|
||
|
||
\notsprout{
|
||
$b \bchoose x : y$ means $x$ when $b = 1$, or $y$ when $b = 0$.
|
||
}
|
||
|
||
The binary relations $<$, $\leq$, $=$, $\geq$, and $>$ have their conventional
|
||
meanings on integers and rationals, and are defined lexicographically on
|
||
sequences of integers.
|
||
|
||
$\floor{x}$ means the largest integer $\leq x$.
|
||
$\ceiling{x}$ means the smallest integer $\geq x$.
|
||
|
||
$\bitlength(x)$, for $x \typecolon \Nat$, means the smallest integer
|
||
$\ell$ such that $2^\ell > x$.
|
||
|
||
The symbol $\bot$ is used to indicate unavailable information, or a failed
|
||
decryption or validity check.
|
||
|
||
The following integer constants will be instantiated in \crossref{constants}:
|
||
\notsprout{\begin{formulae} \item}
|
||
$\MerkleDepthSprout$, \sapling{$\MerkleDepthSapling$,}
|
||
$\NOld$, $\NNew$, $\MerkleHashLengthSprout$, \sapling{$\MerkleHashLengthSapling$,}
|
||
$\hSigLength$, $\PRFOutputLength$, \sapling{$\PRGOutputLength$,} $\NoteCommitRandLength$,
|
||
$\RandomSeedLength$, $\AuthPrivateLength$, \sapling{$\AuthPrivateSeedLength$, $\DiversifierLength$,}
|
||
$\NoteAddressPreRandLength$, $\MAXMONEY$, $\SlowStartInterval$, $\HalvingInterval$,
|
||
$\MaxBlockSubsidy$, $\NumFounderAddresses$, $\PoWAveragingWindow$, $\PoWLimit$,
|
||
$\PoWMedianBlockSpan$, $\PoWDampingFactor$, $\PoWTargetSpacing$.
|
||
\notsprout{\end{formulae}}
|
||
|
||
\sprout{The bit sequence constant $\UncommittedSprout \typecolon \bitseq{\MerkleHashLengthSprout}$,}
|
||
\notsprout{The bit sequence constants $\UncommittedSprout \typecolon \bitseq{\MerkleHashLengthSprout}$
|
||
and $\UncommittedSapling \typecolon \bitseq{\MerkleHashLengthSapling}$,}
|
||
and rational constants $\FoundersFraction$, $\PoWMaxAdjustDown$, and
|
||
$\PoWMaxAdjustUp$ will also be defined in that section.
|
||
|
||
|
||
\introsection
|
||
\nsection{Concepts}
|
||
|
||
\nsubsection{Payment Addresses and Keys} \label{addressesandkeys}
|
||
|
||
Users who wish to receive payments under this scheme first generate a
|
||
random \spendingKey\sprout{ $\AuthPrivate$}.
|
||
\notsprout{In \Sprout this is called $\AuthPrivate$ \sapling{and in \Sapling it is
|
||
called $\AuthPrivateSeed$}.}
|
||
|
||
\introlist
|
||
The following diagram depicts the relations between key
|
||
components\notsprout{ in \Sprout}\sapling{ and \Sapling}.
|
||
Arrows point from a component to any other component(s) that can be derived
|
||
from it.
|
||
|
||
\begin{center}
|
||
\sprout{\includegraphics[scale=.7]{key_components}}
|
||
\sapling{\includegraphics[scale=.5]{key_components_sapling}}
|
||
\end{center}
|
||
|
||
\sproutspecific{
|
||
The \receivingKey $\TransmitPrivate$, the \incomingViewingKey
|
||
$\InViewingKey = (\AuthPublic, \TransmitPrivate)$, and the \paymentAddress
|
||
$\PaymentAddress = (\AuthPublic, \TransmitPublic)$ are derived from
|
||
$\AuthPrivate$, as described in \crossref{sproutkeycomponents}.
|
||
}
|
||
|
||
\saplingonward{
|
||
The \authSigningKey $\AuthSignPrivate$,
|
||
the \authProvingKey $(\AuthSignPublic, \AuthProvePrivate)$,
|
||
the \fullViewingKey $(\AuthSignPublic, \AuthProvePublic)$,
|
||
the \incomingViewingKey $\InViewingKey$, and
|
||
each \diversifiedPaymentAddress $\DiversifiedPaymentAddress = (\Diversifier, \DiversifiedTransmitPublic)$
|
||
are derived from $\AuthPrivateSeed$, as described in \crossref{saplingkeycomponents}.
|
||
}
|
||
|
||
The composition of \paymentAddresses, \changed{\incomingViewingKeys,}
|
||
\sapling{\fullViewingKeys,} and \spendingKeys is a cryptographic protocol
|
||
detail that should not normally be exposed to users. However, user-visible
|
||
operations should be provided to obtain a
|
||
\paymentAddress\changed{ or \incomingViewingKey}\sapling{ or \fullViewingKey}
|
||
from a \spendingKey.
|
||
|
||
Users can accept payment from multiple parties with a single \paymentAddress
|
||
and the fact that these payments are destined to
|
||
the same payee is not revealed on the \blockchain, even to the
|
||
paying parties. \emph{However} if two parties collude to compare a
|
||
\paymentAddress they can trivially determine they are the same. In the
|
||
case that a payee wishes to prevent this they should create a distinct
|
||
\paymentAddress for each payer.
|
||
|
||
\saplingonward{
|
||
\Sapling provides a mechanism to allow the efficient creation of
|
||
\diversifiedPaymentAddresses with the same spending authority. A group of
|
||
such addresses shares the same \fullViewingKey and \incomingViewingKey, and
|
||
so creating as many unlinkable addresses as needed does not increase the cost
|
||
of scanning the \blockchain for relevant \transactions.
|
||
}
|
||
|
||
\pnote{
|
||
It is conventional in cryptography to refer to the key used to encrypt
|
||
a message in an asymmetric encryption scheme as the \quotedterm{public key}.
|
||
However, the public key used as the \transmissionKey component of an address
|
||
($\TransmitPublic$\sapling{ or $\DiversifiedTransmitPublic$}) need not be
|
||
publically distributed; it has the same distribution as the \paymentAddress itself.
|
||
As mentioned above, limiting the distribution of the \paymentAddress is important
|
||
for some use cases. This also helps to reduce reliance of the overall protocol
|
||
on the security of the cryptosystem used for \note encryption
|
||
(see \crossref{inband}), since an adversary would have to know
|
||
$\TransmitPublic$\sapling{ or some $\DiversifiedTransmitPublic$} in order to
|
||
exploit a hypothetical weakness in that cryptosystem.
|
||
}
|
||
|
||
\introsection
|
||
\nsubsection{\Notes} \label{notes}
|
||
|
||
\sprout{
|
||
A \note (denoted $\NoteTuple{}$) is a tuple $\changed{(\AuthPublic, \Value,
|
||
\NoteAddressRand, \NoteCommitRand)}$. It represents that a value $\Value$ is
|
||
spendable by the recipient who holds the \spendingKey $\AuthPrivate$ corresponding
|
||
to $\AuthPublic$, as described in the previous section.
|
||
}
|
||
\notsprout{
|
||
A \note (denoted $\NoteTuple{}$) can be a \Sprout \note\sapling{ or a
|
||
\Sapling \note}. In either case it represents that a value $\Value$ is
|
||
spendable by the recipient who holds the \spendingKey corresponding
|
||
to a given \paymentAddress.
|
||
}
|
||
|
||
A \SproutOrNothing \note is a tuple $\changed{(\AuthPublic,
|
||
\Value, \NoteAddressRand, \NoteCommitRand)}$, where:
|
||
\begin{itemize}
|
||
\item $\AuthPublic \typecolon \PRFOutput$ is the \payingKey of the
|
||
recipient's \paymentAddress;
|
||
\item $\Value \typecolon \range{0}{\MAXMONEY}$ is an integer
|
||
representing the value of the \note in \zatoshi
|
||
($1$ \ZEC = $10^8$ \zatoshi);
|
||
\item $\NoteAddressRand \typecolon \PRFOutput$
|
||
is used as input to $\PRFnf{\AuthPrivate}$ to derive the
|
||
\nullifier of the \note;
|
||
\item $\NoteCommitRand \typecolon \NoteCommitSproutTrapdoor$
|
||
is a random \commitmentTrapdoor as defined in \crossref{abstractcommit}.
|
||
\end{itemize}
|
||
|
||
Let $\NoteTypeSprout$ be the type of a \SproutOrNothing \note, i.e.
|
||
\begin{formulae}
|
||
\item $\NoteTypeSprout := \changed{\PRFOutput \times \range{0}{\MAXMONEY} \times \PRFOutput
|
||
\times \NoteCommitSproutTrapdoor}$.
|
||
\end{formulae}
|
||
|
||
\sapling{
|
||
\vspace{2ex}
|
||
A \Sapling \note is a tuple $(\Diversifier, \DiversifiedTransmitPublic,
|
||
\Value, \NoteCommitRand)$, where:
|
||
\begin{itemize}
|
||
\item $\Diversifier \typecolon \DiversifierType$
|
||
is the \diversifier of the recipient's \paymentAddress;
|
||
\item $\DiversifiedTransmitPublic \typecolon \bitseq{\ellJ}$
|
||
is the \diversifiedTransmissionKey of the recipient's \paymentAddress;
|
||
\item $\Value \typecolon \range{0}{\MAXMONEY}$ is an integer
|
||
representing the value of the \note in \zatoshi;
|
||
\item $\NoteCommitRand \typecolon \NoteCommitSaplingTrapdoor$
|
||
is a random \commitmentTrapdoor as defined in \crossref{abstractcommit}.
|
||
\end{itemize}
|
||
|
||
Let $\NoteTypeSapling$ be the type of a \Sapling \note, i.e.
|
||
|
||
\begin{formulae}
|
||
\item $\NoteTypeSapling := \DiversifierType \times \bitseq{\ellJ} \times \range{0}{\MAXMONEY}
|
||
\times \bitseq{\ellJ} \times \NoteCommitSaplingTrapdoor$.
|
||
\end{formulae}
|
||
}
|
||
|
||
Creation of new \notes is described in \crossref{send}. When \notes are sent,
|
||
only a commitment (see \crossref{abstractcommit}) to the above values is disclosed
|
||
publically, and added to a data structure called the \noteCommitmentTree.
|
||
This allows the value and recipient to be kept private, while the commitment is
|
||
used by the \zeroKnowledgeProof when the \note is spent, to check that it exists
|
||
on the \blockchain.
|
||
|
||
\vspace{2ex}
|
||
A \notsprout{\Sprout} \noteCommitment is computed as
|
||
\begin{formulae}
|
||
\item $\NoteCommitmentSprout(\NoteTuple{}) =
|
||
\NoteCommitSprout{\NoteCommitRand}(\AuthPublic, \Value, \NoteAddressRand)$,
|
||
\end{formulae}
|
||
\vspace{-1.5ex}
|
||
where $\NoteCommitSprout{}$ is instantiated in \crossref{concretesproutcommit}.
|
||
|
||
\sapling{
|
||
\vspace{2ex}
|
||
Let $\GroupJHash{}$ and $U$ be as defined in \crossref{concretegrouphashjubjub}.
|
||
|
||
A \Sapling \noteCommitment is computed as
|
||
|
||
\begin{formulae}
|
||
\item $\DiversifiedTransmitBase := \GroupJHash{U}(\ascii{Zcash\_gd}, \Diversifier)$
|
||
\item $\NoteCommitmentSapling(\NoteTuple{}) :=
|
||
\NoteCommitSapling{\NoteCommitRand}(\reprJOf{\DiversifiedTransmitBase}, \DiversifiedTransmitPublic, \Value)$
|
||
\end{formulae}
|
||
\vspace{-1.5ex}
|
||
where $\NoteCommitSapling{}$ is instantiated in \crossref{concretewindowedcommit}.
|
||
|
||
Notice that the above definition of a \Sapling \note does not have a
|
||
$\NoteAddressRand$ field. There is in fact a $\NoteAddressRand$ value associated
|
||
with each \Sapling \note, but this only be computed once its position in the
|
||
\noteCommitmentTree is known (see \crossref{blockchain} and \crossref{transactions}).
|
||
We refer to the combination of a \note and its \notePosition $\NotePosition$, as a
|
||
\positionedNote.
|
||
|
||
For a \positionedNote, we can compute the value
|
||
$\NoteAddressRand \typecolon \bitseq{\ellJ}$ as described in \crossref{commitmentsandnullifiers}.
|
||
}
|
||
|
||
\vspace{2ex}
|
||
A \nullifier (denoted $\nf$) is derived from the $\NoteAddressRand$ value
|
||
of a \note and the recipient's
|
||
\spendingKey $\AuthPrivate$\sapling{ or \fullViewingKey $(\AuthSignPublic, \AuthProvePublic)$},
|
||
using a \pseudoRandomFunction (see \crossref{abstractprfs}). This computation
|
||
is described in \crossref{commitmentsandnullifiers}.
|
||
|
||
A \note is spent by proving knowledge of $\NoteAddressRand$ and
|
||
$\AuthPrivate$\sapling{ or $(\AuthSignPrivate, \AuthProvePrivate)$}
|
||
in zero knowledge while publically disclosing its \nullifier $\nf$,
|
||
allowing $\nf$ to be used to prevent double-spending.
|
||
|
||
|
||
\nsubsubsection{\NotePlaintexts{} and \Memos} \label{noteptconcept}
|
||
|
||
Transmitted \notes are stored on the \blockchain in encrypted form, together with
|
||
a \noteCommitment $\cm$.
|
||
|
||
The \notePlaintexts in a \joinSplitDescription are encrypted to the
|
||
respective \transmissionKeys $\TransmitPublicNew{\allNew}$.
|
||
Each \notsprout{\Sprout} \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
||
$(\Value, \NoteAddressRand, \NoteCommitRand\changed{, \Memo})$.
|
||
|
||
\saplingonward{
|
||
The \notePlaintext in each \outputDescription is encrypted to the
|
||
\diversifiedTransmissionKey $\DiversifiedTransmitPublic$.
|
||
Each \Sapling \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
||
$(\Diversifier, \Value, \NoteCommitRand, \Memo)$.
|
||
}
|
||
|
||
\changed{
|
||
$\Memo$ represents a \memo associated with this \note. The usage of the
|
||
\memo is by agreement between the sender and recipient of the \note.
|
||
}
|
||
|
||
Other fields are as defined in \crossref{notes}.
|
||
|
||
The result of encryption forms part of a \notesCiphertext (see \crossref{inband}
|
||
for further details).
|
||
|
||
|
||
\nsubsection{The Block Chain} \label{blockchain}
|
||
|
||
At a given point in time, each \fullValidator is aware of a set of candidate
|
||
\blocks. These form a tree rooted at the \genesisBlock, where each node
|
||
in the tree refers to its parent via the $\hashPrevBlock$ \blockHeader field
|
||
(see \crossref{blockheader}).
|
||
|
||
A path from the root toward the leaves of the tree consisting of a sequence
|
||
of one or valid \blocks consistent with consensus rules, is called a
|
||
\validBlockchain.
|
||
|
||
Each \block in a \blockchain has a \blockHeight. The \blockHeight of the
|
||
\genesisBlock is 0, and the \blockHeight of each subsequent \block in the
|
||
\blockchain increments by 1.
|
||
|
||
In order to choose the \bestValidBlockchain in its view of the
|
||
overall \block tree, a node sums the work, as defined in \crossref{workdef}, of
|
||
all \blocks in each chain, and considers the \validBlockchain with greatest
|
||
total work to be best. To break ties between leaf \blocks, a node will prefer the
|
||
\block that it received first.
|
||
|
||
The consensus protocol is designed to ensure that for any given \blockHeight,
|
||
the vast majority of nodes should eventually agree on their \bestValidBlockchain
|
||
up to that height.
|
||
|
||
|
||
\nsubsection{Transactions and Treestates} \label{transactions}
|
||
|
||
Each \block contains one or more \transactions.
|
||
|
||
Inputs to a \transaction insert value into a \transparentValuePool, and outputs
|
||
remove value from this pool. As in \Bitcoin, the remaining value in the pool is
|
||
available to miners as a fee.
|
||
|
||
\vspace{-3ex}
|
||
\consensusrule{
|
||
The remaining value in the \transparentValuePool{} \MUST be nonnegative.
|
||
}
|
||
\vspace{2ex}
|
||
|
||
\sprout{To each \transaction there is associated an initial \treestate.}
|
||
\notsprout{To each \transaction there are associated initial \treestates
|
||
for \Sprout\sapling{ and for \Sapling}.}
|
||
|
||
\introlist
|
||
\sprout{A}\sapling{Each} \treestate consists of:
|
||
|
||
\begin{itemize}
|
||
\item a \noteCommitmentTree (\crossref{merkletree});
|
||
\item a \nullifierSet (\crossref{nullifierset}).
|
||
\end{itemize}
|
||
|
||
Validation state associated with \transparentTransfers, such as the UTXO
|
||
(Unspent Transaction Output) set, is not described in this document; it is
|
||
used in essentially the same way as in \Bitcoin.
|
||
|
||
An \anchor is a Merkle tree root of a \noteCommitmentTree\sapling{ (either the
|
||
\Sprout tree or the \Sapling tree)}. It uniquely identifies a \noteCommitmentTree
|
||
state given the assumed security properties of the Merkle tree's
|
||
\hashFunction. Since the \nullifierSet is always updated together with the
|
||
\noteCommitmentTree, this also identifies a particular state of the associated
|
||
\nullifierSet.
|
||
|
||
\introlist
|
||
In a given \blockchain, \sapling{for each of \Sprout and \Sapling,}
|
||
\treestates are chained as follows:
|
||
|
||
\begin{itemize}
|
||
\item The input \treestate of the first \block is the empty \treestate.
|
||
\item The input \treestate of the first \transaction of a \block is the final
|
||
\treestate of the immediately preceding \block.
|
||
\item The input \treestate of each subsequent \transaction in a \block is the
|
||
output \treestate of the immediately preceding \transaction.
|
||
\item The final \treestate of a \block is the output \treestate of its last
|
||
\transaction.
|
||
\end{itemize}
|
||
|
||
\joinSplitDescriptions also have interstitial input and output
|
||
\treestates\notsprout{ for \Sprout}, explained in the following section.
|
||
\sapling{There is no equivalent of interstitial \treestates for \Sapling.}
|
||
|
||
|
||
\nsubsection{\JoinSplitTransfers{} and Descriptions} \label{joinsplit}
|
||
|
||
A \joinSplitDescription is data included in a \transaction that describes a \joinSplitTransfer,
|
||
i.e.\ a \shielded value transfer.
|
||
\sprout{This kind of value transfer is}
|
||
\notsprout{In \Sprout, this kind of value transfer was}
|
||
the primary \Zcash-specific operation performed by \transactions.
|
||
|
||
A \joinSplitTransfer spends $\NOld$ \notes $\nOld{\allOld}$ and \transparent input
|
||
$\vpubOld$, and creates $\NNew$ \notes $\nNew{\allNew}$ and \transparent output
|
||
$\vpubNew$.
|
||
It is associated with an instance of a \joinSplitStatement (\crossref{joinsplitstatement}),
|
||
for which it provides a \zkSNARKProof.
|
||
|
||
Each \transaction has a \sequenceOfJoinSplitDescriptions.
|
||
|
||
The \changed{total $\vpubNew$ value adds to, and the total} $\vpubOld$
|
||
value subtracts from the \transparentValuePool of the containing \transaction.
|
||
|
||
The \anchor of each \joinSplitDescription in a \transaction{} refers to a
|
||
\SproutOrNothing \treestate. For the first \joinSplitDescription, this \MUST be
|
||
the output \SproutOrNothing \treestate of a previous \block.
|
||
|
||
\changed{
|
||
For each \joinSplitDescription in a \transaction, an interstitial output \treestate is
|
||
constructed which adds the \noteCommitments and \nullifiers specified in that
|
||
\joinSplitDescription to the input \treestate referred to by its \anchor.
|
||
This interstitial output \treestate is available for use as the \anchor of subsequent
|
||
\joinSplitDescriptions in the same \transaction.
|
||
|
||
Interstitial \treestates are necessary because when a \transaction is constructed,
|
||
it is not known where it will eventually appear in a mined \block. Therefore the
|
||
\anchors that it uses must be independent of its eventual position.
|
||
}
|
||
|
||
\begin{consensusrules}
|
||
\item The input and output values of each \joinSplitTransfer{} \MUST balance
|
||
exactly.
|
||
\changed{
|
||
\item The \anchor of each \joinSplitDescription in a \transaction{} \MUST refer
|
||
to either some earlier \block's final \SproutOrNothing \treestate, or to
|
||
the interstitial output \treestate of any prior \joinSplitDescription in
|
||
the same \transaction.
|
||
}
|
||
\end{consensusrules}
|
||
|
||
|
||
\sapling{
|
||
\nsubsection{\SpendTransfers, \OutputTransfers, and their Descriptions} \label{spendsandoutputs}
|
||
|
||
\joinSplitTransfers are not used for \Sapling \notes. Instead, there is a
|
||
separate \spendTransfer for each \shieldedInput, and a separate \outputTransfer
|
||
for each \shieldedOutput.
|
||
|
||
\spendDescriptions and \outputDescriptions are data included in a transaction
|
||
that describe \spendTransfers and \outputTransfers, respectively.
|
||
|
||
A \spendTransfer spends a \note $\nOld{}$. Its \spendDescription includes a
|
||
\xPedersenValueCommitment to the value of the \note.
|
||
It is associated with an instance of a \spendStatement (\crossref{spendstatement})
|
||
for which it provides a \zkSNARKProof.
|
||
|
||
An \outputTransfer creates a \note $\nNew{}$. Similarly, its \outputDescription
|
||
includes a \xPedersenValueCommitment to the \note value.
|
||
It is associated with an instance of an \outputStatement (\crossref{outputstatement})
|
||
for which it provides a \zkSNARKProof.
|
||
|
||
Each \transaction has a sequence of \spendDescriptions and a sequence of
|
||
\outputDescriptions.
|
||
|
||
To ensure balance, we use a homomorphic property of \xPedersenCommitments that
|
||
allows them to be added and subtracted, as elliptic curve points. The result
|
||
of adding two \xPedersenValueCommitments, committing to values $\Value_1$ and
|
||
$\Value_2$, is a new \xPedersenValueCommitment that commits to $\Value_1 + \Value_2$.
|
||
Subtraction works similarly.
|
||
|
||
Therefore, balance can be enforced by adding all of the \valueCommitments for
|
||
\shieldedInputs, subtracting all of the \valueCommitments for \shieldedOutputs,
|
||
and checking that the result commits to a value consistent with the net \transparent
|
||
value change (see \crossref{saplingbalance} for a full specification).
|
||
This approach allows all of the \zkSNARK statements to be independent of
|
||
each other, potentially increasing opportunities for precomputation.
|
||
|
||
A \spendDescription also includes an \anchor, which refers to the output
|
||
\Sapling \treestate of a previous \block.
|
||
|
||
\pnote{
|
||
Interstitial \treestates are not necessary for \Sapling, because a \spendTransfer
|
||
in a given \transaction cannot spend any of the \shieldedOutputs of the same
|
||
\transaction. This is not an onerous restriction because, unlike \Sprout where
|
||
each \joinSplitTransfer must balance individually, in \Sapling it is only necessary
|
||
for the whole \transaction to balance.
|
||
}
|
||
|
||
\begin{consensusrules}
|
||
\item The \transaction{} \MUST balance as specified in \crossref{saplingbalance}.
|
||
\item The \anchor of each \spendDescription in a \transaction{} \MUST refer
|
||
to some earlier \block's final \Sapling \treestate.
|
||
\end{consensusrules}
|
||
}
|
||
|
||
|
||
\nsubsection{\NoteCommitmentTrees} \label{merkletree}
|
||
|
||
\begin{center}
|
||
\includegraphics[scale=.4]{incremental_merkle}
|
||
\end{center}
|
||
|
||
The \noteCommitmentTree is an \incrementalMerkleTree of fixed depth used to store
|
||
\noteCommitments that \joinSplitTransfers\sapling{ and \spendTransfers} produce.
|
||
Just as the \term{unspent transaction output set} (UTXO set) used in \Bitcoin,
|
||
it is used to express the existence of value and the capability to spend it.
|
||
However, unlike the UTXO set, it is \emph{not} the job of this tree to protect
|
||
against double-spending, as it is append-only.
|
||
|
||
A \merkleRoot of this tree is associated with each \treestate, as described in
|
||
\crossref{transactions}.
|
||
|
||
Each \merkleNode in the \incrementalMerkleTree is associated with a \merkleHash of
|
||
size $\MerkleHashLengthSprout$ \sapling{ or $\MerkleHashLengthSapling$} bits.
|
||
The \merkleLayer numbered $h$, counting from \merkleLayer $0$ at the \merkleRoot, has
|
||
$2^h$ \merkleNodes with \merkleIndices $0$ to $2^h-1$ inclusive.
|
||
The \merkleHash associated with the \merkleNode at \merkleIndex $i$ in \merkleLayer $h$
|
||
is denoted $\MerkleNode{h}{i}$.
|
||
|
||
|
||
\nsubsection{\NullifierSets} \label{nullifierset}
|
||
|
||
Each \fullValidator maintains a \nullifierSet logically associated with each \treestate.
|
||
As valid \transactions containing \joinSplitTransfers \sapling{ or \spendTransfers} are
|
||
processed, the \nullifiers revealed in \joinSplitDescriptions \sapling{ and \spendDescriptions}
|
||
are inserted into the \nullifierSet associated with the new \treestate.
|
||
|
||
\xNullifiers are enforced to be unique within a \validBlockchain, in order to
|
||
prevent double-spends.
|
||
|
||
\consensusrule{
|
||
A \nullifier{} \MUSTNOT repeat either within a \transaction, or across
|
||
\transactions in a \validBlockchain.
|
||
}
|
||
|
||
\sapling{\pnote{
|
||
\Sprout and \Sapling \nullifiers are considered disjoint, even if they have
|
||
the same bit pattern.
|
||
}}
|
||
|
||
|
||
\nsubsection{Block Subsidy and Founders' Reward} \label{subsidyconcepts}
|
||
|
||
Like \Bitcoin, \Zcash creates currency when \blocks are mined. The value created on
|
||
mining a \block is called the \blockSubsidy. It is composed of a \minerSubsidy and a
|
||
\foundersReward. As in \Bitcoin, the miner of a \block also receives \transactionFees.
|
||
|
||
The calculations of the \blockSubsidy, \minerSubsidy, and \foundersReward depend on
|
||
the \blockHeight, as defined in \crossref{blockchain}.
|
||
|
||
These calculations are described in \crossref{subsidies}.
|
||
|
||
|
||
\nsubsection{\CoinbaseTransactions}
|
||
|
||
The first \transaction in a block must be a \coinbaseTransaction, which should
|
||
collect and spend any \minerSubsidy and \transactionFees paid by \transactions
|
||
included in this \block. The \coinbaseTransaction must also pay the \foundersReward
|
||
as described in \crossref{foundersreward}.
|
||
|
||
|
||
\nsection{Abstract Protocol}
|
||
|
||
\nsubsection{Abstract Cryptographic Schemes}
|
||
|
||
\nsubsubsection{\HashFunctions} \label{abstracthashes}
|
||
|
||
\sprout{
|
||
$\MerkleCRH \typecolon \MerkleHashSprout \times \MerkleHashSprout \rightarrow \MerkleHashSprout$
|
||
is a collision-resistant \hashFunction used in \crossref{merklepath}.
|
||
It is instantiated in \crossref{merklecrh}.
|
||
}
|
||
\notsprout{
|
||
The functions $\MerkleCRHSprout \typecolon \MerkleLayerSprout \times \MerkleHashSprout \times \MerkleHashSprout
|
||
\rightarrow \MerkleHashSprout$
|
||
\sapling{and (for \Sapling),
|
||
$\MerkleCRHSapling \typecolon \MerkleLayerSapling \times \MerkleHashSapling \times \MerkleHashSapling
|
||
\rightarrow \MerkleHashSapling$
|
||
}
|
||
are \hashFunctions used in \crossref{merklepath}.
|
||
\sapling{$\MerkleCRHSapling$ is collision-resistant on all its arguments, and}
|
||
$\MerkleCRHSprout$ is collision-resistant except on its first argument.
|
||
Both of these functions are instantiated in \crossref{merklecrh}.
|
||
}
|
||
|
||
\changed{
|
||
$\hSigCRH{} \typecolon \bitseq{\RandomSeedLength} \times \typeexp{\PRFOutput}{\NOld} \times \JoinSplitSigPublic \rightarrow \hSigType$
|
||
is a collision-resistant \hashFunction used in \crossref{joinsplitdesc}.
|
||
It is instantiated in \crossref{hsigcrh}.
|
||
|
||
$\EquihashGen{} \typecolon (n \typecolon \PosInt) \times \PosInt \times \byteseqs \times \PosInt \rightarrow \bitseq{n}$
|
||
is another \hashFunction, used in \crossref{equihash} to generate
|
||
input to the Equihash solver. The first two arguments, representing
|
||
the Equihash parameters $n$ and $k$, are written subscripted.
|
||
It is instantiated in \crossref{equihashgen}.
|
||
}
|
||
|
||
\introsection
|
||
\nsubsubsection{\PseudoRandomFunctions} \label{abstractprfs}
|
||
|
||
$\PRF{x}{}$ is a \pseudoRandomFunction keyed by $x$.
|
||
|
||
Let $\AuthPrivateLength$, $\NoteAddressPreRandLength$, $\hSigLength$, and
|
||
$\PRFOutputLength$ be as defined in \crossref{constants}.
|
||
|
||
\sapling{Let $\ellJ$ be as defined in \crossref{jubjub}.}
|
||
|
||
\sprout{\changed{Four} \emph{independent} $\PRF{x}{}$ are needed in our protocol:}
|
||
|
||
\notsprout{For \Sprout, \changed{four} \emph{independent} $\PRF{x}{}$ are needed:}
|
||
|
||
\begin{tabular}{@{\hskip 2em}l@{\;}l@{\;}l@{\;}l@{\;}l}
|
||
$\PRFaddr{} $&$\typecolon\; \bitseq{\AuthPrivateLength} $&$\times\; \range{0}{255} $& &$\rightarrow \PRFOutput $\\
|
||
$\PRFnf{} $&$\typecolon\; \bitseq{\AuthPrivateLength} $&$\times\; \PRFOutput $& &$\rightarrow \PRFOutput $\\
|
||
$\PRFpk{} $&$\typecolon\; \bitseq{\AuthPrivateLength} $&$\times\; \setofOld $&$\times\; \hSigType $&$\rightarrow \PRFOutput $\\
|
||
$\PRFrho{} $&$\typecolon\; \bitseq{\NoteAddressPreRandLength} $&$\times\; \setofNew $&$\times\; \hSigType $&$\rightarrow \PRFOutput $
|
||
\end{tabular}
|
||
|
||
These are used in \crossref{joinsplitstatement}; $\PRFaddr{}$ is also used to
|
||
derive a \paymentAddress from a \spendingKey in \crossref{sproutkeycomponents}.
|
||
|
||
\sapling{
|
||
For \Sapling, one additional $\PRF{x}{}$ is needed:
|
||
|
||
\begin{tabular}{@{\hskip 2em}l@{\hskip 1.05em}l@{\hskip 0.85em}l@{\;}l@{\hskip 6.25em}l}
|
||
$\PRFnr{} $&$\typecolon\; \bitseq{\ellJ} $&$\times\; \bitseq{\ellJ} $& &$\rightarrow \PRFOutput$
|
||
\end{tabular}
|
||
|
||
It is used in \crossref{spendstatement}.}
|
||
|
||
\sprout{They}\notsprout{All of these \pseudoRandomFunctions} are instantiated in \crossref{concreteprfs}.
|
||
|
||
\begin{securityrequirements}
|
||
\item Security definitions for \pseudoRandomFunctions are given in \cite[section 4]{BDJR2000}.
|
||
\item In addition to being \pseudoRandomFunctions, it is required that
|
||
$\PRFnf{x}$,\changed{ $\PRFaddr{x}$, \sprout{and} $\PRFrho{x}$}\sapling{, and $\PRFnr{x}$}
|
||
be collision-resistant across all $x$ --- i.e.\ finding $(x, y) \neq (x', y')$
|
||
such that $\PRFnf{x}(y) = \PRFnf{x'}(y')$ should not be feasible\changed{, and
|
||
similarly for $\PRFaddr{}$ and $\PRFrho{}$\sapling{ and $\PRFnr{}$}}.
|
||
\end{securityrequirements}
|
||
|
||
\pnote{$\PRFnf{}$ was called $\PRFsn{}$ in \Zerocash \cite{BCG+2014}.}
|
||
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsubsection{\PseudoRandomGenerators} \label{abstractprgs}
|
||
|
||
$\PRG{x}{}$ is a \pseudoRandomGenerator keyed by $x$.
|
||
|
||
One $\PRG{x}{}$ is needed for \Sapling:
|
||
|
||
\begin{formulae}
|
||
\item $\PRGExpandSeed{} \typecolon \bitseq{\AuthPrivateSeedLength} \rightarrow \PRGOutput$
|
||
\end{formulae}
|
||
|
||
It is used to derive \Sapling key components from a \spendingKey in
|
||
\crossref{saplingkeycomponents}, and is instantiated in \crossref{concreteprgs}.
|
||
|
||
\securityrequirement{
|
||
Security definitions for \pseudoRandomGenerators are given in \cite[section 1.2]{SS2005}.
|
||
}
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{\SymmetricEncryption} \label{abstractsym}
|
||
|
||
Let $\Sym$ be an \symmetricEncryptionScheme with keyspace $\Keyspace$, encrypting
|
||
plaintexts in $\Plaintext$ to produce ciphertexts in $\Ciphertext$.
|
||
|
||
$\SymEncrypt{} \typecolon \Keyspace \times \Plaintext \rightarrow \Ciphertext$
|
||
is the encryption algorithm.
|
||
|
||
$\SymDecrypt{} \typecolon \Keyspace \times \Ciphertext \rightarrow
|
||
\Plaintext \union \setof{\bot}$ is the corresponding decryption algorithm, such that
|
||
for any $\Key \in \Keyspace$ and $\Ptext \in \Plaintext$,
|
||
$\SymDecrypt{\Key}(\SymEncrypt{\Key}(\Ptext)) = \Ptext$.
|
||
$\bot$ is used to represent the decryption of an invalid ciphertext.
|
||
|
||
\securityrequirement{
|
||
$\Sym$ must be one-time (INT-CTXT $\wedge$ IND-CPA)-secure. \quotedterm{One-time} here
|
||
means that an honest protocol participant will almost surely encrypt only one message
|
||
with a given key; however, the attacker may make many adaptive chosen ciphertext
|
||
queries for a given key. The security notions INT-CTXT and IND-CPA are as defined in
|
||
\cite{BN2007}.
|
||
}
|
||
|
||
\nsubsubsection{\KeyAgreement} \label{abstractkeyagreement}
|
||
|
||
A \keyAgreementScheme is a cryptographic protocol in which two parties agree
|
||
a shared secret, each using their private key and the other party's public key.
|
||
|
||
A \keyAgreementScheme $\KA$ defines a type of public keys $\KAPublic$, a type
|
||
of private keys $\KAPrivate$, and a type of shared secrets $\KASharedSecret$.
|
||
|
||
Let $\KAFormatPrivate \typecolon \PRFOutput \rightarrow \KAPrivate$ be a function
|
||
that converts a bit string of length $\PRFOutputLength$ to a $\KA$ private key.
|
||
|
||
Let $\KADerivePublic \typecolon \KAPrivate \times \KAPublic \rightarrow \KAPublic$
|
||
be a function that derives the $\KA$ public key corresponding to a given $\KA$
|
||
private key and base point.
|
||
|
||
Let $\KAAgree \typecolon \KAPrivate \times \KAPublic \rightarrow \KASharedSecret$
|
||
be the agreement function.
|
||
|
||
\sapling{Optional:} Let $\KABase \typecolon \KAPublic$ be a public base point.
|
||
|
||
\pnote{
|
||
The range of $\KADerivePublic$ may be a strict subset of $\KAPublic$.
|
||
}
|
||
|
||
\begin{securityrequirements}
|
||
\item $\KAFormatPrivate$ must preserve sufficient entropy from its input to be used
|
||
as a secure $\KA$ private key.
|
||
\item The key agreement and the KDF defined in the next section must together
|
||
satisfy a suitable adaptive security assumption along the lines of
|
||
\cite[section 3]{Bern2006} or \cite[Definition 3]{ABR1999}.
|
||
\end{securityrequirements}
|
||
|
||
More precise formalization of these requirements is beyond the scope of this
|
||
specification.
|
||
|
||
|
||
\nsubsubsection{\KeyDerivation} \label{abstractkdf}
|
||
|
||
A \keyDerivationFunction is defined for a particular \keyAgreementScheme and
|
||
\symmetricEncryptionScheme; it takes the shared secret produced by the key
|
||
agreement and additional arguments, and derives a key suitable for the encryption
|
||
scheme.
|
||
|
||
Let $\KDF \typecolon \setofNew \times \hSigType \times \KASharedSecret
|
||
\times \KAPublic \times \KAPublic \rightarrow \Keyspace$ be a
|
||
\keyDerivationFunction suitable for use with $\KA$, deriving keys
|
||
for $\SymEncrypt{}$.
|
||
|
||
\securityrequirement{
|
||
In addition to adaptive security of the key agreement and KDF,
|
||
the following security property is required:
|
||
|
||
\notsprout{
|
||
\todo{adapt this definition to handle \Sapling, or maybe just remove it.}
|
||
|
||
Let $\TransmitBase := \todo{?}$
|
||
}
|
||
\sprout{Let $\TransmitBase := \KABase$.}
|
||
|
||
Let $\TransmitPrivateSup{1}$ and $\TransmitPrivateSup{2}$ each be chosen uniformly and
|
||
independently at random from $\KAPrivate$.
|
||
|
||
Let $\TransmitPublicSup{j} := \KADerivePublic(\TransmitPrivateSup{j}, \TransmitBase)$.
|
||
|
||
\introlist
|
||
An adversary can adaptively query a function
|
||
$Q \typecolon \range{1}{2} \times \hSigType \rightarrow
|
||
\KAPublic \times \Keyspace_{\allNew}$ where $Q_j(\hSig)$ is defined as follows:
|
||
\begin{enumerate}
|
||
\item Choose $\EphemeralPrivate$ uniformly at random from $\KAPrivate$.
|
||
\item Let $\EphemeralPublic := \KADerivePublic(\EphemeralPrivate, \TransmitBase)$.
|
||
\item For $i \in \setofNew$, let $\Key_i :=
|
||
\KDF(i, \hSig, \KAAgree(\EphemeralPrivate, \TransmitPublicSup{j}), \EphemeralPublic, \TransmitPublicSup{j}))$.
|
||
\item Return $(\EphemeralPublic, \Key_{\allNew})$.
|
||
\end{enumerate}
|
||
|
||
Then the adversary must make another query to $Q_j$ with random unknown
|
||
$j \in \range{1}{2}$, and guess $j$ with probability greater than chance.
|
||
}
|
||
|
||
If the adversary's advantage is negligible, then the asymmetric encryption scheme
|
||
constructed from $\KA$, $\KDF$ and $\Sym$ in \crossref{inband} will be key-private
|
||
as defined in \cite{BBDP2001}.
|
||
|
||
\pnote{
|
||
The given definition only requires ciphertexts to be indistinguishable
|
||
between \transmissionKeys that are outputs of $\KADerivePublic$ (which
|
||
includes all keys generated as in \crossref{sproutkeycomponents}). If a
|
||
\transmissionKey not in that range is used, it may be distinguishable.
|
||
This is not considered to be a significant security weakness.
|
||
}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Signature} \label{abstractsig}
|
||
|
||
A signature scheme $\Sig$ defines:
|
||
|
||
\begin{itemize}
|
||
\item a type of signing keys $\SigPrivate$;
|
||
\item a type of verifying keys $\SigPublic$;
|
||
\item a type of messages $\SigMessage$;
|
||
\item a type of signatures $\SigSignature$;
|
||
\item a randomized key pair generation algorithm $\SigGen \typecolon () \rightarrowR \SigPrivate \times \SigPublic$;
|
||
\item a randomized signing algorithm $\SigSign{} \typecolon \SigPrivate \times \SigMessage \rightarrowR \SigSignature$;
|
||
\item a verifying algorithm $\SigVerify{} \typecolon \SigPublic \times \SigMessage \times \SigSignature \rightarrow \bit$;
|
||
\end{itemize}
|
||
|
||
such that for any key pair $(\sk, \vk) \leftarrowR \SigGen()$, and
|
||
any $m \typecolon \SigMessage$ and $s \typecolon \SigSignature \leftarrowR \SigSign{\sk}(m)$,
|
||
$\SigVerify{\vk}(m, s) = 1$.
|
||
|
||
\Zcash uses \sprout{two}\sapling{three} signature schemes:
|
||
|
||
\begin{itemize}
|
||
\item one used for signatures that can be verified by script operations such as
|
||
\ScriptOP{CHECKSIG} and \ScriptOP{CHECKMULTISIG} as in \Bitcoin;
|
||
\item one called $\JoinSplitSig$ (instantiated in \crossref{concretejssig}),
|
||
which is used to sign \transactions that contain at least one
|
||
\joinSplitDescription\sprout{.}\notsprout{;}
|
||
\saplingonwarditem{one called $\SpendAuthSig$ (instantiated
|
||
in \crossref{concretespendauthsig}), which is used to sign authorizations of
|
||
\spendDescriptions.}
|
||
\end{itemize}
|
||
|
||
The following defines only the security properties needed
|
||
for $\JoinSplitSig$\sapling{ and $\SpendAuthSig$}.
|
||
|
||
\securityrequirement{
|
||
$\JoinSplitSig$\sapling{ and $\SpendAuthSig$} must be
|
||
Strongly Unforgeable under (non-adaptive) Chosen Message Attack (SU-CMA),
|
||
as defined for example in \cite[Definition 6]{BDEHR2011}. This allows an adversary
|
||
to obtain signatures on chosen messages, and then requires it to be infeasible
|
||
for the adversary to forge a previously unseen valid \mbox{(message, signature)}
|
||
pair without access to the signing key.
|
||
}
|
||
|
||
\begin{pnotes}
|
||
\item A fresh signature key pair is generated for each \transaction containing
|
||
a \joinSplitDescription\sapling{, and for each \spendDescription}.
|
||
Since each key pair is only used for one signature (see \crossref{nonmalleability}),
|
||
a one-time signature scheme would suffice for $\JoinSplitSig$\sapling{ and $\SpendAuthSig$}.
|
||
This is also the reason why only security against \emph{non-adaptive}
|
||
chosen message attack is needed.
|
||
In fact the instantiation of $\JoinSplitSig$
|
||
\sprout{uses a scheme}\sapling{and $\SpendAuthSig$ use schemes}
|
||
designed for security under adaptive attack even when multiple signatures
|
||
are signed under the same key.
|
||
\item SU-CMA security requires it to be infeasible for the adversary, not
|
||
knowing the private key, to forge a distinct signature on a previously
|
||
seen message. That is, \joinSplitSignatures\sapling{ and \spendAuthSignatures}
|
||
are intended to be nonmalleable in the sense of \cite{BIP-62}.
|
||
\end{pnotes}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsubsection{Signature with Re-Randomizable Keys} \label{abstractsigrerand}
|
||
|
||
A signature scheme with re-randomizable keys $\Sig$ is a signature scheme that
|
||
additionally defines:
|
||
|
||
\begin{itemize}
|
||
\item a type of randomizers $\SigRandom$;
|
||
\item a public key randomization algorithm $\SigRandomizePublic \typecolon \SigPublic \times \SigRandom \rightarrow \SigPublic$;
|
||
\item a private key randomization algorithm $\SigRandomizePrivate \typecolon \SigPrivate \times \SigRandom \rightarrow \SigPrivate$
|
||
\item a distinguished ``identity'' randomizer $\SigRandomnessId \typecolon \SigRandom$
|
||
\end{itemize}
|
||
|
||
\vspace{-1ex}
|
||
such that if $(\pk \typecolon \SigPublic, \sk \typecolon \SigPrivate)$ is a
|
||
valid $\Sig$ key pair, then:
|
||
\vspace{1ex}
|
||
|
||
\begin{itemize}
|
||
\item $\left(\SigRandomizePublic(\pk, \SigRandomness), \SigRandomizePrivate(\sk, \SigRandomness)\right)$
|
||
is also a valid $\Sig$ key pair for any $\SigRandomness \typecolon \SigRandom$;
|
||
\item $\SigRandomizePrivate(\paramdot, \SigRandomness) \typecolon \SigPrivate \rightarrow \SigPrivate$
|
||
is injective and easily invertible for any $\SigRandomness \typecolon \SigRandom$;
|
||
\item For \emph{any} key pair $(\pk, \sk)$ returned by $\SigGen()$, the distribution of
|
||
\begin{formulae}
|
||
\item $\left(\SigRandomizePublic(\pk, \SigRandomness), \SigRandomizePrivate(\sk, \SigRandomness)\right) :
|
||
\SigRandomness \leftarrowR \SigRandom$
|
||
\end{formulae}
|
||
\vspace{-0.ex} is identical to the distribution of $\SigGen()$.
|
||
\item $\left(\SigRandomizePublic(\pk, \SigRandomnessId), \SigRandomizePrivate(\sk, \SigRandomnessId)\right) = (\pk, \sk)$.
|
||
\end{itemize}
|
||
|
||
The following security requirement for such signature schemes is based on that
|
||
given in \cite[section 3]{FKMSSS2016}. Note that we require Strong Unforgeability
|
||
under Re-randomized Keys, not Existential Unforgeability under Re-randomized Keys
|
||
(the latter is just called ``Unforgeability under Re-randomized Keys'' in
|
||
\cite[Definition 8]{FKMSSS2016}).
|
||
|
||
\introsection
|
||
\securityrequirement{\textbf{Strong Unforgeability under Re-randomized Keys (SUFRK-CMA)}
|
||
|
||
Let $\Oracle \typecolon \SigPrivate \times \SigMessage \times \SigRandom \rightarrow \SigSignature$
|
||
be a generator of signing oracles.
|
||
|
||
A signing oracle $\Oracle_{\sk}$ for private key $\sk$ has state
|
||
$Q \typecolon \powerset{\SigMessage \times \SigSignature}$ initialized to $\setof{}$
|
||
that records queried messages and corresponding signatures.
|
||
|
||
\begin{formulae}
|
||
\item $\Oracle_{\sk} :=$ var $Q \leftarrow \setof{}$ in $\fun{(m \typecolon \SigMessage, \SigRandomness \typecolon \SigRandom)}{}$
|
||
\item \tab let $\sigma = \SigSign{\SigRandomizePrivate(\sk, \SigRandomness)}(m)$
|
||
\item \tab $Q \leftarrow Q \union \setof{(m, \sigma)}$
|
||
\item \tab return $\sigma \typecolon \SigSignature$.
|
||
\end{formulae}
|
||
|
||
For random $(\pk, \sk) \leftarrowR \SigGen()$, it must be infeasible for an adversary
|
||
given $\pk$ and a new instance of $\Oracle_{\sk}$ to find $(m^*, \sigma^*, \SigRandomness^*)$
|
||
such that $\SigVerify{\SigRandomizePublic(\pk, \SigRandomness^*)}(m^*, \sigma^*) = 1$ and
|
||
$(m^*, \sigma^*) \not\in \Oracle_{\sk}\mathsf{.}Q$.
|
||
}
|
||
|
||
\begin{pnotes}
|
||
\item The requirement for $\SigRandomnessId$ simplifies the definition of SUFRK-CMA
|
||
by removing the need for two oracles (since the oracle for original keys,
|
||
called $\Oracle_1$ in \cite{FKMSSS2016}, is a special case of the oracle for
|
||
randomized keys).
|
||
\item The fact that
|
||
$\left(\SigRandomizePublic(\pk, \SigRandomness), \SigRandomizePrivate(\sk, \SigRandomness)\right) :
|
||
\SigRandomness \leftarrowR \SigRandom$ is identically distributed to $\SigGen()$,
|
||
implies that the combination of a re-randomized public key and signature(s)
|
||
under that key do not reveal the key from which it was re-randomized.
|
||
\item Since $\SigRandomizePrivate(\paramdot, \SigRandomness)$ is injective and
|
||
easily invertible, knowledge of $\SigRandomizePrivate(\sk, \SigRandomness)$
|
||
\emph{and} $\SigRandomness$ implies knowledge of $\sk$.
|
||
\end{pnotes}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Commitment} \label{abstractcommit}
|
||
|
||
A \commitmentScheme is a function that, given a random \commitmentTrapdoor
|
||
and an input, can be used to commit to the input in such a way that:
|
||
|
||
\begin{itemize}
|
||
\item no information is revealed about it without the \trapdoor (\quotedterm{hiding}),
|
||
\item given the \trapdoor and input, the commitment can be verified to \quotedterm{open}
|
||
to that input and no other (\quotedterm{binding}).
|
||
\end{itemize}
|
||
|
||
\vspace{-3ex}
|
||
A \commitmentScheme $\CommitAlg$ defines a type of inputs $\CommitInput$,
|
||
a type of commitments $\CommitOutput$, and a type of \commitmentTrapdoors
|
||
$\CommitTrapdoor$.
|
||
|
||
Let $\CommitAlg \typecolon \CommitTrapdoor \times \CommitInput \rightarrow \CommitOutput$
|
||
be a function satisfying the security requirements below.
|
||
|
||
\begin{securityrequirements}
|
||
\item \textbf{Computational hiding:} For all $x, x' \typecolon \CommitInput$,
|
||
the distributions $\{\; \Commit{r}(x) \;|\; r \leftarrowR \CommitTrapdoor \;\}$
|
||
and $\{\; \Commit{r}(x') \;|\; r \leftarrowR \CommitTrapdoor \;\}$ are
|
||
computationally indistinguishable.
|
||
\item \textbf{Computational binding:} It is infeasible to find
|
||
$x, x' \typecolon \CommitInput$ and
|
||
$r, r' \typecolon \CommitTrapdoor$
|
||
such that $x \neq x'$ and $\Commit{r}(x) = \Commit{r'}(x')$.
|
||
\end{securityrequirements}
|
||
|
||
\pnote{
|
||
If it were feasible to find $x \typecolon \CommitInput$ and
|
||
$r, r' \typecolon \CommitTrapdoor$ such that $r \neq r'$ and
|
||
$\Commit{r}(x) = \Commit{r'}(x)$, this would not by itself contradict
|
||
the computational binding security requirement.
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{\RepresentedGroup} \label{abstractgroup}
|
||
|
||
A \representedGroup $\GroupG{}$ consists of:
|
||
|
||
\begin{itemize}
|
||
\item a subgroup order parameter $\ParamG{r} \typecolon \PosInt$, which must be prime;
|
||
\item a cofactor parameter $\ParamG{h} \typecolon \PosInt$;
|
||
\item a group $\GroupG{}$ of order $\ParamG{h} \mult \ParamG{r}$, written additively
|
||
with operation $+ \typecolon \GroupG{} \times \GroupG{} \rightarrow \GroupG{}$,
|
||
and additive identity $\ZeroG{}$;
|
||
\item a generator $\GenG{}$ of the subgroup of $\GroupG{}$ of order $\ParamG{r}$;
|
||
\item a bit-length parameter $\ellG{} \typecolon \Nat$;
|
||
\item a representation function $\reprG{} \typecolon
|
||
\GroupG{} \rightarrow \bitseq{\ellG{}}$;
|
||
\item an abstraction function $\abstG{} \typecolon
|
||
\bitseq{\ellG{}} \rightarrow \GroupG{} \union \setof{\bot}$;
|
||
\end{itemize}
|
||
\vspace{-2ex}
|
||
such that $\abstG{}$ is the left inverse of $\reprG{}$, i.e.
|
||
for all $P \in \GroupG{}$, $\abstG{}(\reprG{}(P)) = P$, and
|
||
for all $S$ not in the image of $\reprG{}$, $\abstG{}(S) = \bot$.
|
||
|
||
% Do we actually need \GenG? It is natural to include it for some groups
|
||
% and not others.
|
||
|
||
We extend the $\vsum{}{}$ notation to addition on group elements.
|
||
|
||
\vspace{-3ex}
|
||
For $G \typecolon \GroupG{}$ and $k \typecolon \Nat$ (or $k \typecolon \GF{\ParamG{r}}$)
|
||
we write $\scalarmult{k}{G}$ for $\vsum{i = 1}{k} G$.
|
||
\vspace{1ex}
|
||
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsubsection{\HashExtractor} \label{abstractextractor}
|
||
|
||
A \hashExtractor for a \representedGroup $\GroupG{}$ is a function
|
||
$\ExtractG \typecolon \GroupG{} \rightarrow T$ for some type $T$,
|
||
such that $\ExtractG$ is injective on the subgroup of $\GroupG{}$ of order
|
||
$\ParamG{r}$.
|
||
|
||
\pnote{
|
||
Unlike the representation function $\reprG{}$, $\ExtractG$ need not have an
|
||
efficiently computable left inverse.
|
||
}
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\introlist
|
||
\nsubsubsection{\GroupHash} \label{abstractgrouphash}
|
||
|
||
Given a represented group $\GroupG{}$ and a type $\CRSType$, we define a
|
||
\term{family of group hashes into\, $\GroupG{}$} as a function
|
||
|
||
\begin{formulae}
|
||
\item $\GroupGHash{} \typecolon \CRSType \times \bitseq{\ell} \rightarrow \GroupG{}$
|
||
\end{formulae}
|
||
\vspace{-1ex}
|
||
with the following security requirement.
|
||
|
||
\securityrequirement{\textbf{Discrete Logarithm Independence}
|
||
|
||
For a randomly selected member $\GroupGHash{\CRS}$ of the family, it is infeasible to find
|
||
a sequence of distinct inputs $m_{\alln} \typecolon \typeexp{\bitseq{\ell}}{n}$
|
||
and a sequence of nonzero scalars $x_{\alln} \typecolon \typeexp{\GFstar{\ParamG{r}}}{n}$
|
||
such that $\vsum{i = 1}{n}\left(\scalarmult{x_i}{\GroupGHash{\CRS}(m_i)}\right) = \ZeroG{}$.
|
||
}
|
||
|
||
\begin{pnotes}
|
||
\item This property implies (and is stronger than) collision-resistance,
|
||
since a collision $(m_1, m_2)$ for $\GroupGHash{\CRS}$ trivially gives a
|
||
discrete logarithm relation with $x_1 = 1$ and $x_2 = -1$.
|
||
\item An alternative approach is to model $\GroupGHash{\CRS}$ as a random
|
||
oracle, and assume that the Discrete Logarithm Problem is hard in
|
||
the group. We prefer to avoid the Random Oracle Model and instead make
|
||
a more specific standard-model assumption, which is effectively no
|
||
stronger than the assumptions made in the random oracle approach.
|
||
\item $\CRS$ is a \commonRandomString; we choose it verifiably at random,
|
||
\emph{after} fixing the concrete group hash algorithm to be used.
|
||
If we publish the algorithm and the method of choosing the
|
||
\commonRandomString before the $\CRS$ could be known, then this
|
||
mitigates the possibility that the group hash algorithm could have
|
||
been backdoored.
|
||
\end{pnotes}
|
||
}
|
||
|
||
\introlist
|
||
\nsubsubsection{\RepresentedPairing} \label{abstractpairing}
|
||
|
||
A \representedPairing $\GroupP{}$ consists of:
|
||
|
||
\begin{itemize}
|
||
\item a group order parameter $\ParamP{r} \typecolon \PosInt$ which must be prime;
|
||
\item two \representedGroups $\GroupP{1..2}$, both of order $\ParamP{r}$;
|
||
\item a group $\GroupP{T}$ of order $\ParamP{r}$, written multiplicatively with operation
|
||
$\mult \typecolon \GroupP{T} \times \GroupP{T} \rightarrow \GroupP{T}$
|
||
and multiplicative identity $\ParamP{\mathbf{1}}$;
|
||
\item a pairing function
|
||
$\PairingP \typecolon \GroupP{1} \times \GroupP{2} \rightarrow \GroupP{T}$
|
||
satisfying:
|
||
|
||
\begin{itemize}
|
||
\item (Bilinearity)\; for all $a, b \typecolon \GFstar{r}$,
|
||
$P \typecolon \GroupP{1}$, and $Q \typecolon \GroupP{2}$,
|
||
$\PairingP(\scalarmult{a}{P}, \scalarmult{b}{Q}) = \PairingP(P, Q)^{a \mult b}$, and
|
||
\item (Nondegeneracy)\; there does not exist $P \typecolon \GroupP{1} \setminus \ZeroP{1}$
|
||
such that for all $Q \typecolon \GroupP{2},
|
||
\PairingP(P, Q) = \ParamP{\mathbf{1}}$;
|
||
\end{itemize}
|
||
\end{itemize}
|
||
|
||
\nsubsubsection{\ZeroKnowledgeProvingSystem} \label{abstractzk}
|
||
|
||
A \zeroKnowledgeProvingSystem is a cryptographic protocol that allows
|
||
proving a particular \statement, dependent on \primary and \auxiliaryInputs,
|
||
in zero knowledge --- that is, without revealing information about the
|
||
\auxiliaryInputs other than that implied by the \statement. The type of
|
||
\zeroKnowledgeProvingSystem needed by \Zcash is a \ppzkSNARK.
|
||
|
||
\introlist
|
||
A \ppzkSNARK instance $\ZK$ defines:
|
||
|
||
\begin{itemize}
|
||
\item a type of \zkProvingKeys, $\ZKProvingKey$;
|
||
\item a type of \zkVerifyingKeys, $\ZKVerifyingKey$;
|
||
\item a type of \primaryInputs $\ZKPrimary$;
|
||
\item a type of \auxiliaryInputs $\ZKAuxiliary$;
|
||
\item a type of proofs $\ZKProof$;
|
||
\item a type $\ZKSatisfying \subseteq \ZKPrimary \times \ZKAuxiliary$ of inputs satisfying
|
||
the \statement;
|
||
\item a randomized key pair generation algorithm $\ZKGen \typecolon () \rightarrowR \ZKProvingKey \times \ZKVerifyingKey$;
|
||
\item a proving algorithm $\ZKProve{} \typecolon \ZKProvingKey \times \ZKSatisfying \rightarrow \ZKProof$;
|
||
\item a verifying algorithm $\ZKVerify{} \typecolon \ZKVerifyingKey \times \ZKPrimary \times \ZKProof \rightarrow \bit$;
|
||
\end{itemize}
|
||
|
||
The security requirements below are supposed to hold with overwhelming
|
||
probability for $(\pk, \vk) \leftarrowR \ZKGen()$.
|
||
|
||
\begin{securityrequirements}
|
||
\item \textbf{Completeness:} An honestly generated proof will convince a verifier:
|
||
for any $(x, w) \in \ZKSatisfying$, if $\ZKProve{\pk}(x, w)$ outputs $\Proof{}$,
|
||
then $\ZKVerify{\vk}(x, \Proof{}) = 1$.
|
||
\item \textbf{Knowledge Soundness:} For any adversary $\Adversary$ able to find an
|
||
$x \typecolon \ZKPrimary$ and proof $\Proof{} \typecolon \ZKProof$ such that $\ZKVerify{\vk}(x, \Proof{}) = 1$,
|
||
there is an efficient extractor $E_{\Adversary}$ such that if $E_{\Adversary}(\vk, \pk)$
|
||
returns $w$, then the probability that $(x, w) \not\in \ZKSatisfying$ is negligable.
|
||
\item \textbf{Statistical Zero Knowledge:} An honestly generated proof is statistical
|
||
zero knowledge. That is, there is a feasible stateful simulator $\Simulator$ such that,
|
||
for all stateful distinguishers $\Distinguisher$, the following two probabilities are
|
||
negligibly close:
|
||
\vspace{0.5ex}
|
||
|
||
$\;\;\Prob{
|
||
(x, w) \in \ZKSatisfying \\
|
||
\Distinguisher(\Proof{}) = 1
|
||
}{
|
||
(\pk, \vk) \leftarrowR \ZKGen() \\
|
||
(x, w) \leftarrowR \Distinguisher(\pk, \vk) \\
|
||
\Proof{} \leftarrowR \ZKProve{\pk}(x, w)
|
||
}
|
||
\text{\; and \;}
|
||
\Prob{
|
||
(x, w) \in \ZKSatisfying \\
|
||
\Distinguisher(\Proof{}) = 1
|
||
}{
|
||
(\pk, \vk) \leftarrowR \Simulator() \\
|
||
(x, w) \leftarrowR \Distinguisher(\pk, \vk) \\
|
||
\Proof{} \leftarrowR \Simulator(x)
|
||
}$
|
||
\end{securityrequirements}
|
||
|
||
These definitions are derived from those in \cite[Appendix C]{BCTV2014}, adapted to
|
||
state concrete security for a fixed circuit, rather than asymptotic security for
|
||
arbitrary circuits. ($\ZKProve{}$ corresponds to $P$, $\ZKVerify{}$ corresponds to $V$,
|
||
and $\ZKSatisfying$ corresponds to $\mathcal{R}_C$ in the notation of that appendix.)
|
||
|
||
The Knowledge Soundness definition is a way to formalize the property that it is
|
||
infeasible to find a new proof $\Proof{}$ where $\ZKVerify{\vk}(x, \Proof{}) = 1$ without
|
||
\emph{knowing} an \auxiliaryInput $w$ such that $(x, w) \in \ZKSatisfying$.
|
||
Note that Knowledge Soundness implies Soundness --- i.e.\ the property that it is
|
||
infeasible to find a new proof $\Proof{}$ where $\ZKVerify{\vk}(x, \Proof{}) = 1$ without
|
||
\emph{there existing} an \auxiliaryInput $w$ such that $(x, w) \in \ZKSatisfying$.
|
||
|
||
It is possible to replay proofs, but informally, a proof for a given $(x, w)$ gives
|
||
no information that helps to find a proof for other $(x, w)$.
|
||
|
||
\sprout{
|
||
The \provingSystem is instantiated in \crossref{phgr}.
|
||
$\JoinSplit$ refers to this \provingSystem with the $\BNCurve$ pairing,
|
||
specialized to the \joinSplitStatement given in \crossref{joinsplitstatement}.
|
||
In this case we omit the key subscripts on $\JoinSplitProve$ and $\JoinSplitVerify$,
|
||
taking them to be the particular \provingKey and \verifyingKey defined by the
|
||
\joinSplitParameters in \crossref{sproutparameters}.
|
||
}
|
||
\sapling{
|
||
\Zcash uses two \provingSystems:
|
||
\begin{itemize}
|
||
\item $\PHGR$ (\crossref{phgr}) is used with the
|
||
$\BNCurve$ pairing (\crossref{bnpairing}),
|
||
to prove and verify the \Sprout \joinSplitStatement
|
||
(\crossref{joinsplitstatement}).
|
||
\item $\Groth$ (\crossref{groth}) is used with the
|
||
$\BLSCurve$ pairing (\crossref{blspairing}),
|
||
to prove and verify the \Sapling \spendStatement
|
||
(\crossref{spendstatement}) and \outputStatement
|
||
(\crossref{outputstatement}).
|
||
\end{itemize}
|
||
|
||
These specializations are referred to as
|
||
$\JoinSplit$ for the \Sprout \joinSplitStatement,
|
||
$\Spend$ for the \Sapling \spendStatement, and
|
||
$\Output$ for the \Sapling \outputStatement.
|
||
|
||
We omit the key subscripts on $\JoinSplitProve$ and
|
||
$\JoinSplitVerify$, taking them to be the $\PHGR$ \provingKey
|
||
and \verifyingKey defined in \crossref{sproutparameters}.
|
||
|
||
Similarly, we omit the key subscripts on $\SpendProve$,
|
||
$\SpendVerify$, $\OutputProve$, and $\OutputVerify$, taking
|
||
them to be the $\Groth$ \provingKeys and
|
||
\verifyingKeys defined in \crossref{saplingparameters}.
|
||
}
|
||
|
||
\nsubsection{\KeyComponents} \label{keycomponents}
|
||
|
||
\notsprout{\nsubsubsection{\Sprout{} \KeyComponents}} \label{sproutkeycomponents}
|
||
|
||
Let $\PRFaddr{}$ be a \pseudoRandomFunction, instantiated in \crossref{concreteprfs}.
|
||
|
||
Let $\KASprout$ be a \keyAgreementScheme, instantiated in \crossref{concretesproutkeyagreement}.
|
||
|
||
A new \SproutOrNothing \spendingKey $\AuthPrivate$ is generated by choosing a bit sequence
|
||
uniformly at random from $\bitseq{\AuthPrivateLength}$.
|
||
|
||
\introlist
|
||
\changed{
|
||
$\AuthPublic$, $\TransmitPrivate$ and $\TransmitPublic$ are derived from
|
||
$\AuthPrivate$
|
||
as follows:}
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\AuthPublic$ &$:= \changed{\PRFaddr{\AuthPrivate}(0)}$ \\
|
||
$\TransmitPrivate$ &$:= \changed{\KASproutFormatPrivate(\PRFaddr{\AuthPrivate}(1))}$ \\
|
||
$\TransmitPublic$ &$:= \changed{\KASproutDerivePublic(\TransmitPrivate, \KASproutBase)}$.
|
||
\end{tabular}
|
||
|
||
\sapling{
|
||
\nsubsubsection{\Sapling{} \KeyComponents} \label{saplingkeycomponents}
|
||
|
||
Let $\PRGExpandSeed{}$ be a \pseudoRandomGenerator, instantiated in \crossref{concreteprgs}.
|
||
|
||
Let $\KASapling$ be a \keyAgreementScheme, instantiated in \crossref{concretesaplingkeyagreement}.
|
||
|
||
Let $\CRHivk$ be a \hashFunction, instantiated in \crossref{concretecrhivk}.
|
||
|
||
Let $\FindGroupJHash{U}$ be as defined in \crossref{concretegrouphashjubjub}.
|
||
|
||
Let $\AuthSignBase = \FindGroupJHashOf{\ascii{Zcash\_G\_}, \ascii{}}$ and
|
||
let $\AuthProveBase = \FindGroupJHashOf{\ascii{Zcash\_H\_}, \ascii{}}$.
|
||
|
||
Let $\reprJ$ be the representation function for the $\JubjubCurve$ \representedGroup,
|
||
instantiated in \crossref{jubjub}.
|
||
|
||
Let $\LEBStoOSP{} \typecolon (\ell \typecolon \Nat) \times \bitseq{\ell} \rightarrow \byteseq{\ceiling{\ell/8}}$
|
||
be defined as in \crossref{endian}.
|
||
|
||
\vspace{2ex}
|
||
A new \Sapling \spendingKey $\AuthPrivateSeed$ is generated by choosing a bit sequence
|
||
uniformly at random from $\bitseq{\AuthPrivateSeedLength}$.
|
||
|
||
This is expanded using $\PRGExpandSeed{}$ to a $512$-bit \expandedSeed,
|
||
then split into two integers,
|
||
$\PreAuthSignPrivate \typecolon \range{0}{2^{256}-1}$ and
|
||
$\PreAuthProvePrivate \typecolon \range{0}{2^{256}-1}$:
|
||
\vspace{1.5ex}
|
||
}
|
||
|
||
\newsavebox{\expandedseedbox}
|
||
\begin{lrbox}{\expandedseedbox}
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\sapling{
|
||
\bitbox{256}{$256$-bit $\PreAuthSignPrivate$} &
|
||
\bitbox{256}{$256$-bit $\PreAuthProvePrivate$}
|
||
}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\crhivkinputbox}
|
||
\begin{lrbox}{\crhivkinputbox}
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\sapling{
|
||
\bitbox{256}{$256$-bit $\LEBStoOSPOf{256}{\reprJOf{\AuthSignPublic}}$} &
|
||
\bitbox{256}{$256$-bit $\LEBStoOSPOf{256}{\reprJOf{\AuthProvePublic}}$}
|
||
}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\begin{formulae}
|
||
\item $\Justthebox{\expandedseedbox} := \PRGExpandSeed{\AuthPrivateSeed}()$ \\
|
||
\end{formulae}
|
||
|
||
\todo{Check byte and bit order. Also, we shouldn't use bit layout diagrams in
|
||
the abstract protocol; they are only defined in \crossref{endian}.}
|
||
|
||
\introlist
|
||
$\AuthSignPrivate$, $\AuthProvePrivate$, $\AuthSignPublic$, $\AuthProvePublic$,
|
||
and $\InViewingKey$ are then derived as follows:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\AuthSignPrivate$ &$:= \PreAuthSignPrivate \bmod \JubjubScalarThreshold$ \\
|
||
$\AuthProvePrivate$ &$:= \PreAuthProvePrivate \bmod \JubjubScalarThreshold$ \\
|
||
$\AuthSignPublic$ &$:= \scalarmult{\AuthSignPrivate}{\AuthSignBase}$ \\
|
||
$\AuthProvePublic$ &$:= \scalarmult{\AuthProvePrivate}{\AuthProveBase}$ \\
|
||
$\InViewingKey$ &$:= \CRHivkBox{\crhivkinputbox}$.
|
||
\end{tabular}
|
||
|
||
\vspace{2ex}
|
||
As explained in \crossref{addressesandkeys}, \Sapling allows the efficient
|
||
creation of multiple \diversifiedPaymentAddresses with the same spending
|
||
authority. A group of such addresses shares the same \fullViewingKey and
|
||
\incomingViewingKey.
|
||
|
||
To create a new \diversifiedPaymentAddress given an \incomingViewingKey
|
||
$\InViewingKey$, first choose a \diversifier $\Diversifier$ uniformly at
|
||
random from $\DiversifierType$.
|
||
Then calculate:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\DiversifiedTransmitBase$ &$:= \GroupJHash{U}(\ascii{Zcash\_gd}, \Diversifier)$ \\
|
||
$\DiversifiedTransmitPublic$ &$:= \reprJOf{\KASaplingDerivePublic(\InViewingKey, \DiversifiedTransmitBase)}$.
|
||
\end{tabular}
|
||
|
||
The resulting \diversifiedPaymentAddress is $(\Diversifier, \DiversifiedTransmitPublic)$.
|
||
|
||
\begin{pnotes}
|
||
\item The protocol does not prevent using the \diversifier $\Diversifier$ to produce
|
||
\quotedterm{vanity} addresses that start with a meaningful string when
|
||
encoded in Bech32 (see \crossref{saplingpaymentaddrencoding}).
|
||
Users and writers of software that generates addresses should be aware that
|
||
this provides weaker privacy properties than a randomly chosen \diversifier,
|
||
since a vanity address can obviously be distinguished, and might leak more
|
||
information than intended as to who created it.
|
||
\item Similarly, address generators \MAY encode information in the \diversifier
|
||
that can be recovered by the recipient of a payment to determine which
|
||
\diversifiedPaymentAddress was used. It is \RECOMMENDED that such \diversifiers
|
||
be randomly chosen unique byte sequences used to index into a database, rather
|
||
than directly encoding the needed data.
|
||
\end{pnotes}
|
||
}
|
||
|
||
|
||
\nsubsection{\JoinSplitDescriptions} \label{joinsplitdesc}
|
||
|
||
A \joinSplitTransfer, as specified in \crossref{joinsplit}, is encoded in
|
||
\transactions as a \joinSplitDescription.
|
||
|
||
Each \transaction includes a sequence of zero or more \joinSplitDescriptions.
|
||
When this sequence is non-empty, the \transaction also includes encodings of a
|
||
$\JoinSplitSig$ public verification key and signature.
|
||
|
||
\introlist
|
||
A \joinSplitDescription consists of $(\vpubOld, \vpubNew, \rt, \nfOld{\allOld},
|
||
\cmNew{\allNew}, \EphemeralPublic, \RandomSeed, \h{\allOld}, \ProofJoinSplit,
|
||
\TransmitCiphertext{\allNew})$
|
||
|
||
where
|
||
\begin{itemize}
|
||
\item \changed{$\vpubOld \typecolon \range{0}{\MAXMONEY}$ is
|
||
the value that the \joinSplitTransfer removes from the \transparentValuePool};
|
||
\item $\vpubNew \typecolon \range{0}{\MAXMONEY}$ is
|
||
the value that the \joinSplitTransfer inserts into the \transparentValuePool;
|
||
\item $\rt \typecolon \MerkleHash$ is an \anchor, as defined in
|
||
\crossref{blockchain}, for the output \treestate of either
|
||
a previous \block, or a previous \joinSplitTransfer in this
|
||
\transaction.
|
||
\item $\nfOld{\allOld} \typecolon \typeexp{\PRFOutput}{\NOld}$ is
|
||
the sequence of \nullifiers for the input \notes;
|
||
\item $\cmNew{\allNew} \typecolon \typeexp{\NoteCommitSproutOutput}{\NNew}$ is
|
||
the sequence of \noteCommitments for the output \notes;
|
||
\item \changed{$\EphemeralPublic \typecolon \KASproutPublic$ is
|
||
a key agreement public key, used to derive the key for encryption
|
||
of the \notesCiphertext (\crossref{inband})};
|
||
\item \changed{$\RandomSeed \typecolon \RandomSeedType$ is
|
||
a seed that must be chosen independently at random for each
|
||
\joinSplitDescription};
|
||
\item $\h{\allOld} \typecolon \typeexp{\PRFOutput}{\NOld}$ is
|
||
a sequence of tags that bind $\hSig$ to each
|
||
$\AuthPrivate$ of the input \notes;
|
||
\item $\ProofJoinSplit \typecolon \JoinSplitProof$ is
|
||
the \zeroKnowledgeProof for the \joinSplitStatement;
|
||
\item $\TransmitCiphertext{\allNew} \typecolon \typeexp{\Ciphertext}{\NNew}$ is
|
||
a sequence of ciphertext components for the encrypted output \notes.
|
||
\end{itemize}
|
||
|
||
The $\ephemeralKey$ and $\encCiphertexts$ fields together form the \notesCiphertext.
|
||
|
||
\introlist
|
||
The value $\hSig$ is also computed from \changed{$\RandomSeed$, $\nfOld{\allOld}$, and} the
|
||
$\joinSplitPubKey$ of the containing \transaction:
|
||
|
||
\begin{formulae}
|
||
\item $\hSig := \hSigCRH(\changed{\RandomSeed, \nfOld{\allOld},\,} \joinSplitPubKey)$.
|
||
\end{formulae}
|
||
|
||
$\hSigCRH$ is instantiated in \crossref{hsigcrh}.
|
||
|
||
\begin{consensusrules}
|
||
\item Elements of a \joinSplitDescription{} \MUST have the types given
|
||
above (for example: $0 \leq \vpubOld \leq \MAXMONEY$ and $0 \leq \vpubNew \leq \MAXMONEY$).
|
||
\item Either $\vpubOld$ or $\vpubNew$ \MUST be zero.
|
||
\item The proof $\Proof{\JoinSplit}$ \MUST be valid given a \primaryInput formed
|
||
from the other fields and $\hSig$.
|
||
I.e.\ it must be the case that $\JoinSplitVerify{}((\rt, \nfOld{\allOld}, \cmNew{\allNew},
|
||
\vpubOld, \vpubNew, \hSig, \h{\allOld}), \Proof{\JoinSplit}) = 1$.
|
||
\end{consensusrules}
|
||
|
||
|
||
\sapling{
|
||
\nsubsection{\SpendDescriptions} \label{spenddesc}
|
||
|
||
A \spendTransfer, as specified in \crossref{spendsandoutputs}, is encoded in
|
||
\transactions as a \spendDescription.
|
||
|
||
Each \transaction includes a sequence of zero or more \spendDescriptions.
|
||
|
||
Unlike \joinSplitSignatures of which there is at most one per \transaction,
|
||
\emph{each} \spendDescription is authorized by a signature, called the
|
||
\spendAuthSignature.
|
||
|
||
\introlist
|
||
A \spendDescription consists of $(\cv, \rt, \nf, \ProofSpend, \spendAuthSig)$
|
||
|
||
where
|
||
\begin{itemize}
|
||
\item $\cv \typecolon \bitseq{\ellJ}$ is the \valueCommitment to the value of the input \note;
|
||
\item $\rt \typecolon \MerkleHashSapling$ is an \anchor, as defined in
|
||
\crossref{blockchain}, for the output \treestate of a previous \block.
|
||
\item $\nf \typecolon \bitseq{\ellJ}$ is the \nullifier for the input \note;
|
||
\item $\ProofSpend \typecolon \SpendProof$ is
|
||
the \zeroKnowledgeProof for the \spendStatement;
|
||
\item $\spendAuthSig \typecolon \SpendAuthSigSignature$ is a signature authorizing this spend.
|
||
\end{itemize}
|
||
|
||
\begin{consensusrules}
|
||
\item Elements of a \spendDescription{} \MUST have the types given above.
|
||
\item The proof $\Proof{\Spend}$ \MUST be valid given a \primaryInput formed
|
||
from the other fields except $\spendAuthSig$.
|
||
I.e.\ it must be the case that $\SpendVerify{}((\cv, \rt, \nf), \Proof{\Spend}) = 1$.
|
||
\item The \spendAuthSignature{} \MUST be a valid $\SpendAuthSig$ signature using
|
||
$\nf$ as the public key, over \todo{...}
|
||
\end{consensusrules}
|
||
|
||
|
||
\nsubsection{\OutputDescriptions} \label{outputdesc}
|
||
|
||
An \outputTransfer, as specified in \crossref{spendsandoutputs}, is encoded in
|
||
\transactions as an \outputDescription.
|
||
|
||
Each \transaction includes a sequence of zero or more \outputDescriptions.
|
||
There are no signatures associated with \outputDescriptions.
|
||
|
||
\introlist
|
||
An \outputDescription consists of $(\cv, \cm, \EphemeralPublic, \TransmitCiphertext{}, \ProofOutput)$
|
||
|
||
where
|
||
\begin{itemize}
|
||
\item $\cv \typecolon \bitseq{\ellJ}$ is the \valueCommitment to the value of the output \note;
|
||
\item $\cm \typecolon \bitseq{\ellJ}$ is the \noteCommitment for the output \note;
|
||
\item $\EphemeralPublic \typecolon \KASaplingPublic$ is
|
||
a key agreement public key, used to derive the key for encryption
|
||
of the \notesCiphertext (\crossref{inband});
|
||
\item $\TransmitCiphertext{} \typecolon \Ciphertext$ is
|
||
a ciphertext component for the encrypted output \note.
|
||
\item $\ProofOutput \typecolon \OutputProof$ is
|
||
the \zeroKnowledgeProof for the \outputStatement.
|
||
\end{itemize}
|
||
|
||
\begin{consensusrules}
|
||
\item Elements of an \outputDescription{} \MUST have the types given above.
|
||
\item The proof $\Proof{\Output}$ \MUST be valid given a \primaryInput formed
|
||
from the other fields except $\TransmitCiphertext{}$.
|
||
I.e.\ it must be the case that $\SpendVerify{}((\cv, \cm, \EphemeralPublic), \Proof{\Output}) = 1$.
|
||
\end{consensusrules}
|
||
}
|
||
|
||
|
||
\introlist
|
||
\nsubsection{Sending \Notes} \label{send}
|
||
|
||
\notsprout{\nsubsubsection{Sending \Notes{} (\Sprout)}} \label{sproutsend}
|
||
|
||
In order to send \shielded value, the sender constructs a \transaction
|
||
containing one or more \joinSplitDescriptions. This involves first generating
|
||
a new $\JoinSplitSig$ key pair:
|
||
|
||
\begin{formulae}
|
||
\item $(\joinSplitPrivKey, \joinSplitPubKey) \leftarrowR \JoinSplitSigGen()$.
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
For each \joinSplitDescription, the sender chooses $\RandomSeed$ uniformly at
|
||
random on $\bitseq{\RandomSeedLength}$, and selects
|
||
the input \notes. At this point there is sufficient information to compute $\hSig$,
|
||
as described in the previous section. \changed{The sender also chooses $\NoteAddressPreRand$
|
||
uniformly at random on $\bitseq{\NoteAddressPreRandLength}$.}
|
||
Then it creates each output \note with index $i \typecolon \setofNew$ as follows:
|
||
|
||
\begin{itemize}
|
||
\item Choose $\NoteCommitRandNew{i}$ uniformly at random on $\bitseq{\NoteCommitRandLength}$.
|
||
\changed{
|
||
\item Compute $\NoteAddressRandNew{i} = \PRFrho{\NoteAddressPreRand}(i, \hSig)$.
|
||
}
|
||
\item Encrypt the \note to the recipient \transmissionKey $\TransmitPublicNew{i}$,
|
||
as described in \crossref{inband}, giving the ciphertext component
|
||
$\TransmitCiphertext{i}$.
|
||
\end{itemize}
|
||
|
||
In order to minimize information leakage, the sender \SHOULD randomize the order
|
||
of the input \notes and of the output \notes. Other considerations relating to
|
||
information leakage from the structure of \transactions are beyond the
|
||
scope of this specification.
|
||
|
||
\introlist
|
||
After generating all of the \joinSplitDescriptions, the sender obtains the
|
||
$\dataToBeSigned$ (\crossref{nonmalleability}), and signs it with
|
||
the private \joinSplitSigningKey:
|
||
|
||
\begin{formulae}
|
||
\item $\joinSplitSig \leftarrowR \JoinSplitSigSign{\text{\small\joinSplitPrivKey}}(\dataToBeSigned)$
|
||
\end{formulae}
|
||
|
||
Then the encoded \transaction including $\joinSplitSig$ is submitted to the network.
|
||
|
||
|
||
\nsubsubsection{\DummyNotes\notsprout{ (\Sprout)}} \label{dummynotes}
|
||
|
||
The fields in a \joinSplitDescription allow for $\NOld$ input \notes, and
|
||
$\NNew$ output \notes. In practice, we may wish to encode a \joinSplitTransfer
|
||
with fewer input or output \notes. This is achieved using \dummyNotes.
|
||
|
||
\introlist
|
||
\changed{
|
||
A \dummy input \note, with index $i$ in the \joinSplitDescription, is constructed
|
||
as follows:
|
||
|
||
\begin{itemize}
|
||
\item Generate a new random \spendingKey $\AuthPrivateOld{i}$ and derive its
|
||
\payingKey $\AuthPublicOld{i}$.
|
||
\item Set $\vOld{i} := 0$.
|
||
\item Choose $\NoteAddressRandOld{i}$ uniformly at random on $\PRFOutput$.
|
||
\item Choose $\NoteCommitRandOld{i}$ uniformly at random on $\bitseq{\NoteCommitRandLength}$.
|
||
\item Compute $\nfOld{i} := \PRFnf{\AuthPrivateOld{i}}(\NoteAddressRandOld{i})$.
|
||
\item Construct a \dummy \merklePath $\treepath{i}$ for use in the
|
||
\auxiliaryInput to the \joinSplitStatement (this will not be checked).
|
||
\item When generating the \joinSplitProof\!\!, set $\EnforceMerklePath{i}$ to $0$.
|
||
\end{itemize}
|
||
}
|
||
|
||
A \dummy output \note is constructed as normal but with zero value, and
|
||
sent to a random \paymentAddress.
|
||
|
||
\sapling{
|
||
\nsubsubsection{Sending \Notes{} (\Sapling)} \label{saplingsend}
|
||
|
||
In order to send \shielded value, the sender constructs a \transaction
|
||
containing one or more \shieldedOutputs.
|
||
|
||
Let $\OutputIndexType$ be the type $\range{0}{2^{32}-1}$.
|
||
|
||
\introlist
|
||
For each \outputDescription with index $\OutputIndex \typecolon \OutputIndexType$, the sender
|
||
selects a value $\ValueNew{\OutputIndex}$ and a destination \Sapling \paymentAddress
|
||
$(\Diversifier, \DiversifiedTransmitPublic)$, and then performs the following steps:
|
||
|
||
\begin{enumerate}
|
||
\item Check that $\DiversifiedTransmitPublic$ is a valid compressed representation of
|
||
an Edwards point on the $\JubjubCurve$ curve and this point is not of small order
|
||
(i.e. $\abstJOf{\DiversifiedTransmitPublic} \neq \bot$ and
|
||
$\scalarmult{8}{\abstJOf{\DiversifiedTransmitPublic}} \neq \ZeroJ$).
|
||
|
||
\item Calculate $\DiversifiedTransmitBase = \GroupJHash{U}(\ascii{Zcash\_gd}, \Diversifier)$
|
||
and check that $\DiversifiedTransmitBase \neq \bot$.
|
||
|
||
\item Choose $\EphemeralPrivate$ uniformly at random on $\range{0}{\ParamJ{r} - 1}$.
|
||
\todo{any advantage in making this $\range{0}{\JubjubScalarThreshold - 1}$?}
|
||
|
||
\item Choose independent random commitment trapdoors:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\ValueCommitRandNew{\OutputIndex}$ &$\typecolon \ValueCommitTrapdoor$ \\
|
||
$\NoteCommitRandNew{\OutputIndex}$ &$\typecolon \NoteCommitSaplingTrapdoor$
|
||
\end{tabular}
|
||
|
||
\item Calculate
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\cvNew{\OutputIndex}$ &$:= \ValueCommit{\ValueCommitRandNew{\OutputIndex}}(\ValueNew{\OutputIndex})$ \\
|
||
$\cmNew{\OutputIndex}$ &$:=
|
||
\NoteCommitSapling{\NoteCommitRandNew{\OutputIndex}}(\reprJOf{\DiversifiedTransmitBase),
|
||
\DiversifiedTransmitPublic,
|
||
\ValueNew{\OutputIndex}}$ \\
|
||
$\EphemeralPublic$ &$:= \KASaplingDerivePublic(\EphemeralPrivate, \DiversifiedTransmitBase)$.
|
||
\end{tabular}
|
||
|
||
\item Calculate $\DHSecret{} \typecolon \AffineEdwardsJubjub$ using an
|
||
Edwards scalar multiplication with cofactor 8:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$\DHSecret{}$ &$:= \KASaplingAgree(\EphemeralPrivate, \DiversifiedTransmitPublic)$
|
||
\end{tabular}
|
||
|
||
\item Let $\Key := \KDFSapling(\OutputIndex, \DHSecret{}, \EphemeralPublic)$.
|
||
|
||
\item Let $\Ptext$ be the raw encoding of the \notePlaintext
|
||
$(\Diversifier, \ValueNew{\OutputIndex}, \NoteCommitRandNew{\OutputIndex}, \Memo)$.
|
||
|
||
\item Encrypt $\Ptext$ using the IETF version of $\SymSpecific$, with empty associated data,
|
||
all zero $96$-bit nonce, and $256$-bit key $\Key$, giving $\Ctext$.
|
||
|
||
\item Generate a proof $\ProofOutput$ for the \outputCircuit described below.
|
||
|
||
\item Return $(\cvNew{\OutputIndex}, \cmNew{\OutputIndex}, \EphemeralPublic, \Ctext, \ProofOutput)$.
|
||
|
||
% \item Encrypt the \note to the recipient \transmissionKey $\TransmitPublicNew{i}$,
|
||
% as described in \crossref{inbandsapling}, giving the ciphertext component
|
||
% $\TransmitCiphertext{i}$.
|
||
\end{enumerate}
|
||
|
||
In order to minimize information leakage, the sender \SHOULD randomize the order
|
||
of the input \notes and of the output \notes. Other considerations relating to
|
||
information leakage from the structure of \transactions are beyond the
|
||
scope of this specification.
|
||
|
||
The encoded \transaction is submitted to the network.
|
||
}
|
||
|
||
\nsubsection{Merkle path validity} \label{merklepath}
|
||
|
||
\sprout{
|
||
The depth of the \noteCommitmentTree is $\MerkleDepth$ (defined in \crossref{constants}).
|
||
}
|
||
\notsprout{
|
||
Let $\MerkleDepth$ be $\MerkleDepthSprout$ for the \Sprout \noteCommitmentTree\sapling{,
|
||
or $\MerkleDepthSapling$ for the \Sapling \noteCommitmentTree}. These constants are
|
||
defined in \crossref{constants}.
|
||
|
||
Similarly, let $\MerkleCRH$ be $\MerkleCRHSprout$ for \Sprout\sapling{, or $\MerkleDepthSapling$
|
||
for \Sapling}.
|
||
|
||
The following discussion applies independently to the \Sprout and \Sapling \noteCommitmentTrees.
|
||
}
|
||
|
||
Each \merkleNode in the \incrementalMerkleTree is associated with a \merkleHash,
|
||
which is a bit sequence. The \merkleLayer numbered $h$, counting from
|
||
\merkleLayer $0$ at the \merkleRoot, has $2^h$ \merkleNodes with \merkleIndices
|
||
$0$ to $2^h-1$ inclusive.
|
||
|
||
Let $\MerkleNode{h}{i}$ be the \merkleHash associated with the \merkleNode at
|
||
\merkleIndex $i$ in \merkleLayer $h$.
|
||
|
||
The \merkleNodes at \merkleLayer $\MerkleDepth$ are called \merkleLeafNodes.
|
||
When a \noteCommitment is added to the tree, it occupies the \merkleLeafNode
|
||
\merkleHash $\MerkleNode{\MerkleDepth}{i}$ for the next available $i$.
|
||
|
||
As-yet unused \merkleLeafNodes are associated with a distinguished \merkleHash
|
||
$\UncommittedSprout$ \sapling{ or $\UncommittedSapling$}.
|
||
It is assumed to be infeasible to find a preimage \note $\NoteTuple{}$ such that
|
||
$\NoteCommitmentSprout(\NoteTuple{}) = \UncommittedSprout$.
|
||
\sapling{(No similar assumption is needed for \Sapling because we use a representation
|
||
for $\UncommittedSapling$ that cannot occur as an output of $\NoteCommitmentSapling$,
|
||
and explicitly check when a \note is spent that this representation is not given as
|
||
its purported \noteCommitment.)}
|
||
|
||
\introlist
|
||
The \merkleNodes at \merkleLayers $0$ to $\MerkleDepth-1$ inclusive are called
|
||
\merkleInternalNodes, and are associated with $\MerkleCRH$ outputs.
|
||
\MerkleInternalNodes are computed from their children in the next \merkleLayer
|
||
as follows: for $0 \leq h < \MerkleDepth$ and $0 \leq i < 2^h$,
|
||
|
||
\begin{formulae}
|
||
\item $\MerkleNode{h}{i} := \MerkleCRH(\MerkleNode{h+1}{2i}, \MerkleNode{h+1}{2i+1})$.
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
A \merklePath from \merkleLeafNode $\MerkleNode{\MerkleDepth}{i}$ in the
|
||
\incrementalMerkleTree is the sequence
|
||
|
||
\begin{formulae}
|
||
\item $\listcomp{\MerkleNode{h}{\MerkleSibling(h, i)} \for
|
||
h \from \MerkleDepth \downto 1}$,
|
||
\end{formulae}
|
||
|
||
where
|
||
\begin{formulae}
|
||
\item $\MerkleSibling(h, i) := \floor{\frac{i}{2^{\MerkleDepth-h}}} \xor 1$
|
||
\end{formulae}
|
||
|
||
Given such a \merklePath, it is possible to verify that \merkleLeafNode
|
||
$\MerkleNode{\MerkleDepth}{i}$ is in a tree with a given \merkleRoot $\rt = \MerkleNode{0}{0}$.
|
||
|
||
\nsubsection{Non-malleability} \label{nonmalleability}
|
||
|
||
\Bitcoin defines several \sighashTypes that cover various parts of a transaction.
|
||
\changed{In \Zcash, all of these \sighashTypes are extended to cover the \Zcash-specific
|
||
fields $\nJoinSplit$, $\vJoinSplit$, and (if present) $\joinSplitPubKey$, described in
|
||
\crossref{txnencoding}. They \emph{do not} cover the field $\joinSplitSig$.
|
||
|
||
\consensusrule{
|
||
If $\nJoinSplit > 0$, the \transaction{} \MUSTNOT use \sighashTypes other than
|
||
$\SIGHASHALL$.
|
||
}
|
||
}
|
||
|
||
Let $\dataToBeSigned$ be the hash of the \transaction{} \changed{using the $\SIGHASHALL$
|
||
\sighashType}. \changed{This \emph{excludes} all of the $\scriptSig$ fields in
|
||
the non-\Zcash-specific parts of the \transaction.}
|
||
|
||
In order to ensure that a \joinSplitDescription is cryptographically bound to the
|
||
\transparent inputs and outputs corresponding to $\vpubNew$ and $\vpubOld$, and
|
||
to the other \joinSplitDescriptions in the same \transaction, an ephemeral $\JoinSplitSig$
|
||
key pair is generated for each \transaction, and the $\dataToBeSigned$ is
|
||
signed with the private signing key of this key pair. The corresponding public
|
||
verification key is included in the \transaction encoding as $\joinSplitPubKey$.
|
||
|
||
$\JoinSplitSig$ is instantiated in \crossref{concretejssig}.
|
||
|
||
\changed{
|
||
If $\nJoinSplit$ is zero, the $\joinSplitPubKey$ and $\joinSplitSig$ fields are
|
||
omitted. Otherwise, a \transaction has a correct \joinSplitSignature if and only if
|
||
$\JoinSplitSigVerify{\text{\small\joinSplitPubKey}}(\dataToBeSigned, \joinSplitSig) = 1$.
|
||
% FIXME: distinguish pubkey and signature from their encodings.
|
||
}
|
||
|
||
Let $\hSig$ be computed as specified in \crossref{joinsplitdesc}, and let
|
||
$\PRFpk{}$ be as defined in \crossref{abstractprfs}.
|
||
|
||
For each $i \in \setofOld$, the creator of a \joinSplitDescription calculates
|
||
$\h{i} = \PRFpk{\AuthPrivateOld{i}}(i, \hSig)$.
|
||
|
||
The correctness of $\h{\allOld}$ is enforced by the \joinSplitStatement
|
||
given in \crossref{sproutnonmalleablejs}. This ensures that a holder of
|
||
all of the $\AuthPrivateOld{\allOld}$ for every \joinSplitDescription in the
|
||
\transaction has authorized the use of the private signing key corresponding
|
||
to $\joinSplitPubKey$ to sign this \transaction.
|
||
|
||
\saplingonward{
|
||
\todo{Specify the \spendAuthSignature.}
|
||
}
|
||
|
||
|
||
\nsubsection{Balance} \label{balance} \label{saplingbalance}
|
||
|
||
A \joinSplitTransfer can be seen, from the perspective of the \transaction, as
|
||
an input \changed{and an output simultaneously}.
|
||
\changed{$\vpubOld$ takes value from the \transparentValuePool and}
|
||
$\vpubNew$ adds value to the \transparentValuePool. As a result, \changed{$\vpubOld$ is
|
||
treated like an \emph{output} value, whereas} $\vpubNew$ is treated like an
|
||
\emph{input} value.
|
||
|
||
\changed{
|
||
Unlike original \Zerocash \cite{BCG+2014}, \Zcash does not have
|
||
a distinction between Mint and Pour operations. The addition of $\vpubOld$ to a
|
||
\joinSplitDescription subsumes the functionality of both Mint and Pour. Also,
|
||
a difference in the number of real input \notes does not by itself cause two
|
||
\joinSplitDescriptions to be distinguishable.
|
||
|
||
As stated in \crossref{joinsplitdesc}, either $\vpubOld$ or $\vpubNew$ \MUST be zero.
|
||
No generality is lost because, if a \transaction in which both $\vpubOld$ and
|
||
$\vpubNew$ were nonzero were allowed, it could be replaced by an equivalent one
|
||
in which $\minimum(\vpubOld, \vpubNew)$ is subtracted from both of these values.
|
||
This restriction helps to avoid unnecessary distinctions between \transactions
|
||
according to client implementation.
|
||
}
|
||
|
||
\sapling{\todo{Add details of balance checking for \Sapling \transactions.}}
|
||
|
||
|
||
\nsubsection{\NoteCommitments{} and \Nullifiers} \label{commitmentsandnullifiers}
|
||
|
||
A \transaction that contains one or more
|
||
\joinSplitDescriptions\sapling{ or \spendDescriptions}, when entered
|
||
into the \blockchain, appends to the \noteCommitmentTree with all constituent
|
||
\noteCommitments. All of the constituent \nullifiers are also entered into the
|
||
\nullifierSet of the associated \treestate. A \transaction is not valid if it
|
||
attempts to add a \nullifier to the \nullifierSet that already exists in the set.
|
||
|
||
\sprout{Each}\notsprout{In \Sprout, each} \note has a $\NoteAddressRand$ component.
|
||
|
||
\sapling{
|
||
In \Sapling, each \positionedNote has an associated $\NoteAddressRand$ value which
|
||
is computed from its \noteCommitment $\cm$ and \notePosition $\NotePosition$
|
||
as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\NoteAddressRand := \MixingPedersenHash(\ascii{Zcashrho}, \cm, \NotePosition)$.
|
||
\end{formulae}
|
||
|
||
$\MixingPedersenHash$ is defined in \crossref{concretemixinghash}.
|
||
}
|
||
|
||
Let $\PRFnf{}{}$\sapling{ and $\PRFnr{}{}$} be as instantiated in \crossref{concreteprfs}.
|
||
|
||
\sprout{The \nullifier of a \note}\notsprout{For a \Sprout \note, the \nullifier}
|
||
is derived as $\PRFnf{\AuthPrivate}(\NoteAddressRand)$.
|
||
|
||
% TODO: \scalarmults should only be in the concrete section
|
||
\sapling{
|
||
For a \Sapling \note, the \nullifier is derived as
|
||
$\scalarmult{\PRFnr{\AuthProvePublic}(\NoteAddressRand)}{\scalarmult{8}{\AuthSignPublic}}$.
|
||
}
|
||
|
||
\introsection
|
||
|
||
\nsubsection{\ZkSNARKStatements} \label{snarkstatements}
|
||
|
||
\nsubsubsection{\JoinSplitStatement{} \notsprout{(\Sprout)}} \label{joinsplitstatement}
|
||
|
||
A valid instance of $\ProofJoinSplit$ assures that given a \term{primary input}:
|
||
|
||
\begin{formulae}
|
||
\item $(\rt \typecolon \MerkleHashSprout,\\
|
||
\hparen\nfOld{\allOld} \typecolon \typeexp{\PRFOutput}{\NOld},\vspace{0.4ex}\\
|
||
\hparen\cmNew{\allNew} \typecolon \typeexp{\NoteCommitSproutOutput}{\NNew},\vspace{0.8ex}\\
|
||
\hparen\changed{\vpubOld \typecolon \range{0}{2^{64}-1},}\vspace{0.4ex}\\
|
||
\hparen\vpubNew \typecolon \range{0}{2^{64}-1},\\
|
||
\hparen\hSig \typecolon \hSigType,\\
|
||
\hparen\h{\allOld} \typecolon \typeexp{\PRFOutput}{\NOld})$,
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
the prover knows an \term{auxiliary input}:
|
||
|
||
\begin{formulae}
|
||
\item $(\treepath{\allOld} \typecolon \typeexp{\typeexp{\MerkleHashSprout}{\MerkleDepthSprout}
|
||
\times \NotePositionTypeSprout}{\NOld},\\
|
||
\hparen\nOld{\allOld} \typecolon \typeexp{\NoteTypeSprout}{\NOld},\\
|
||
\hparen\AuthPrivateOld{\allOld} \typecolon \typeexp{\bitseq{\AuthPrivateLength}}{\NOld},\\
|
||
\hparen\nNew{\allNew} \typecolon \typeexp{\NoteTypeSprout}{\NNew}\changed{,}\vspace{0.8ex}\\
|
||
\hparen\changed{\NoteAddressPreRand \typecolon \bitseq{\NoteAddressPreRandLength},}\\
|
||
\hparen\changed{\EnforceMerklePath{\allOld} \typecolon \bitseq{\NOld}})$,
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
where:
|
||
|
||
\begin{formulae}
|
||
\item for each $i \in \setofOld$: $\nOld{i} = (\AuthPublicOld{i},
|
||
\vOld{i}, \NoteAddressRandOld{i}, \NoteCommitRandOld{i})$;
|
||
\item for each $i \in \setofNew$: $\nNew{i} = (\AuthPublicNew{i},
|
||
\vNew{i}, \NoteAddressRandNew{i}, \NoteCommitRandNew{i})$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
such that the following conditions hold:
|
||
|
||
\subparagraph{Merkle path validity} \label{sproutmerklepathvalidity}
|
||
|
||
for each $i \in \setofOld$ \changed{$\mid$ $\EnforceMerklePath{i} = 1$}:
|
||
$\treepath{i}$ must be a valid \merklePath of depth given by $\MerkleDepthSprout$, as defined in
|
||
\crossref{merklepath}, from $\NoteCommitmentSprout(\nOld{i})$ to \noteCommitmentTree root $\rt$.
|
||
|
||
\textbf{Note:} Merkle path validity covers both conditions 1. (a) and 1. (d) of the NP statement
|
||
given in \cite[section 4.2]{BCG+2014}.
|
||
|
||
\changed{
|
||
\subparagraph{Merkle path enforcement} \label{sproutmerklepathenforcement}
|
||
|
||
for each $i \in \setofOld$, if $\vOld{i} \neq 0$ then $\EnforceMerklePath{i} = 1$.
|
||
}
|
||
|
||
\subparagraph{Balance} \label{sproutbalance}
|
||
|
||
$\changed{\vpubOld\; +} \vsum{i=1}{\NOld} \vOld{i} = \vpubNew + \vsum{i=1}{\NNew} \vNew{i} \in \range{0}{2^{64}-1}$.
|
||
|
||
\subparagraph{\Nullifier{} integrity} \label{sproutnullifierintegrity}
|
||
|
||
for each $i \in \setofOld$:
|
||
$\nfOld{i} = \PRFnf{\AuthPrivateOld{i}}(\NoteAddressRandOld{i})$.
|
||
|
||
\subparagraph{Spend authority} \label{sproutspendauthority}
|
||
|
||
for each $i \in \setofOld$:
|
||
$\AuthPublicOld{i} = \changed{\PRFaddr{\AuthPrivateOld{i}}(0)}$.
|
||
|
||
\subparagraph{Non-malleability} \label{sproutnonmalleablejs}
|
||
|
||
for each $i \in \setofOld$:
|
||
$\h{i} = \PRFpk{\AuthPrivateOld{i}}(i, \hSig)$.
|
||
|
||
\changed{
|
||
\subparagraph{Uniqueness of $\NoteAddressRandNew{i}$} \label{sproutuniquerho}
|
||
|
||
for each $i \in \setofNew$:
|
||
$\NoteAddressRandNew{i} = \PRFrho{\NoteAddressPreRand}(i, \hSig)$.
|
||
}
|
||
|
||
\subparagraph{Note commitment integrity} \label{sproutcommitmentintegrity}
|
||
|
||
for each $i \in \setofNew$: $\cmNew{i}$ = $\NoteCommitSprout(\nNew{i})$.
|
||
|
||
\vspace{2.5ex}
|
||
For details of the form and encoding of proofs, see \crossref{phgr}.
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsubsection{\SpendStatement{} (\Sapling)} \label{spendstatement}
|
||
|
||
A valid instance of $\ProofSpend$ assures that given a \term{primary input}:
|
||
|
||
\begin{formulae}
|
||
\item $(\rt \typecolon \MerkleHashSapling,\\
|
||
\hparen\cvOld{} \typecolon \ValueCommitOutput,\\
|
||
\hparen\nfOld{} \typecolon \GroupJ)$,
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
the prover knows an \term{auxiliary input}:
|
||
|
||
\begin{formulae}
|
||
\item $(\treepath{} \typecolon \typeexp{\MerkleHash}{\MerkleDepthSapling} \times \NotePositionTypeSapling,\\
|
||
\hparen\nOld{} \typecolon \NoteTypeSapling,\\
|
||
\hparen\cmOld{} \typecolon \MerkleHashSapling,\\
|
||
\hparen\ValueCommitRandOld \typecolon \ValueCommitTrapdoor,\\
|
||
\hparen\DiversifiedTransmitBase \typecolon \KASaplingPublic,\\
|
||
\hparen\DiversifiedTransmitPublic \typecolon \KASaplingPublic,\\
|
||
\hparen\NoteCommitRandOld \typecolon \NoteCommitSaplingTrapdoor,\\
|
||
\hparen\AuthSignPublic \typecolon \KASaplingPublic,\\
|
||
\hparen\AuthProvePrivate \typecolon \KASaplingPrivate)$
|
||
\end{formulae}
|
||
|
||
where $\nOld{} = (\Diversifier, \DiversifiedTransmitPublic, \vOld{}, \NoteCommitRandOld{})$
|
||
|
||
\introlist
|
||
such that the following conditions hold:
|
||
|
||
\subparagraph{Note commitment integrity} \label{saplingnotecommitmentintegrity}
|
||
|
||
$\cmOld{} \neq \UncommittedSapling$, and $\pack(\cmOld{}) = \NoteCommitmentSapling(\nOld{})$.
|
||
|
||
\subparagraph{Merkle path validity} \label{saplingmerklepathvalidity}
|
||
|
||
$\treepath{}$ must be a valid \merklePath of depth $\MerkleDepthSapling$, as defined in
|
||
\crossref{merklepath}, from $\cmOld{}$ to \noteCommitmentTree root $\rt$.
|
||
|
||
\subparagraph{Value commitment integrity} \label{saplingvaluecommitmentintegrity}
|
||
|
||
$\cvOld{} = \ValueCommit{\ValueCommitRandOld{}}(\vOld{})$.
|
||
|
||
\subparagraph{Point validity checks} \label{saplingpointvalidity}
|
||
|
||
$\AuthSignPublic, \DiversifiedTransmitBase \in \GroupJ$.
|
||
|
||
$\scalarmult{8}{\AuthSignPublic} \neq \ZeroJ$.
|
||
|
||
$\scalarmult{8}{\DiversifiedTransmitBase} \neq \ZeroJ$.
|
||
|
||
\subparagraph{\Nullifier{} integrity} \label{saplingnullifierintegrity}
|
||
|
||
|
||
|
||
$\nfOld{} = \scalarmult{\PRFnr{\AuthProvePublic}(\NoteAddressRand)}{\scalarmult{8}{\AuthSignPublic}}$.
|
||
|
||
where
|
||
|
||
\begin{formulae}
|
||
\item $\AuthProvePublic = \scalarmult{\AuthProvePrivate}{\AuthProveBase}$
|
||
\item $\NoteAddressRand = \MixingPedersenHash(\cmOld{}, \NotePosition)$
|
||
\end{formulae}
|
||
|
||
\subparagraph{Spend authority} \label{saplingspendauthority}
|
||
|
||
for each $i \in \setofOld$:
|
||
$\AuthPublicOld{i} = \PRFaddr{\AuthPrivateOld{i}}(0)$.
|
||
|
||
\vspace{2.5ex}
|
||
For details of the form and encoding of \spendStatement proofs, see \crossref{groth}.
|
||
|
||
\introsection
|
||
\nsubsubsection{\OutputStatement{} (\Sapling)} \label{outputstatement}
|
||
|
||
\todo{}
|
||
|
||
For details of the form and encoding of \outputStatement proofs, see \crossref{groth}.
|
||
}
|
||
|
||
|
||
\nsubsection{In-band secret distribution} \label{inband}
|
||
|
||
The secrets that need to be transmitted to a recipient of funds in order for
|
||
them to later spend, are $\Value$, $\NoteAddressRand$, $\NoteCommitRand$\sapling{,
|
||
and in the case of \Sapling $\Diversifier$ and $\DiversifiedTransmitPublic$}.
|
||
\changed{A \memo (\crossref{noteptconcept}) is also transmitted.}
|
||
|
||
In order to the transmit these secrets securely to a recipient
|
||
\emph{without} requiring an out-of-band communication channel, the
|
||
\transmissionKey $\TransmitPublic$\sapling{ or $\DiversifiedTransmitPublic$}
|
||
is used to encrypt them. The recipient's possession of the associated
|
||
\incomingViewingKey $\InViewingKey$ is used to reconstruct the original
|
||
\note\changed{ and \memo}.
|
||
|
||
All of the resulting ciphertexts are combined to form a \notesCiphertext.
|
||
|
||
Let $\Sym$ be the \encryptionScheme instantiated in \crossref{concretesym}.
|
||
|
||
\introlist
|
||
For both encryption and decryption,
|
||
|
||
\sprout{
|
||
\begin{itemize}
|
||
\item Let $\KDFSprout$ be the \keyDerivationFunction instantiated in \crossref{concretesproutkdf}.
|
||
\item Let $\KASprout$ be the \keyAgreementScheme instantiated in \crossref{concretesproutkeyagreement}.
|
||
\item Let $\hSig$ be the value computed for this \joinSplitDescription in \crossref{joinsplitdesc}.
|
||
\end{itemize}
|
||
}
|
||
\notsprout{
|
||
\begin{itemize}
|
||
\item Let $\KDFSprout$\sapling{ and $\KDFSapling$} be the \keyDerivationFunctions instantiated in
|
||
\crossref{concretesproutkdf}.
|
||
\item Let $\KASprout$\sapling{ and $\KASapling$} be the \keyAgreementSchemes instantiated in
|
||
\crossref{concretekaandkdf}.
|
||
\item \sproutspecific{Let $\hSig$ be the value computed for this \joinSplitDescription in
|
||
\crossref{joinsplitdesc}.}
|
||
\end{itemize}
|
||
}
|
||
|
||
\nsubsubsection{Encryption}
|
||
|
||
Let $\TransmitPublicNew{\allNew}$ be the \transmissionKeys
|
||
for the intended recipient addresses of each new \note.
|
||
|
||
Let $\NotePlaintext{\allNew}$ be the \notePlaintexts as defined in \crossref{notept}.
|
||
|
||
\introlist
|
||
Then to encrypt:
|
||
|
||
\begin{itemize}
|
||
\changed{
|
||
\item Generate a new $\KASprout$ (public, private) key pair
|
||
$(\EphemeralPublic, \EphemeralPrivate)$.
|
||
\item For $i \in \setofNew$,
|
||
\begin{itemize}
|
||
\item Let $\TransmitPlaintext{i}$ be the raw encoding of $\NotePlaintext{i}$.
|
||
\item Let $\DHSecret{i} := \KASproutAgree(\EphemeralPrivate,
|
||
\TransmitPublicNew{i})$.
|
||
\item Let $\TransmitKey{i} := \KDFSprout(i, \hSig, \DHSecret{i}, \EphemeralPublic,
|
||
\TransmitPublicNew{i})$.
|
||
\item Let $\TransmitCiphertext{i} :=
|
||
\SymEncrypt{\TransmitKey{i}}(\TransmitPlaintext{i})$.
|
||
\end{itemize}
|
||
}
|
||
\end{itemize}
|
||
|
||
The resulting \notesCiphertext is $\changed{(\EphemeralPublic,
|
||
\TransmitCiphertext{\allNew})}$.
|
||
|
||
\pnote{
|
||
It is technically possible to replace $\TransmitCiphertext{i}$ for a given \note
|
||
with a random (and undecryptable) dummy ciphertext, relying instead on out-of-band
|
||
transmission of the \note to the recipient. In this case the ephemeral key \MUST
|
||
still be generated as a random public key (rather than a random bit sequence) to ensure
|
||
indistinguishability from other \joinSplitDescriptions. This mode of operation raises
|
||
further security considerations, for example of how to validate a \note received
|
||
out-of-band, which are not addressed in this document.
|
||
}
|
||
|
||
\nsubsubsection{Decryption by a Recipient}
|
||
|
||
Let $\InViewingKey = (\AuthPublic, \TransmitPrivate)$ be the recipient's \incomingViewingKey,
|
||
and let $\TransmitPublic$ be the corresponding \transmissionKey derived from
|
||
$\TransmitPrivate$ as specified in \crossref{keycomponents}.
|
||
|
||
Let $\cmNew{\allNew}$ be the \noteCommitments of each output coin.
|
||
|
||
\introlist
|
||
Then for each $i \in \setofNew$, the recipient will attempt to decrypt that ciphertext
|
||
component as follows:
|
||
|
||
\changed{
|
||
\begin{formulae}
|
||
\item let $\DHSecret{i} = \KASproutAgree(\TransmitPrivate, \EphemeralPublic)$
|
||
\item let $\TransmitKey{i} = \KDFSprout(i, \hSig, \DHSecret{i}, \EphemeralPublic,
|
||
\TransmitPublic)$
|
||
\item return $\DecryptNote(\TransmitKey{i}, \TransmitCiphertext{i}, \cmNew{i},
|
||
\AuthPublic).$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
$\DecryptNote(\TransmitKey{i}, \TransmitCiphertext{i}, \cmNew{i}, \AuthPublic)$
|
||
is defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item let $\TransmitPlaintext{i} =
|
||
\SymDecrypt{\TransmitKey{i}}(\TransmitCiphertext{i})$
|
||
\item if $\TransmitPlaintext{i} = \bot$, return $\bot$
|
||
\item extract $\NotePlaintext{i} = (\ValueNew{i},
|
||
\NoteAddressRandNew{i}, \NoteCommitRandNew{i}, \Memo_i)$ from $\TransmitPlaintext{i}$
|
||
\item if $\NoteCommitSprout((\AuthPublic, \ValueNew{i}, \NoteAddressRandNew{i},
|
||
\NoteCommitRandNew{i})) \neq \cmNew{i}$, return $\bot$, else return $\NotePlaintext{i}$.
|
||
\end{formulae}
|
||
}
|
||
|
||
To test whether a \note is unspent in a particular \blockchain also requires
|
||
the \spendingKey $\AuthPrivate$; the coin is unspent if and only if
|
||
$\nf = \PRFnf{\AuthPrivate}(\NoteAddressRand)$ is not in the \nullifierSet
|
||
for that \blockchain.
|
||
|
||
\begin{pnotes}
|
||
\item The decryption algorithm corresponds to step 3 (b) i. and ii.
|
||
(first bullet point) of the $\Receive$ algorithm shown in \cite[Figure 2]{BCG+2014}.
|
||
\item A \note can change from being unspent to spent as a node's view of the best
|
||
\blockchain is extended by new \transactions. Also, \blockchain reorganizations
|
||
can cause a node to switch to a different best \blockchain that does not
|
||
contain the \transaction in which a \note was output.
|
||
\end{pnotes}
|
||
|
||
See \crossref{inbandrationale} for further discussion of the security and
|
||
engineering rationale behind this encryption scheme.
|
||
|
||
|
||
\nsection{Concrete Protocol}
|
||
|
||
\nsubsection{Caution}
|
||
|
||
\todo{Explain the kind of things that can go wrong with linkage between
|
||
abstract and concrete protocol. E.g. \crossref{internalh}}
|
||
|
||
\nsubsection{Integers, Bit Sequences, and Endianness} \label{boxnotation} \label{endian}
|
||
|
||
All integers in \emph{\Zcash-specific} encodings are unsigned, have a fixed
|
||
bit length, and are encoded in little-endian byte order \emph{unless otherwise
|
||
specified}.
|
||
|
||
The following functions convert between sequences of bits, sequences of bytes,
|
||
and integers:
|
||
|
||
\begin{itemize}
|
||
\item $\ItoLEBSP{} \typecolon (\ell \typecolon \Nat) \times \range{0}{2^\ell\!-\!1} \rightarrow \bitseq{\ell}$,
|
||
such that $\ItoLEBSP{\ell}(x)$ is the sequence of $\ell$ bits representing $x$ in
|
||
little-endian order;
|
||
\item $\ItoBEBSP{} \typecolon (\ell \typecolon \Nat) \times \range{0}{2^\ell\!-\!1} \rightarrow \bitseq{\ell}$
|
||
such that $\ItoBEBSP{u}(\ell)$ is the sequence of $\ell$ bits representing $x$ in
|
||
big-endian order.
|
||
\item $\ItoLEOSPvar \typecolon \Nat \rightarrow \byteseqs$,
|
||
such that $\ItoLEOSPvar(i)$ is the shortest little-endian encoding of $i$
|
||
as a byte sequence, i.e. so that the encoding does not end in a zero
|
||
byte. ($\ItoLEOSPvar(0) = []$.)
|
||
\item $\LEOStoIP{} \typecolon (k \typecolon \Nat) \times \byteseq{k} \rightarrow \range{0}{256^k\!-\!1}$
|
||
such that $\LEOStoIP{k}(S)$ is the integer represented in little-endian order by the
|
||
byte sequence $S$ of length $k$.
|
||
\item $\LEBStoOSP{} \typecolon (\ell \typecolon \Nat) \times \bitseq{\ell} \rightarrow \byteseq{\ceiling{\ell/8}}$
|
||
defined as follows: pad the input on the right with $8 \mult \ceiling{\ell/8} - \ell$ zero bits
|
||
so that its length is a multiple of 8 bits. Then convert each group of 8 bits to a byte
|
||
value with the \emph{least} significant bit first, and concatenate the resulting bytes
|
||
in the same order as the groups.
|
||
\end{itemize}
|
||
|
||
In bit layout diagrams, each box of the diagram represents a sequence of bits.
|
||
Diagrams are read from left-to-right, with lines read from top-to-bottom;
|
||
the breaking of boxes across lines has no significance.
|
||
The bit length $\ell$ is given explicitly in each box, except for the case of a
|
||
single bit, or for the notation $\zeros{\ell}$ representing the sequence of $\ell$
|
||
zero bits.
|
||
|
||
The entire diagram represents the sequence of \emph{bytes} formed by first
|
||
concatenating these bit sequences, and then treating each subsequence of 8 bits
|
||
as a byte with the bits ordered from \emph{most significant} to
|
||
\emph{least significant}. Thus the \emph{most significant} bit in each byte
|
||
is toward the left of a diagram. Where bit fields are used, the text will
|
||
clarify their position in each case.
|
||
|
||
\introlist
|
||
\nsubsection{Constants} \label{constants}
|
||
|
||
Define:
|
||
|
||
\begin{formulae}
|
||
\item $\MerkleDepthSprout \typecolon \Nat := \changed{29}$
|
||
\sapling{
|
||
\item $\MerkleDepthSapling \typecolon \Nat := 29$
|
||
}
|
||
\item $\NOld \typecolon \Nat := 2$
|
||
\item $\NNew \typecolon \Nat := 2$
|
||
\item $\MerkleHashLengthSprout \typecolon \Nat := 256$
|
||
\sapling{
|
||
\item $\MerkleHashLengthSapling \typecolon \Nat := 255$
|
||
}
|
||
\item $\hSigLength \typecolon \Nat := 256$
|
||
\item $\PRFOutputLength \typecolon \Nat := 256$
|
||
\item $\PRGOutputLength \typecolon \Nat := 512$
|
||
\item $\NoteCommitRandLength \typecolon \Nat := \changed{256}$
|
||
\item $\changed{\RandomSeedLength \typecolon \Nat := 256}$
|
||
\item $\AuthPrivateLength \typecolon \Nat := \changed{252}$
|
||
\sapling{
|
||
\item $\AuthPrivateSeedLength \typecolon \Nat := 256$
|
||
\item $\DiversifierLength \typecolon \Nat := 88$
|
||
}
|
||
\item $\changed{\NoteAddressPreRandLength \typecolon \Nat := 252}$
|
||
\item $\UncommittedSprout \typecolon \bitseq{\MerkleHashLengthSprout} := \zeros{\MerkleHashLengthSprout}$
|
||
\sapling{
|
||
\item $\UncommittedSapling \typecolon \bitseq{\MerkleHashLengthSapling} := \ones{\MerkleHashLengthSapling}$
|
||
}
|
||
\item $\MAXMONEY \typecolon \Nat := \changed{2.1 \smult 10^{15}}$ (\zatoshi)
|
||
\item $\SlowStartInterval \typecolon \Nat := 20000$
|
||
\item $\HalvingInterval \typecolon \Nat := 840000$
|
||
\item $\MaxBlockSubsidy \typecolon \Nat := 1.25 \smult 10^9$ (\zatoshi)
|
||
\item $\NumFounderAddresses \typecolon \Nat := 48$
|
||
\item $\FoundersFraction \typecolon \Rat := \frac{1}{5}$
|
||
\item $\PoWLimit \typecolon \Nat := \begin{cases}
|
||
2^{243} - 1,&\squash\text{for the production network} \\
|
||
2^{251} - 1,&\squash\text{for the test network}
|
||
\end{cases}$
|
||
\item $\PoWAveragingWindow \typecolon \Nat := 17$
|
||
\item $\PoWMedianBlockSpan \typecolon \Nat := 11$
|
||
\item $\PoWMaxAdjustDown \typecolon \Rat := \frac{32}{100}$
|
||
\item $\PoWMaxAdjustUp \typecolon \Rat := \frac{16}{100}$
|
||
\item $\PoWDampingFactor \typecolon \Nat := 4$
|
||
\item $\PoWTargetSpacing \typecolon \Nat := 150$ (seconds).
|
||
\end{formulae}
|
||
|
||
|
||
\introlist
|
||
\nsubsection{Concrete Cryptographic Schemes}
|
||
|
||
\nsubsubsection{\HashFunctions}
|
||
|
||
\nsubsubsubsection{SHA-256 and SHA256Compress \HashFunctions} \label{concretesha256}
|
||
|
||
SHA-256 is defined by \cite{NIST2015}.
|
||
|
||
\Zcash uses the full \shaHashFunction to instantiate $\NoteCommitmentSprout$.
|
||
|
||
\begin{formulae}
|
||
\item $\SHAFull \typecolon \byteseqs \rightarrow \byteseq{32}$
|
||
\end{formulae}
|
||
|
||
It also uses the \shaCompressFunction, $\SHACompress$. This operates
|
||
on a single $512$-bit block and \emph{excludes} the padding step specified
|
||
in \cite[section 5.1]{NIST2015}; i.e.\ the input to $\SHACompress$ is what
|
||
\cite[section 5.2]{NIST2015} refers to as ``the message and its padding''.
|
||
The Initial Hash Value is the same as for full $\SHAFull$.
|
||
|
||
\introlist
|
||
\Zcash uses $\SHACompress$ to instantiate several \pseudoRandomFunctions and
|
||
$\MerkleCRHSprout$.
|
||
|
||
\begin{formulae}
|
||
\item $\SHACompress \typecolon \bitseq{512} \rightarrow \bitseq{256}$
|
||
\end{formulae}
|
||
|
||
\todo{Specify bit order.}
|
||
|
||
|
||
\nsubsubsubsection{\BlakeTwo{} \HashFunction} \label{concreteblake2}
|
||
|
||
BLAKE2 is defined by \cite{ANWW2013}.
|
||
\sprout{\Zcash uses only the $\BlakeTwobGeneric$ variant.}
|
||
\sapling{\Zcash uses both the $\BlakeTwobGeneric$ and $\BlakeTwosGeneric$
|
||
variants.}
|
||
|
||
$\BlakeTwob{\ell}(p, x)$ refers to unkeyed $\BlakeTwob{\ell}$
|
||
in sequential mode, with an output digest length of $\ell/8$ bytes,
|
||
$16$-byte personalization string $p$, and input $x$.
|
||
|
||
\introlist
|
||
$\BlakeTwobGeneric$ is used to instantiate $\hSigCRH$, $\EquihashGen{}$,
|
||
and $\KDFSprout$.
|
||
\nuzero{From \NUZero onward, it is used to compute \sighashTxHashes.}
|
||
\sapling{For \Sapling, it is also used to instantiate $\KDFSapling$ and
|
||
$\PRGExpandSeed{}$, and in the $\EdJubjub$ \signatureScheme which
|
||
instantiates $\SpendAuthSig$.}
|
||
|
||
\begin{formulae}
|
||
\item $\BlakeTwob{\ell} \typecolon \byteseq{16} \times \byteseqs \rightarrow \byteseq{\ell/8}$
|
||
\end{formulae}
|
||
|
||
\vspace{-3ex}
|
||
\pnote{
|
||
$\BlakeTwob{\ell}$ is not the same as $\BlakeTwob{512}$ truncated to
|
||
$\ell$ bits, because the digest length is encoded in the parameter
|
||
block.
|
||
}
|
||
|
||
\sapling{
|
||
\vspace{3ex}
|
||
$\BlakeTwos{\ell}(p, x)$ refers to unkeyed $\BlakeTwos{\ell}$
|
||
in sequential mode, with an output digest length of $\ell/8$ bytes,
|
||
$8$-byte personalization string $p$, and input $x$.
|
||
|
||
$\BlakeTwosGeneric$ is used to instantiate $\PRFnr{}$, $\CRHivk$, and
|
||
$\GroupJHash{}$.
|
||
|
||
\begin{formulae}
|
||
\item $\BlakeTwos{\ell} \typecolon \byteseq{8} \times \byteseqs \rightarrow \byteseq{\ell/8}$
|
||
\end{formulae}
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsubsection{\MerkleTree{} \HashFunction} \label{merklecrh}
|
||
|
||
\newsavebox{\merklebox}
|
||
\begin{lrbox}{\merklebox}
|
||
\begin{bytefield}[bitwidth=0.04em]{512}
|
||
\bitbox{256}{$256$-bit $\mathsf{left}$} &
|
||
\bitbox{256}{$256$-bit $\mathsf{right}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sprout{
|
||
$\MerkleCRHSprout$ is used to hash \incrementalMerkleTree \merkleHashes.
|
||
|
||
Let $\SHACompress$ be as specified in \crossref{concretesha256}.
|
||
|
||
$\MerkleCRHSprout \typecolon \MerkleHashSprout \times \MerkleHashSprout \rightarrow \MerkleHashSprout$
|
||
is defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\MerkleCRHSprout(\mathsf{left}, \mathsf{right}) := \SHACompressBox{\merklebox}$.
|
||
\end{formulae}
|
||
|
||
\pnote{
|
||
$\SHACompress$ is not the same as the $\SHAFull$ function, which hashes arbitrary-length
|
||
byte sequences.
|
||
}
|
||
}
|
||
\notsprout{
|
||
$\MerkleCRHSprout$ and $\MerkleCRHSapling$ are used to hash
|
||
\incrementalMerkleTree \merkleHashes for \Sprout and \Sapling respectively.
|
||
|
||
\subsubsubsubsection{$\MerkleCRHSprout$ \HashFunction} \label{merklecrhsprout}
|
||
|
||
Let $\SHACompress$ be as specified in \crossref{concretesha256}.
|
||
|
||
$\MerkleCRHSprout \typecolon \MerkleLayerSprout \times \MerkleHashSprout \times \MerkleHashSprout
|
||
\rightarrow \MerkleHashSprout$ is defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\MerkleCRHSprout(\mathsf{layer}, \mathsf{left}, \mathsf{right}) := \SHACompressBox{\merklebox}$.
|
||
\end{formulae}
|
||
|
||
\vspace{-4ex}
|
||
\begin{pnotes}
|
||
\item The $\mathsf{layer}$ argument does not affect the output.
|
||
\item $\SHACompress$ is not the same as the $\SHAFull$ function, which hashes arbitrary-length
|
||
byte sequences.
|
||
\end{pnotes}
|
||
}
|
||
|
||
\vspace{-2ex}
|
||
\securityrequirement{
|
||
$\SHACompress$ must be collision-resistant, and it must be infeasible to find a preimage $x$
|
||
such that $\SHACompress(x) = \zeros{256}$.
|
||
}
|
||
|
||
\sapling{
|
||
\subsubsubsubsection{$\MerkleCRHSapling$ \HashFunction} \label{merklecrhsapling}
|
||
|
||
Let $\PedersenHash$ be as specified in \crossref{concretepedersenhash}.
|
||
|
||
$\MerkleCRHSapling \typecolon \MerkleLayerSapling \times \MerkleHashSapling \times \MerkleHashSapling
|
||
\rightarrow \MerkleHashSapling$ is defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\MerkleCRHSapling(\mathsf{layer}, \mathsf{left}, \mathsf{right}) := \PedersenHash(\ascii{Zcash\_PH},
|
||
l \bconcat \mathsf{left} \bconcat \mathsf{right})$
|
||
\item \tab where $l = \ItoLEBSP{6}(\MerkleDepthSapling - 1 - \mathsf{layer})$.
|
||
\end{formulae}
|
||
|
||
\vspace{-2ex}
|
||
\securityrequirement{
|
||
$\PedersenHash$ must be collision-resistant.
|
||
}
|
||
}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsubsection{\hSigText{} \HashFunction} \label{hsigcrh}
|
||
|
||
\newsavebox{\hsigbox}
|
||
\begin{lrbox}{\hsigbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.04em]{1024}
|
||
\bitbox{256}{$256$-bit $\RandomSeed$} &
|
||
\bitbox{256}{\hfill $256$-bit $\nfOld{\mathrm{1}}$\hfill...\;} &
|
||
\bitbox{256}{$256$-bit $\nfOld{\NOld}$} &
|
||
\bitbox{300}{$256$-bit $\joinSplitPubKey$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
$\hSigCRH$ is used to compute the value $\hSig$ in \crossref{joinsplitdesc}.
|
||
|
||
\changed{
|
||
\begin{formulae}
|
||
\item $\hSigCRH(\RandomSeed, \nfOld{\allOld}, \joinSplitPubKey) := \BlakeTwob{256}(\ascii{ZcashComputehSig},\; \hSigInput)$
|
||
\end{formulae}
|
||
|
||
where
|
||
\begin{formulae}
|
||
\item $\hSigInput := \Justthebox{\hsigbox}$.
|
||
\end{formulae}
|
||
}
|
||
|
||
$\BlakeTwob{256}(p, x)$ is defined in \crossref{concreteblake2}.
|
||
|
||
\securityrequirement{
|
||
$\BlakeTwob{256}(\ascii{ZcashComputehSig}, x)$ must be collision-resistant.
|
||
}
|
||
|
||
|
||
\newsavebox{\crhivkbox}
|
||
\begin{lrbox}{\crhivkbox}
|
||
\begin{bytefield}[bitwidth=0.05em]{512}
|
||
\bitbox{256}{$256$-bit $\reprJOf{\AuthSignPublic}$} &
|
||
\bitbox{256}{$256$-bit $\reprJOf{\AuthProvePublic}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\introlist
|
||
\nsubsubsubsection{\CRHivkText{} \HashFunction} \label{concretecrhivk}
|
||
|
||
$\CRHivk$ is used to derive the \incomingViewingKey $\InViewingKey$
|
||
for a \Sapling \paymentAddress.
|
||
For its use when generating an address see \crossref{saplingkeycomponents},
|
||
and for its use in the \spendStatement see \crossref{spendstatement}.
|
||
|
||
\introlist
|
||
It is defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\CRHivk(\AuthSignPublic, \AuthProvePublic) :=
|
||
\LEOStoIP{256}(\BlakeTwos{256}(\ascii{Zcashivk},\; \crhInput)) \bmod 2^{251}$
|
||
\end{formulae}
|
||
|
||
where
|
||
\begin{formulae}
|
||
\item $\crhInput := \Justthebox{\crhivkbox}$
|
||
\end{formulae}
|
||
|
||
\vspace{2ex}
|
||
$\BlakeTwos{256}(p, x)$ refers to unkeyed $\BlakeTwos{256}$
|
||
\cite{ANWW2013} in sequential mode, with an output digest length of
|
||
$32$ bytes, $8$-byte personalization string $p$, and input $x$.
|
||
|
||
\securityrequirement{
|
||
$\LEOStoIP{256}(\BlakeTwos{256}(\ascii{Zcashivk}, x)) \bmod 2^{251}$
|
||
must be collision-resistant on a $512$-bit input $x$. Note that this
|
||
does not follow from collision-resistance of $\BlakeTwos{256}$
|
||
(and the best possible concrete security is that of a $251$-bit hash
|
||
rather than a $256$-bit hash), but it is a reasonable assumption
|
||
given the design and structure of $\BlakeTwosGeneric$.
|
||
}
|
||
|
||
\pnote{
|
||
The variable output digest length feature of $\BlakeTwosGeneric$ does
|
||
not support arbitrary bit lengths, otherwise that would have been
|
||
used rather than external truncation. However, the protocol-specific
|
||
personalization string together with truncation achieve essentially
|
||
the same effect as using that feature.
|
||
}
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\introlist
|
||
\nsubsubsubsection{\PedersenHashFunction} \label{concretepedersenhash}
|
||
|
||
$\PedersenHash$ is an algebraic hash function with collision resistance
|
||
(for fixed input length) derived from assumed hardness of the
|
||
Discrete Logarithm Problem on the $\JubjubCurve$ curve.
|
||
It is based on the work of David Chaum, Ivan Damgård, Jeroen van de Graaf,
|
||
Jurjen Bos, George Purdy, Eugène van Heijst and Birgit Pfitzmann in
|
||
\cite{CDG1987}, \cite{BCP1988} and \cite{CvHP1991},
|
||
and of Mihir Bellare, Oded Goldreich, and Shafi Goldwasser in \cite{BGG1995},
|
||
with optimizations for efficient instantiation in \zkSNARKCircuits
|
||
by Sean Bowe and Daira Hopwood.
|
||
|
||
$\PedersenHash$ is used in the \incrementalMerkleTree over \noteCommitments
|
||
(\crossref{merkletree}) and in the definition of \xPedersenCommitments
|
||
(\crossref{concretewindowedcommit}).
|
||
|
||
Let $\GroupJ$ be as defined in \crossref{jubjub}.
|
||
|
||
Let $\ExtractJ$ be as defined in \crossref{concreteextractorjubjub}.
|
||
|
||
Let $\FindGroupJHash$ be as defined in \crossref{concretegrouphashjubjub}.
|
||
|
||
Let $c := 63$.
|
||
|
||
\newsavebox{\gencountbox}
|
||
\begin{lrbox}{\gencountbox}
|
||
\begin{bytefield}[bitwidth=0.28em]{32}
|
||
\bitbox{32}{$32$-bit $\floor{\frac{i-1}{c}}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\introlist
|
||
\vspace{2ex}
|
||
Define $\PedersenGenAlg \typecolon \byteseq{8} \times \Nat \rightarrow \GroupJ$ by:
|
||
|
||
\begin{formulae}
|
||
\item $\PedersenGen{D}{i} := \FindGroupJHashOf{D, \Justthebox{\gencountbox}}$.
|
||
\end{formulae}
|
||
|
||
\newcommand{\sj}[1]{s^{\kern 0.02em j}_{#1}}
|
||
|
||
\vspace{2ex}
|
||
\introsection
|
||
Define $\PedersenHashToPoint(D \typecolon \byteseq{8}, M \typecolon \bitseq{\PosInt})$ as follows:
|
||
|
||
\begin{formulae}
|
||
\item Pad $M$ to a multiple of $3$ bits by appending zero bits, giving $M'$.
|
||
\item Let $n = \ceiling{\hfrac{\length(M')}{3 \mult c}}$.
|
||
\item Split $M'$ into $n$ \quotedterm{segments} $M_\barerange{1}{n}$
|
||
so that $M' = \concatbits(M_\barerange{1}{n})$, and
|
||
each of $M_\barerange{1}{n-1}$ is of length $3 \smult c$ bits.
|
||
($M_n$ may be shorter.)
|
||
\item Return $\vsum{i=1}{n} \scalarmult{\PedersenEncode{M_i}}{\PedersenGen{D}{i}} \typecolon \GroupJ$.
|
||
\end{formulae}
|
||
|
||
where
|
||
$\PedersenEncode{\paramdot} \typecolon \bitseq{3 \mult \range{1}{c}} \rightarrow
|
||
\rangenozero{-\hfrac{\ParamJ{r}-1}{2}}{\hfrac{\ParamJ{r}-1}{2}}$ is defined as:
|
||
|
||
\begin{formulae}
|
||
\item Let $k_i = \length(M_i)/3$.
|
||
\item Split $M_i$ into $3$-bit \quotedterm{chunks} $m_\barerange{1}{k_i}$
|
||
so that $M_i = \concatbits(m_\barerange{1}{k_i})$.
|
||
\item Write each $m_j$ as $[\sj{0}, \sj{1}, \sj{2}]$, and let
|
||
$\enc(m_j) = (1 - 2 \smult \sj{2}) \mult (1 + \sj{0} + 2 \smult \sj{1})$.
|
||
\item Let $\PedersenEncode{M_i} = \vsum{j=1}{k_i} \enc(m_j) \mult 2^{4 \mult (j-1)}$.
|
||
\end{formulae}
|
||
|
||
Finally, define $\PedersenHash \typecolon \byteseq{8} \times \bitseq{\PosInt} \rightarrow \bitseq{255}$ by:
|
||
|
||
\begin{formulae}
|
||
\item $\PedersenHash(D, M) := \ItoLEBSP{255}(\ExtractJ(\PedersenHashToPoint(D, M)))$.
|
||
\end{formulae}
|
||
|
||
See \crossref{cctpedersenhash} for rationale and efficient circuit implementation
|
||
of these functions.
|
||
|
||
\securityrequirement{
|
||
$\PedersenHash$ and $\PedersenHashToPoint$ are required to be collision-resistant
|
||
between inputs of fixed length, for a given personalization input $D$.
|
||
No other security properties commonly associated with \hashFunctions are needed.
|
||
}
|
||
|
||
\vspace{2ex}
|
||
\begin{theorem} \label{thmpedersenencodeinjective}
|
||
The encoding function $\PedersenEncode{\paramdot}$ is injective.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
We first check that the range of
|
||
$\vsum{j=1}{k_i} \enc(m_j) \mult 2^{4 \mult (j-1)}$ is a subset of
|
||
the allowable range $\rangenozero{-\hfrac{\ParamJ{r}-1}{2}}{\hfrac{\ParamJ{r}-1}{2}}$.
|
||
The range of this expression is a subset of
|
||
$\rangenozero{-\PedersenRangeOffset}{\PedersenRangeOffset}$ where
|
||
$\PedersenRangeOffset = 4 \mult \vsum{i=0}{c-1} 2^{4 \mult i} = 4 \mult \hfrac{2^{4 \mult c}}{15}$.
|
||
|
||
When $c = 63$, we have
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$4 \mult \hfrac{2^{4 \mult c}}{15}$ &$= \hexint{444444444444444444444444444444444444444444444444444444444444444}$ \\
|
||
& \\[-2ex]
|
||
$\hfrac{\ParamJ{r}-1}{2}$ &$= \hexint{73EDA753299D7D483339D80809A1D8053341049E6640841684B872F6B7B965B}$
|
||
\end{tabular}
|
||
|
||
so the required condition is met. This implies that there is no ``wrap around''
|
||
and so $\vsum{j=1}{k_i} \enc(m_j) \mult 2^{4 \mult (j-1)}$ may be treated as an
|
||
integer expression.
|
||
|
||
$\enc$ is injective. In order to prove that $\PedersenEncode{\paramdot}$ is injective,
|
||
consider $\PedersenEncodeNonneg{\paramdot} \typecolon \bitseq{3 \mult \range{1}{c}} \rightarrow
|
||
\range{0}{2 \smult \PedersenRangeOffset}$ such that
|
||
$\PedersenEncodeNonneg{M_i} = \PedersenEncode{M_i} + \PedersenRangeOffset$.
|
||
With $k_i$ and $m_j$ defined as above, we have
|
||
$\PedersenEncodeNonneg{M_i} = \vsum{j=1}{k_i} \enc'(m_j) \mult 2^{4 \mult (j-1)}$
|
||
where $\enc'(m_j) = \enc(m_j) + 4$ is in $\range{0}{8}$ and $\enc'$ is injective.
|
||
Express this sum in hexadecimal; then each $m_j$ affects only one hex digit, and
|
||
it is easy to see that $\PedersenEncodeNonneg{\paramdot}$ is injective.
|
||
Therefore so is $\PedersenEncode{\paramdot}$.
|
||
\end{proof}
|
||
|
||
Since the security proof from \cite[Appendix A]{BGG1995}
|
||
depends only on the encoding being injective and its range not including
|
||
zero, the proof can be adapted straightforwardly to show that $\PedersenHashToPoint$
|
||
is collision-resistant under the same assumptions and security bounds.
|
||
Because $\ItoLEBSP{255}$ and $\ExtractJ$ are injective, it follows that
|
||
$\PedersenHash$ is equally collision-resistant.
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{Mixing Pedersen \HashFunction} \label{concretemixinghash}
|
||
|
||
A mixing \xPedersenHash is used to compute $\NoteAddressRand$ from
|
||
$\cm$ and $\NotePosition$ in \crossref{commitmentsandnullifiers}. It takes as
|
||
input a \xPedersenCommitment $P$, and hashes it with another input $x$.
|
||
|
||
We define $\MixingPedersenHash{D} \typecolon \byteseq{8} \times \GroupJ \times \range{0}{\ParamJ{r}-1}
|
||
\rightarrow \GroupJ$ by:
|
||
|
||
\begin{formulae}
|
||
\item $\MixingPedersenHash(D, P, x) := P + \scalarmult{x}{\FindGroupJHashOf{D, \ascii{}}}$.
|
||
\end{formulae}
|
||
|
||
\securityrequirement{
|
||
Fix $D_1, D_2 \typecolon \byteseq{8}$ with $D_1 \neq D_2$, and consider the function
|
||
\begin{formulae}
|
||
\item $\fun{(r, M, x) \typecolon \range{0}{\ParamJ{r}-1} \times \bitseq{\PosInt} \times
|
||
\range{0}{\ParamJ{r}-1}}{\MixingPedersenHash(D_2, x, \WindowedPedersenCommit{r}(D_1, M)) \typecolon \GroupJ}$.
|
||
\end{formulae}
|
||
This function must be collision-resistant on $(r, M, x)$.
|
||
}
|
||
|
||
See \crossref{cctmixinghash} for rationale and efficient circuit implementation
|
||
of this function.
|
||
}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsubsection{Equihash Generator} \label{equihashgen}
|
||
|
||
$\EquihashGen{n, k}$ is a specialized \hashFunction that maps an input
|
||
and an index to an output of length $n$ bits. It is used in \crossref{equihash}.
|
||
|
||
\newsavebox{\powtagbox}
|
||
\begin{lrbox}{\powtagbox}
|
||
\begin{bytefield}[bitwidth=0.16em]{128}
|
||
\bitbox{64}{64-bit $\ascii{ZcashPoW}$} &
|
||
\bitbox{32}{32-bit $n$} &
|
||
\bitbox{32}{32-bit $k$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\powcountbox}
|
||
\begin{lrbox}{\powcountbox}
|
||
\begin{bytefield}[bitwidth=0.16em]{32}
|
||
\bitbox{32}{32-bit $g$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
Let $\powtag := \Justthebox{\powtagbox}$.
|
||
|
||
Let $\powcount(g) := \Justthebox{\powcountbox}$.
|
||
|
||
\vspace{2ex}
|
||
\introlist
|
||
% Blech. Dijkstra was right \cite{EWD831}.
|
||
Let $\EquihashGen{n, k}(S, i) := T_\barerange{h+1}{h+n}$, where
|
||
\begin{formulae}
|
||
\item $m := \floor{\frac{512}{n}}$;
|
||
\item $h := (i-1 \bmod m) \mult n$;
|
||
\item $T := \BlakeTwob{(\mathnormal{n \mult m})}(\powtag,\, S \bconcat \powcount(\floor{\frac{i-1}{m}}))$.
|
||
\end{formulae}
|
||
|
||
Indices of bits in $T$ are 1-based.
|
||
|
||
$\BlakeTwob{\ell}(p, x)$ is defined in \crossref{concreteblake2}.
|
||
|
||
\securityrequirement{
|
||
$\BlakeTwob{\ell}(\powtag, x)$ must generate output that is sufficiently
|
||
unpredictable to avoid short-cuts to the Equihash solution process.
|
||
It would suffice to model it as a random oracle.
|
||
}
|
||
|
||
\pnote{
|
||
When $\EquihashGen{}$ is evaluated for sequential indices, as
|
||
in the Equihash solving process (\crossref{equihash}),
|
||
the number of calls to $\BlakeTwobGeneric$ can be reduced by a factor of
|
||
$\floor{\frac{512}{n}}$ in the best case (which is a factor of 2 for
|
||
$n = 200$).
|
||
}
|
||
|
||
\introsection
|
||
\nsubsubsection{\PseudoRandomFunctions} \label{concreteprfs}
|
||
|
||
$\PRFaddr{}$, $\PRFnf{}$, $\PRFpk{}$\changed{, and $\PRFrho{}$},
|
||
described in \crossref{abstractprfs}, are all instantiated using the \shaCompressFunction
|
||
defined in \crossref{concretesha256}:
|
||
|
||
\newcommand{\iminusone}{\hspace{0.3pt}\scriptsize{$i$\hspace{0.6pt}-1}}
|
||
|
||
\newsavebox{\addrbox}
|
||
\begin{lrbox}{\addrbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{224}{$252$-bit $x$} &
|
||
\bitbox{56}{$8$-bit $t$} &
|
||
\bitbox{200}{$\zeros{248}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\nfbox}
|
||
\begin{lrbox}{\nfbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{224}{$252$-bit $\AuthPrivate$} &
|
||
\bitbox{256}{$256$-bit $\NoteAddressRand$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\pkbox}
|
||
\begin{lrbox}{\pkbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{18}{\iminusone} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{224}{$252$-bit $\AuthPrivate$} &
|
||
\bitbox{256}{$256$-bit $\hSig$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\rhobox}
|
||
\begin{lrbox}{\rhobox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.06em]{512}
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{18}{\iminusone} &
|
||
\bitbox{18}{$1$} &
|
||
\bitbox{18}{$0$} &
|
||
\bitbox{224}{$252$-bit $\NoteAddressPreRand$} &
|
||
\bitbox{256}{$256$-bit $\hSig$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\vspace{-2ex}
|
||
\begin{equation*}
|
||
\begin{aligned}
|
||
\setchanged \PRFaddr{x}(t) &\setchanged := \SHACompressBox{\addrbox} \\
|
||
\PRFnf{\AuthPrivate}(\NoteAddressRand) &:= \SHACompressBox{\nfbox} \\
|
||
\PRFpk{\AuthPrivate}(i, \hSig) &:= \SHACompressBox{\pkbox} \\
|
||
\setchanged \PRFrho{\NoteAddressPreRand}(i, \hSig) &\setchanged := \SHACompressBox{\rhobox}
|
||
\end{aligned}
|
||
\end{equation*}
|
||
|
||
\begin{securityrequirements}
|
||
\item The \shaCompressFunction must be collision-resistant.
|
||
\item The \shaCompressFunction must be a PRF when keyed by the bits
|
||
corresponding to $x$, $\AuthPrivate$ or $\NoteAddressPreRand$
|
||
in the above diagrams, with input in the remaining bits.
|
||
\end{securityrequirements}
|
||
|
||
\changed{
|
||
\pnote{
|
||
The first four bits --i.e.\ the most significant four bits of the first byte--
|
||
are used to distinguish different uses of $\SHACompress$, ensuring that the functions
|
||
are independent. In addition to the inputs shown here, the bits $\mathtt{1011}$
|
||
in this position are used to distinguish uses of the full $\SHAFull$ hash
|
||
function --- see \crossref{concretesproutcommit}.
|
||
|
||
(The specific bit patterns chosen here were motivated by the possibility of future
|
||
extensions that might have increased $\NOld$ and/or $\NNew$ to 3, or added an
|
||
additional bit to $\AuthPrivate$ to encode a new key type, or that would have
|
||
required an additional PRF.\sapling{ In fact since \Sapling switches to
|
||
non-$\SHACompress$-based cryptographic primitives, these extensions are unlikely to
|
||
be necessary.})
|
||
}
|
||
}
|
||
|
||
\newsavebox{\nrbox}
|
||
\begin{lrbox}{\nrbox}
|
||
\setsapling
|
||
\begin{bytefield}[bitwidth=0.04em]{512}
|
||
\bitbox{256}{$256$-bit $\reprJ(\AuthProvePublic)$} &
|
||
\bitbox{256}{$256$-bit $\reprJ(\NoteAddressRand)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\introlist
|
||
\vspace{2ex}
|
||
$\PRFnr{}$, described in \crossref{abstractprfs}, is instantiated using the
|
||
$\BlakeTwosGeneric$ \hashFunction defined in \crossref{concreteblake2}:
|
||
|
||
Define:
|
||
|
||
\begin{formulae}
|
||
\item $\Hashnr(x) := \BlakeTwos{256}(\ascii{ZcashnrL}, x) \bconcat \BlakeTwos{256}(\ascii{ZcashnrH}, x)$.
|
||
\item $\PRFnr{\AuthProvePublic}(\NoteAddressRand) :=
|
||
\LEOStoIP{512}\!\left(\Hashnr\!\left(\Justthebox{\nrbox}\right)\right) \bmod \ParamS{r}$.
|
||
\end{formulae}
|
||
}
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsubsection{\PseudoRandomGenerators} \label{concreteprgs}
|
||
|
||
$\PRGExpandSeed{}$, described in \crossref{abstractprgs}, maps a
|
||
\Sapling \spendingKey to an \expandedSeed:
|
||
|
||
\begin{formulae}
|
||
\item $\PRGExpandSeed{\AuthPrivateSeed}() := \BlakeTwob{512}(\ascii{Zcash\_ExpandSeed}, \AuthPrivateSeed)$
|
||
\end{formulae}
|
||
|
||
(The \expandedSeed is used to derive the \authSigningKey $\AuthSignPrivate$
|
||
and the \authProvingKey $\AuthProvePrivate$ in \crossref{saplingkeycomponents}.)
|
||
|
||
\securityrequirement{
|
||
$\BlakeTwob{512}(\ascii{Zcash\_ExpandSeed}, x)$ must be a \pseudoRandomGenerator
|
||
\cite{SS2005} producing $512$ output bits given key $x$.
|
||
}
|
||
}
|
||
|
||
\introsection
|
||
\nsubsubsection{\SymmetricEncryption} \label{concretesym}
|
||
|
||
\changed{
|
||
Let $\Keyspace := \bitseq{256}$, $\Plaintext := \byteseqs$, and $\Ciphertext := \byteseqs$.
|
||
|
||
Let $\SymEncrypt{\Key}(\Ptext)$ be authenticated encryption using
|
||
$\SymSpecific$ \cite{RFC-7539} encryption of plaintext $\Ptext \in \Plaintext$,
|
||
with empty ``associated data", all-zero nonce $\zeros{96}$, and $256$-bit key
|
||
$\Key \in \Keyspace$.
|
||
|
||
Similarly, let $\SymDecrypt{\Key}(\Ctext)$ be $\SymSpecific$
|
||
decryption of ciphertext $\Ctext \in \Ciphertext$, with empty
|
||
``associated data", all-zero nonce $\zeros{96}$, and $256$-bit key
|
||
$\Key \in \Keyspace$. The result is either the plaintext byte sequence,
|
||
or $\bot$ indicating failure to decrypt.
|
||
}
|
||
|
||
\pnote{
|
||
The ``IETF" definition of $\SymSpecific$ from \cite{RFC-7539} is
|
||
used; this has a 32-bit block count and a 96-bit nonce, rather than a 64-bit
|
||
block count and 64-bit nonce as in the original definition of $\SymCipher$.
|
||
}
|
||
|
||
\nsubsubsection{\KeyAgreementAndDerivation} \label{concretekaandkdf}
|
||
|
||
\nsubsubsubsection{\SproutOrNothing \KeyAgreement} \label{concretesproutkeyagreement}
|
||
|
||
\changed{
|
||
The \keyAgreementScheme specified in \crossref{abstractkeyagreement} is
|
||
instantiated using Curve25519 \cite{Bern2006} as follows.
|
||
|
||
Let $\KASproutPublic$ and $\KASproutSharedSecret$ be the type of Curve25519 public keys
|
||
(i.e.\ a sequence of $32$ bytes), and let $\KASproutPrivate$ be the type of Curve25519
|
||
secret keys.
|
||
|
||
Let $\CurveMultiply(\bytes{n}, \bytes{q})$ be the result of point
|
||
multiplication of the Curve25519 public key represented by the byte
|
||
sequence $\bytes{q}$ by the Curve25519 secret key represented by the
|
||
byte sequence $\bytes{n}$, as defined in \cite[section 2]{Bern2006}.
|
||
|
||
Let $\KASproutBase := \CurveBase$ be the public byte sequence representing
|
||
the Curve25519 base point.
|
||
|
||
Let $\Clamp(\bytes{x})$ take a 32-byte sequence $\bytes{x}$ as input
|
||
and return a byte sequence representing a Curve25519 private key, with
|
||
bits ``clamped'' as described in \cite[section 3]{Bern2006}:
|
||
``clear bits $0, 1, 2$ of the first byte, clear bit $7$ of the last byte,
|
||
and set bit $6$ of the last byte.'' Here the bits of a byte are numbered
|
||
such that bit $b$ has numeric weight $2^b$.
|
||
|
||
Define $\KASproutFormatPrivate(x) := \Clamp(x)$.
|
||
|
||
Define $\KASproutAgree(n, q) := \CurveMultiply(n, q)$.
|
||
}
|
||
|
||
\introsection
|
||
\nsubsubsubsection{\SproutOrNothing \KeyDerivation} \label{concretesproutkdf}
|
||
|
||
\newsavebox{\kdftagbox}
|
||
\begin{lrbox}{\kdftagbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.16em]{128}
|
||
\bitbox{64}{$64$-bit $\ascii{ZcashKDF}$} &
|
||
\bitbox{32}{$8$-bit $i\!-\!1$} &
|
||
\bitbox{56}{$\zeros{56}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\kdfinputbox}
|
||
\begin{lrbox}{\kdfinputbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.04em]{1024}
|
||
\bitbox{256}{$256$-bit $\hSig$} &
|
||
\bitbox{256}{$256$-bit $\DHSecret{i}$} &
|
||
\bitbox{256}{$256$-bit $\EphemeralPublic$} &
|
||
\bitbox{256}{$256$-bit $\TransmitPublicNew{i}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\changed{
|
||
The \keyDerivationFunction specified in \crossref{abstractkdf} is instantiated
|
||
using $\BlakeTwob{256}$ as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\KDFSprout(i, \hSig, \DHSecret{i}, \EphemeralPublic, \TransmitPublicNew{i}) :=
|
||
\BlakeTwob{256}(\kdftag, \kdfinput)$
|
||
\end{formulae}
|
||
\introlist
|
||
where:
|
||
\begin{formulae}
|
||
\item $\kdftag := \Justthebox{\kdftagbox}$
|
||
\item $\kdfinput := \Justthebox{\kdfinputbox}$.
|
||
\end{formulae}
|
||
}
|
||
|
||
$\BlakeTwob{256}(p, x)$ is defined in \crossref{concreteblake2}.
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\Sapling \KeyAgreement} \label{concretesaplingkeyagreement}
|
||
|
||
The \keyAgreementScheme specified in \crossref{abstractkeyagreement} is
|
||
instantiated using Diffie-Hellman with cofactor multiplication on $\JubjubCurve$
|
||
as follows.
|
||
|
||
Let $\KASaplingPublic$ and $\KASaplingSharedSecret$ be the type of compressed
|
||
$\JubjubCurve$ points $\CompressedEdwardsJubjub$, and let $\KASaplingPrivate$ be
|
||
the type of $\JubjubCurve$ secret keys. \todo{expand this}
|
||
}
|
||
|
||
|
||
\newsavebox{\kdfsaplinginputbox}
|
||
\begin{lrbox}{\kdfsaplinginputbox}
|
||
\begin{bytefield}[bitwidth=0.07em]{544}
|
||
\bitbox{80}{$32$-bit $\OutputIndex$} &
|
||
\bitbox{256}{$256$-bit $\reprJOf{\DHSecret{}}$} &
|
||
\bitbox{256}{$256$-bit $\reprJOf{\EphemeralPublic}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\Sapling \KeyDerivation} \label{concretesaplingkdf}
|
||
|
||
The $\KDFSapling$ \keyDerivationFunction specified in \crossref{abstractkdf}
|
||
is instantiated using $\BlakeTwob{256}$ as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\KDFSapling(\OutputIndex, \DHSecret{}, \EphemeralPublic) :=
|
||
\BlakeTwob{256}(\ascii{Zcash\_SaplingKDF}, \kdfinput)$.
|
||
\end{formulae}
|
||
\introlist
|
||
where:
|
||
\begin{formulae}
|
||
\item $\kdfinput := \Justthebox{\kdfsaplinginputbox}$.
|
||
\end{formulae}
|
||
|
||
$\BlakeTwob{256}(p, x)$ is defined in \crossref{concreteblake2}.
|
||
}
|
||
|
||
|
||
\nsubsubsection{\JoinSplitSignature} \label{concretejssig}
|
||
|
||
$\JoinSplitSig$ is specified in \crossref{abstractsig}.
|
||
|
||
\changed{It is instantiated as $\JoinSplitSigSpecific$ \cite{BDLSY2012},
|
||
with the additional requirements that:
|
||
|
||
\begin{itemize}
|
||
\item $\EdDSAS$ \MUST represent an integer less than
|
||
the prime $\ell = 2^{252} + 27742317777372353535851937790883648493$;
|
||
\item $\EdDSAR$ \MUST represent a point of order $\ell$ on the Ed25519 curve;
|
||
\end{itemize}
|
||
|
||
If these requirements are not met then the signature is considered invalid.
|
||
Note that it is \emph{not} required that the encoding of the $y$-coordinate
|
||
in $\EdDSAR$ is less than $2^{255}-19$.
|
||
|
||
$\JoinSplitSigSpecific$ is defined as using $\JoinSplitSigHashName$ internally.
|
||
|
||
A valid $\JoinSplitSigSpecific$ public key is defined as a point of order $\ell$
|
||
on the Ed25519 curve, in the encoding specified by \cite{BDLSY2012}. Again, it is
|
||
\emph{not} required that the encoding of the y-coordinate of the public key is
|
||
less than $2^{255}-19$.
|
||
}
|
||
|
||
\newsavebox{\sigbox}
|
||
\begin{lrbox}{\sigbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.075em]{512}
|
||
\bitbox{256}{$256$-bit $\EdDSAR$} &
|
||
\bitbox{256}{$256$-bit $\EdDSAS$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\introlist
|
||
\changed{
|
||
The encoding of a signature is:
|
||
}
|
||
\begin{formulae}
|
||
\item $\Justthebox{\sigbox}$
|
||
\end{formulae}
|
||
|
||
\changed{
|
||
where $\EdDSAR$ and $\EdDSAS$ are as defined in \cite{BDLSY2012}.
|
||
|
||
The encoding of a public key is as defined in \cite{BDLSY2012}.
|
||
}
|
||
|
||
\sapling{
|
||
\nsubsubsection{\SpendAuthSignature} \label{concretespendauthsig}
|
||
|
||
$\SpendAuthSig$ is specified in \crossref{abstractsig}.
|
||
|
||
It is instantiated as EdJubjub, which is defined as $\EdDSA$ \cite{BJLSY2015} over the
|
||
$\JubjubCurve$ curve which these additional constraints: \todo{...}
|
||
|
||
\cite{FKMSSS2016}
|
||
}
|
||
|
||
\introlist
|
||
\nsubsubsection{Commitment schemes} \label{concretecommit}
|
||
|
||
\nsubsubsubsection{\SproutOrNothing{} \NoteCommitments} \label{concretesproutcommit}
|
||
|
||
\newsavebox{\cmbox}
|
||
\begin{lrbox}{\cmbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.031em]{840}
|
||
\bitbox{24}{$1$} &
|
||
\bitbox{24}{$0$} &
|
||
\bitbox{24}{$1$} &
|
||
\bitbox{24}{$1$} &
|
||
\bitbox{24}{$0$} &
|
||
\bitbox{24}{$0$} &
|
||
\bitbox{24}{$0$} &
|
||
\bitbox{24}{$0$} &
|
||
\bitbox{256}{$256$-bit $\AuthPublic$} &
|
||
\bitbox{128}{$64$-bit $\Value$} &
|
||
\bitbox{256}{$256$-bit $\NoteAddressRand$} &
|
||
\bitbox{256}{$256$-bit $\NoteCommitRand$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
The commitment scheme $\NoteCommitSprout{}$ specified in \crossref{abstractcommit} is
|
||
instantiated using $\SHAFull$ as follows:
|
||
|
||
\begin{formulae}[leftmargin=1em]
|
||
\item $\NoteCommitSprout{\NoteCommitRand}(\AuthPublic, \Value, \NoteAddressRand) := \SHAFullBox{\cmbox}$.
|
||
\end{formulae}
|
||
|
||
\pnote{
|
||
The leading byte of the $\SHAFull$ input is $\hexint{B0}$.
|
||
}
|
||
|
||
\begin{securityrequirements}
|
||
\item The \shaCompressFunction must be collision-resistant.
|
||
\item The \shaCompressFunction must be a PRF when keyed by the bits corresponding
|
||
to the position of $\NoteCommitRand$ in the second block of $\SHAFull$
|
||
input, with input to the PRF in the remaining bits of the block and
|
||
the chaining variable.
|
||
\end{securityrequirements}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{Windowed Pedersen commitments} \label{concretewindowedcommit}
|
||
|
||
We construct \quotedterm{windowed} \xPedersenCommitments by reusing the \xPedersenHash
|
||
construction from \crossref{concretepedersenhash}, and adding a randomized point
|
||
on the $\JubjubCurve$ curve (see \crossref{jubjub}):
|
||
|
||
\begin{formulae}
|
||
\item $\WindowedPedersenCommit{r}(D, s) :=
|
||
\PedersenHashToPoint(D, s) + \scalarmult{r}{\FindGroupJHashOf{D, \ascii{}}}$.
|
||
\end{formulae}
|
||
|
||
See \crossref{cctwindowedcommit} for rationale and efficient circuit implementation
|
||
of this function.
|
||
|
||
The commitment scheme $\NoteCommitSprout{}$ specified in \crossref{abstractcommit} is
|
||
instantiated using $\WindowedPedersenCommitAlg$ as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\NoteCommitSapling{\NoteCommitRand}(\Diversifier, \DiversifiedTransmitPublic, \Value) :=
|
||
\WindowedPedersenCommit{\NoteCommitRand}(\ascii{Zcash\_cm},
|
||
\Diversifier \bconcat \DiversifiedTransmitPublic \bconcat \ItoLEBSP{64}(\Value))$.
|
||
\end{formulae}
|
||
|
||
\begin{securityrequirements}
|
||
\item $\WindowedPedersenCommitAlg$ must be a computationally binding and at least
|
||
computationally hiding \commitmentScheme, for a given personalization input $D$.
|
||
\item $\NoteCommitSaplingAlg$ must be a computationally binding and at least
|
||
computationally hiding \commitmentScheme.
|
||
\end{securityrequirements}
|
||
|
||
(They are in fact unconditionally hiding \commitmentSchemes.)
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{Homomorphic Pedersen commitments} \label{concretehomomorphiccommit}
|
||
|
||
The windowed Pedersen commitments defined in the preceding section are
|
||
highly efficient, but they do not support the homomorphic property we
|
||
need when instantiating $\ValueCommit{}$ (see \crossref{spendsandoutputs}
|
||
and \crossref{saplingbalance}).
|
||
|
||
In order to support this property, we also define \quotedterm{homomorphic}
|
||
\xPedersenCommitments as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\HomomorphicPedersenCommit{\ValueCommitRand}(D, \Value) :=
|
||
\scalarmult{\Value}{\FindGroupJHashOf{D}, \ascii{v}} + \scalarmult{\ValueCommitRand}{\FindGroupJHashOf{D, \ascii{}}}$
|
||
\end{formulae}
|
||
|
||
|
||
See \crossref{ccthomomorphiccommit} for rationale and efficient circuit implementation
|
||
of this function.
|
||
|
||
The commitment scheme $\ValueCommit{}$ specified in \crossref{abstractcommit} is
|
||
instantiated using $\HomomorphicPedersenCommit{}$ as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\ValueCommit{\ValueCommitRand}(\Value) :=
|
||
\HomomorphicPedersenCommit{\ValueCommitRand}(\ascii{Zcash\_cv}, \Value)$.
|
||
\end{formulae}
|
||
|
||
\begin{securityrequirements}
|
||
\item $\HomomorphicPedersenCommitAlg$ must be a computationally binding and at least
|
||
computationally hiding \commitmentScheme, for a given personalization input $D$.
|
||
\item $\ValueCommitAlg$ must be a computationally binding and at least
|
||
computationally hiding \commitmentScheme.
|
||
\end{securityrequirements}
|
||
|
||
(They are in fact unconditionally hiding \commitmentSchemes.)
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{\RepresentedGroupsAndPairings} \label{concretepairing}
|
||
|
||
\nsubsubsubsection{\BNRepresentedPairing} \label{bnpairing}
|
||
|
||
The \representedPairing $\BNCurve$ is defined in this section.
|
||
|
||
Let $\ParamG{q} := 21888242871839275222246405745257275088696311157297823662689037894645226208583$.
|
||
|
||
Let $\ParamG{r} := 21888242871839275222246405745257275088548364400416034343698204186575808495617$.
|
||
|
||
Let $\ParamG{b} := 3$.
|
||
|
||
(\hairspace $\ParamG{q}$ and $\ParamG{r}$ are prime.)
|
||
|
||
Let $\GroupG{1}$ be the group of points on a Barreto--Naehrig curve $\CurveG{1}$ over
|
||
$\GF{\ParamG{q}}$ with equation $y^2 = x^3 + \ParamG{b}$.
|
||
This curve has embedding degree 12 with respect to $\ParamG{r}$.
|
||
|
||
Let $\GroupG{2}$ be the subgroup of order $r$ in the sextic twist $\CurveG{2}$ of
|
||
$\GroupG{1}$ over $\GF{\ParamGexp{q}{2}}$ with equation $y^2 = x^3 + \frac{\ParamG{b}}{\xi}$,
|
||
where $\xi \typecolon \GF{\ParamGexp{q}{2}}$.
|
||
|
||
We represent elements of $\GF{\ParamGexp{q}{2}}$ as polynomials
|
||
$a_1 \mult t + a_0 \typecolon \GF{\ParamG{q}}[t]$, modulo the irreducible polynomial
|
||
$t^2 + 1$; in this representation, $\xi$ is given by $t + 9$.
|
||
|
||
Let $\GroupG{T}$ be the subgroup of $\ParamGexp{r}{\mathrm{th}}$ roots of unity in
|
||
$\GFstar{\ParamGexp{q}{12}}$.
|
||
|
||
Let $\PairingG$ be the optimized ate pairing of type
|
||
$\GroupG{1} \times \GroupG{2} \rightarrow \GroupG{T}$.
|
||
|
||
For $i \typecolon \range{1}{2}$, let $\ZeroG{i}$ be the point at infinity
|
||
(which is the additive identity) in $\GroupG{i}$, and let
|
||
$\GroupGstar{i} := \GroupG{i} \setminus \setof{\ZeroG{i}}$.
|
||
|
||
Let $\GenG{1} \typecolon \GroupGstar{1} := (1, 2)$.
|
||
|
||
\begin{tabular}{@{}l@{}r@{}l@{}}
|
||
Let $\GenG{2} \typecolon \GroupGstar{2} :=\;$
|
||
% are these the right way round?
|
||
&$(11559732032986387107991004021392285783925812861821192530917403151452391805634$ & $\mult\, t\;+$ \\
|
||
&$ 10857046999023057135944570762232829481370756359578518086990519993285655852781$ & $, $ \\
|
||
&$ 4082367875863433681332203403145435568316851327593401208105741076214120093531$ & $\mult\, t\;+$ \\
|
||
&$ 8495653923123431417604973247489272438418190587263600148770280649306958101930$ & $). $
|
||
\end{tabular}
|
||
|
||
$\GenG{1}$ and $\GenG{2}$ are generators of $\GroupG{1}$ and $\GroupG{2}$ respectively.
|
||
|
||
\newsavebox{\gonebox}
|
||
\begin{lrbox}{\gonebox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.045em]{264}
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$1$} &
|
||
\bitbox{80}{$1$-bit $\tilde{y}$} &
|
||
\bitbox{256}{$256$-bit $\ItoBEBSP{256}(x)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\gtwobox}
|
||
\begin{lrbox}{\gtwobox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.045em]{520}
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$1$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{20}{$1$} &
|
||
\bitbox{80}{$1$-bit $\tilde{y}$} &
|
||
\bitbox{512}{$512$-bit $\ItoBEBSP{512}(x)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
Define $\ItoBEBSP{} \typecolon (\ell \typecolon \Nat) \times \range{0}{2^\ell\!-\!1} \rightarrow
|
||
\bitseq{\ell}$ as in \crossref{endian}.
|
||
|
||
\introlist
|
||
For a point $P \typecolon \GroupGstar{1} = (\xP, \yP)$:
|
||
|
||
\begin{itemize}
|
||
\item The field elements $\xP$ and $\yP \typecolon \GF{q}$ are represented as
|
||
integers $x$ and $y \typecolon \range{0}{q\!-\!1}$.
|
||
\item Let $\tilde{y} = y \bmod 2$.
|
||
\item $P$ is encoded as $\Justthebox{\gonebox}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
For a point $P \typecolon \GroupGstar{2} = (\xP, \yP)$:
|
||
|
||
\begin{itemize}
|
||
\item Define $\FEtoIP \typecolon \GF{\ParamG{q}}[t] / (t^2 + 1) \rightarrow
|
||
\range{0}{\ParamGexp{q}{2}\!-\!1}$ such that
|
||
$\FEtoIP(a_{w,1} \mult t + a_{w,0}) = a_{w,1} \mult q + a_{w,0}$.
|
||
\item Let $x = \FEtoIP(\xP)$, $y = \FEtoIP(\yP)$, and $y' = \FEtoIP(-\yP)$.
|
||
\item Let $\tilde{y} = \begin{cases}
|
||
1, &\caseif y > y' \\
|
||
0, &\caseotherwise.
|
||
\end{cases}$
|
||
\item $P$ is encoded as $\Justthebox{\gtwobox}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{Non-normative notes:}
|
||
|
||
\begin{itemize}
|
||
\item The use of big-endian order by $\ItoBEBSP{}$ is different from the encoding
|
||
of most other integers in this protocol.
|
||
The encodings for $\GroupGstar{1, 2}$ are consistent with the
|
||
definition of $\ECtoOSP{}$ for compressed curve points in
|
||
\cite[section 5.5.6.2]{IEEE2004}. The LSB compressed form
|
||
(i.e.\ $\ECtoOSPXL$) is used for points in $\GroupGstar{1}$,
|
||
and the SORT compressed form (i.e.\ $\ECtoOSPXS$) for points in
|
||
$\GroupGstar{2}$.
|
||
\item The points at infinity $\ZeroG{1, 2}$ never occur in proofs and
|
||
have no defined encodings in this protocol.
|
||
\item Testing $y > y'$ for the compression of $\GroupGstar{2}$ points is equivalent
|
||
to testing whether $(a_{y,1}, a_{y,0}) > (a_{-y,1}, a_{-y,0})$ in lexicographic order.
|
||
\item Algorithms for decompressing points from the above encodings are
|
||
given in \cite[Appendix A.12.8]{IEEE2000} for $\GroupGstar{1}$, and
|
||
\cite[Appendix A.12.11]{IEEE2004} for $\GroupGstar{2}$.
|
||
\item A rational point $P \neq \ZeroG{2}$ on the curve $\CurveG{2}$ can be
|
||
verified to be of order $\ParamG{r}$, and therefore in $\GroupGstar{2}$,
|
||
by checking that $\ParamG{r} \mult P = \ZeroG{2}$.
|
||
\end{itemize}
|
||
|
||
When computing square roots in $\GF{\ParamG{q}}$ or $\GF{\ParamGexp{q}{2}}$ in
|
||
order to decompress a point encoding, the implementation \MUSTNOT assume that
|
||
the square root exists, or that the encoding represents a point on the curve.
|
||
|
||
|
||
\newsavebox{\sonebox}
|
||
\begin{lrbox}{\sonebox}
|
||
\setsapling
|
||
\begin{bytefield}[bitwidth=0.045em]{384}
|
||
\bitbox{20}{$1$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{80}{$1$-bit $\tilde{y}$} &
|
||
\bitbox{381}{$381$-bit $\ItoBEBSP{381}(x)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newsavebox{\stwobox}
|
||
\begin{lrbox}{\stwobox}
|
||
\setsapling
|
||
\begin{bytefield}[bitwidth=0.045em]{768}
|
||
\bitbox{20}{$1$} &
|
||
\bitbox{20}{$0$} &
|
||
\bitbox{80}{$1$-bit $\tilde{y}$} &
|
||
\bitbox{381}{$381$-bit $\ItoBEBSP{381}(x_1)$} &
|
||
\bitbox{384}{$384$-bit $\ItoBEBSP{384}(x_2)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\BLSRepresentedPairing} \label{blspairing}
|
||
|
||
The \representedPairing $\BLSCurve$ is defined in this section. Parameters are taken from
|
||
\cite{Bowe2017}.
|
||
|
||
\introlist
|
||
Let $\ParamS{q} :=\;$\scalebox{0.812}[1]{$4002409555221667393417789825735904156556882819939007885332058136124031650490837864442687629129015664037894272559787$}.
|
||
|
||
Let $\ParamS{r} := 52435875175126190479447740508185965837690552500527637822603658699938581184513$.
|
||
|
||
Let $\ParamS{u} := -15132376222941642752$.
|
||
|
||
Let $\ParamS{b} := 4$.
|
||
|
||
(\hairspace $\ParamS{q}$ and $\ParamS{r}$ are prime.)
|
||
|
||
Let $\GroupS{1}$ be the group of points on a Barreto--Lynn--Scott curve $\CurveS{1}$ over
|
||
$\GF{\ParamS{q}}$ with equation $y^2 = x^3 + \ParamS{b}$.
|
||
This curve has embedding degree 12 with respect to $\ParamS{r}$.
|
||
|
||
Let $\GroupS{2}$ be the subgroup of order $\ParamS{r}$ in the sextic twist $\CurveS{2}$ of
|
||
$\GroupS{1}$ over $\GF{\ParamSexp{q}{2}}$ with equation $y^2 = x^3 + 4(i + 1)$, where
|
||
$i \typecolon \GF{\ParamSexp{q}{2}}$.
|
||
|
||
We represent elements of $\GF{\ParamSexp{q}{2}}$ as polynomials
|
||
$a_1 \mult t + a_0 \typecolon \GF{\ParamS{q}}[t]$, modulo the irreducible polynomial
|
||
$t^2 + 1$; in this representation, $i$ is given by \todo{$?$}.
|
||
|
||
Let $\GroupS{T}$ be the subgroup of $\ParamSexp{r}{\mathrm{th}}$ roots of unity in
|
||
$\GFstar{\ParamSexp{q}{12}}$.
|
||
|
||
Let $\PairingS$ be the optimized ate pairing of type
|
||
$\GroupS{1} \times \GroupS{2} \rightarrow \GroupS{T}$.
|
||
|
||
For $i \typecolon \range{1}{2}$, let $\ZeroS{i}$ be the point at infinity in $\GroupS{i}$,
|
||
and let $\GroupSstar{i} := \GroupS{i} \setminus \setof{\ZeroS{i}}$.
|
||
|
||
\introlist
|
||
Let $\GenS{1} \typecolon \GroupSstar{1} := (1, 2)$.
|
||
|
||
\begin{tabular}{@{}l@{}r@{}l@{}}
|
||
Let $\GenS{2} \typecolon \GroupSstar{2} :=\;$
|
||
% are these the right way round?
|
||
&$(11559732032986387107991004021392285783925812861821192530917403151452391805634$ & $\mult\, t\;+$ \\
|
||
&$ 10857046999023057135944570762232829481370756359578518086990519993285655852781$ & $, $ \\
|
||
&$ 4082367875863433681332203403145435568316851327593401208105741076214120093531$ & $\mult\, t\;+$ \\
|
||
&$ 8495653923123431417604973247489272438418190587263600148770280649306958101930$ & $). $
|
||
\end{tabular}
|
||
|
||
$\GenS{1}$ and $\GenS{2}$ are generators of $\GroupS{1}$ and $\GroupS{2}$ respectively.
|
||
|
||
Define $\ItoBEBSP{} \typecolon (\ell \typecolon \Nat) \times \range{0}{2^\ell\!-\!1} \rightarrow
|
||
\bitseq{\ell}$ as in \crossref{endian}.
|
||
|
||
\introlist
|
||
For a point $P \typecolon \GroupSstar{1} = (\xP, \yP)$:
|
||
|
||
\begin{itemize}
|
||
\item The field elements $\xP$ and $\yP \typecolon \GF{\ParamS{q}}$ are represented as
|
||
integers $x$ and $y \typecolon \range{0}{\ParamS{q}\!-\!1}$.
|
||
\item Let $\tilde{y} = \begin{cases}
|
||
1, &\caseif y > \ParamS{q}-y \\
|
||
0, &\caseotherwise.
|
||
\end{cases}$
|
||
\item $P$ is encoded as $\Justthebox{\sonebox}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
For a point $P \typecolon \GroupSstar{2} = (\xP, \yP)$:
|
||
|
||
\begin{itemize}
|
||
\item Define $\FEtoIPP \typecolon \GF{\ParamS{q}}[t] / (t^2 + 1) \rightarrow
|
||
\typeexp{\range{0}{\ParamS{q}\!-\!1}}{2}$ such that
|
||
$\FEtoIPP(a_{w,1} \mult t + a_{w,0}) = [a_{w,1}, a_{w,0}]$.
|
||
\item Let $x = \FEtoIPP(\xP)$, $y = \FEtoIPP(\yP)$, and $y' = \FEtoIPP(-\yP)$.
|
||
\item Let $\tilde{y} = \begin{cases}
|
||
1, &\caseif y > y' \text{ lexicographically} \\
|
||
0, &\caseotherwise.
|
||
\end{cases}$
|
||
\item $P$ is encoded as $\Justthebox{\stwobox}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{Non-normative notes:}
|
||
|
||
\begin{itemize}
|
||
\item The encodings for $\GroupSstar{1, 2}$ are specific to \Zcash.
|
||
\item The points at infinity $\ZeroS{1, 2}$ never occur in proofs and
|
||
have no defined encodings in this protocol.
|
||
\item Algorithms for decompressing points from the encodings of
|
||
$\GroupSstar{1, 2}$ are defined analogously to those for
|
||
$\GroupGstar{1, 2}$ in \crossref{bnpairing}, taking into account that
|
||
the SORT compressed form (not the LSB compressed form) is used
|
||
for $\GroupGstar{1}$.
|
||
\item A rational point $P \neq \ZeroS{2}$ on the curve $\CurveS{2}$ can be
|
||
verified to be of order $\ParamS{r}$, and therefore in $\GroupSstar{2}$,
|
||
by checking that $\ParamS{r} \mult P = \ZeroS{2}$.
|
||
\end{itemize}
|
||
|
||
When computing square roots in $\GF{\ParamS{q}}$ or $\GF{\ParamSexp{q}{2}}$
|
||
in order to decompress a point encoding, the implementation \MUSTNOT assume
|
||
that the square root exists, or that the encoding represents a point on the
|
||
curve.
|
||
}
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\Jubjub} \label{jubjub}
|
||
|
||
The \representedGroup $\JubjubCurve$ is defined in this section.
|
||
|
||
Let $\ParamJ{q} := \ParamS{r}$, as defined in \crossref{blspairing}.
|
||
|
||
Let $\ParamJ{r} := 6554484396890773809930967563523245729705921265872317281365359162392183254199$.
|
||
|
||
(\hairspace $\ParamJ{q}$ and $\ParamJ{r}$ are prime.)
|
||
|
||
Let $\ParamJ{a} := -1$.
|
||
|
||
Let $\ParamJ{d} := -10240/10241 \pmod{\ParamJ{q}}$.
|
||
|
||
Let $\GroupJ$ be the group of points $(u, \varv)$ on a twisted Edwards curve $\CurveJ$
|
||
over $\GF{\ParamJ{q}}$ with equation $\ParamJ{a} \smult u^2 + \varv^2 = 1 + \ParamJ{d} \smult u^2 \smult \varv^2$.
|
||
The zero point with coordinates $(0, 1)$ is denoted $\ZeroJ$.
|
||
$\GroupJ$ has order $8 \smult \ParamJ{r}$.
|
||
|
||
Let $\ellJ := 256$.
|
||
|
||
Define $\ItoLEBSP{} \typecolon (\ell \typecolon \Nat) \times \range{0}{2^\ell\!-\!1} \rightarrow \bitseq{\ell}$
|
||
as in \crossref{endian}.
|
||
|
||
Define $\reprJ \typecolon \GroupJ \rightarrow \bitseq{\ellJ}$ such
|
||
that $\reprJOf{u, \varv} = \ItoLEBSP{256}(\varv + 2^{255} \smult \tilde{u})$, where
|
||
$\tilde{u} = u \bmod 2$.
|
||
|
||
\todo{Representing this as a bit string is problematic because we normally encode
|
||
most-significant-bit first within a byte, so that would result in the wrong
|
||
(i.e. non-standard) encoding as a byte sequence. It's a tricky specification
|
||
problem that we get away with elsewhere in the spec mostly by luck. Maybe keep
|
||
the representation as an integer?}
|
||
|
||
Let $\abstJ \typecolon \bitseq{\ellJ} \rightarrow \GroupJ \union \setof{\bot}$
|
||
be the left inverse of $\reprJ$ such that if $S$ is not in the range of
|
||
$\reprJ$, then $\abstJOf{S} = \bot$.
|
||
|
||
\introlist
|
||
\subparagraph{Non-normative notes:}
|
||
|
||
\begin{itemize}
|
||
\item The encoding of a compressed twisted Edwards point used here is
|
||
consistent with that used in EdDSA \cite{BJLSY2015} for public keys and
|
||
the $R$ element of a signature.
|
||
\item Algorithms for decompressing points from the encoding of
|
||
$\GroupJ$ are given in \cite[``Encoding and parsing curve points'']{BJLSY2015}.
|
||
\end{itemize}
|
||
|
||
When computing square roots in $\GF{\ParamJ{q}}$ in order to decompress a point encoding,
|
||
the implementation \MUSTNOT assume that the square root exists, or that the encoding
|
||
represents a point on the curve.
|
||
|
||
This specification requires ``strict'' parsing as defined in
|
||
\cite[``Encoding and parsing integers'']{BJLSY2015}.
|
||
|
||
Note that algorithms elsewhere in this specification that use $\JubjubCurve$ may impose
|
||
other conditions on points, for example that they are not the zero point, or are in the
|
||
large prime-order subgroup.
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\HashExtractor{} for \Jubjub} \label{concreteextractorjubjub}
|
||
|
||
Let $\SelectuOf{(u, \varv)} = u$ and let $\SelectvOf{(u, \varv)} = \varv$.
|
||
|
||
Let $\ExtractJ \typecolon \GroupJ \rightarrow \GF{\ParamJ{q}}$ be $\Selectu$.
|
||
|
||
Let $G$ be the subgroup of $\GroupJ$ of order $\ParamJ{r}$ (an odd prime).
|
||
|
||
\facts{The point $(0, 1) = \ZeroJ$, and the point $(0, -1)$ has order $2$ in $\GroupJ$.}
|
||
|
||
% <https://github.com/zcash/zcash/issues/2234#issuecomment-333360977>
|
||
\vspace{2ex}
|
||
\begin{lemma*}
|
||
Let $P = (u, \varv) \in G$. Then $(u, -\varv) \notin G$.
|
||
\end{lemma*}
|
||
|
||
\begin{proof}
|
||
If $P = \ZeroJ$ then $(u, -\varv) = (0, -1) \notin G$.
|
||
Else, $P$ is of odd-prime order. Note that $\varv \neq 0$.
|
||
(If $\varv = 0$ then $a \mult u^2 = 1$, and so applying the doubling formula
|
||
gives $\scalarmult{2}{P} = (0, -1)$, then $\scalarmult{4}{P} = (0, 1) = \ZeroJ$;
|
||
contradiction since then $P$ would not be of odd-prime order.)
|
||
Therefore, $-\varv \neq \varv$.
|
||
Now suppose $(u, -\varv) = Q$ is a point in $G$. Then by applying the
|
||
doubling formula we have $\scalarmult{2}{Q} = -\scalarmult{2}{P}$.
|
||
But also $\scalarmult{2}{(-P)} = -\scalarmult{2}{P}$. Therefore either
|
||
$Q = -P$ (then $\SelectvOf{Q} = \SelectvOf{-P}$; contradiction since
|
||
$-\varv \neq \varv$), or doubling is not injective on $G$ (contradiction
|
||
since $G$ is of odd order \cite{KvE2013}).
|
||
\end{proof}
|
||
|
||
\vspace{0.5ex}
|
||
\begin{theorem} \label{thmselectuinjective}
|
||
$\Selectu$ is injective on $G$.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
By writing the curve equation as
|
||
$\varv^2 = (1 - a \smult u^2) / (1 - d \smult u^2)$, and noting that the
|
||
potentially exceptional case $1 - d \smult u^2 = 0$ does not occur for a
|
||
complete twisted Edwards curve, we see that for a given $u$ there can be at
|
||
most two possible solutions for $\varv$, and that if there are two solutions
|
||
they can be written as $\varv$ and $-\varv$. In that case by the Lemma, at
|
||
most one of $(u, \varv)$ and $(u, -\varv)$ is in $G$. Therefore, $\Selectu$
|
||
is injective on points in $G$.
|
||
\end{proof}
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\GroupHash{} into \Jubjub} \label{concretegrouphashjubjub}
|
||
|
||
%Let $\CRS$ be the $64$-byte \commonRandomString given by the $\SHAd$ hash
|
||
%(expressed as an ASCII lowercase hex string in RPC byte order \cite{Bitc-ByteOrder})
|
||
%of the first \block in the eventual consensus \Bitcoin \blockchain having
|
||
%timestamp at or after 2018-03-01 00:00:00 UTC.
|
||
\todo{Define $\CRS$ using the MPC randomness beacon.}
|
||
|
||
Let $\BlakeTwos{256}$ be as defined in \crossref{concreteblake2}.
|
||
|
||
Let $\abstJ$ be as defined in \crossref{jubjub}.
|
||
|
||
Let $D \typecolon \byteseq{8}$ be an $8$-byte domain separator, and
|
||
let $M \typecolon \byteseqs$ be the hash input.
|
||
|
||
The hash $\GroupJHash{\CRS}(D, M)$ is calculated as follows:
|
||
|
||
\newsavebox{\ghintbox}
|
||
\begin{lrbox}{\ghintbox}
|
||
\begin{bytefield}[bitwidth=0.04em]{256}
|
||
\bitbox{256}{256-bit $p$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\begin{formulae}
|
||
\item $\Justthebox{\ghintbox} := \BlakeTwos{256}(D,\, \CRS \bconcat\, M)$
|
||
\item $P := \abstJOf{p}$
|
||
\item If $P = \bot$ then return $\bot$.
|
||
\item $Q := \scalarmult{8}{P}$
|
||
\item If $Q = \ZeroJ$ then return $\bot$, else return $Q$.
|
||
\end{formulae}
|
||
|
||
Define $\ItoLEOSPvar \typecolon \Nat \rightarrow \byteseqs$ as in \crossref{endian}.
|
||
|
||
Define $\first \typecolon (\Nat \rightarrow T \union \setof{\bot}) \rightarrow T$
|
||
so that $\first(f) = f(i)$ where $i$ is the least nonnegative integer
|
||
such that $f(i) \neq \bot$. (For our use of $\first$, such an $i$ always
|
||
exists.)
|
||
|
||
Let $\FindGroupJHashOf{D, M} =
|
||
\first(\fun{i \typecolon \Nat}{\GroupJHash{\CRS}(D, M \bconcat \ItoLEOSPvar(i)) \typecolon \GroupJ})$.
|
||
|
||
\begin{pnotes}
|
||
\item The $\BlakeTwos{256}$ chaining variable after processing $\CRS$
|
||
may be precomputed.
|
||
\item $\FindGroupJHash$ is designed for use with fixed-length $M$.
|
||
If it is reused in a context where $M$ may be variable-length,
|
||
then an encoding of the length of $M$ should be prepended.
|
||
\end{pnotes}
|
||
}
|
||
|
||
|
||
\nsubsubsection{\ZeroKnowledgeProvingSystems}
|
||
|
||
\nsubsubsubsection{\PHGRProvingSystem} \label{phgr}
|
||
|
||
\Zcash uses \zkSNARKs generated by its fork of \libsnark \cite{libsnark-fork}
|
||
with the \provingSystem described in \cite{BCTV2015}, which is a refinement of
|
||
the systems in \cite{PHGR2013} and \cite{BCGTV2013}.
|
||
|
||
A proof consists of a tuple
|
||
$(\Proof{A} \typecolon \GroupGstar{1},\;
|
||
\Proof{A}' \typecolon \GroupGstar{1},\;
|
||
\Proof{B} \typecolon \GroupGstar{2},\;
|
||
\Proof{B}' \typecolon \GroupGstar{1},\;
|
||
\Proof{C} \typecolon \GroupGstar{1},\;
|
||
\Proof{C}' \typecolon \GroupGstar{1},\;
|
||
\Proof{K} \typecolon \GroupGstar{1},\;
|
||
\Proof{H} \typecolon \GroupGstar{1})$.
|
||
It is computed using the parameters above as described in \cite[Appendix B]{BCTV2015}.
|
||
|
||
\pnote{
|
||
Many details of the \provingSystem are beyond the scope of this protocol
|
||
document. For example, the \quadraticArithmeticProgram verifying the \joinSplitStatement,
|
||
or its expression as a \rankOneConstraintSystem, are not specified in this document.
|
||
In practice it will be necessary to use the specific proving and verification keys
|
||
generated for the \Zcash production \blockchain (see \crossref{sproutparameters}),
|
||
and a \provingSystem implementation that is interoperable with the \Zcash fork of
|
||
\libsnark, to ensure compatibility.
|
||
}
|
||
|
||
\introlist
|
||
\subparagraph{\EncodingOfPHGRProofs} \vspace{1ex} \label{phgrencoding}
|
||
|
||
\newsavebox{\phgrbox}
|
||
\begin{lrbox}{\phgrbox}
|
||
\setchanged
|
||
\begin{bytefield}[bitwidth=0.021em]{2368}
|
||
\bitbox{264}{264-bit $\Proof{A}$} &
|
||
\bitbox{264}{264-bit $\Proof{A}'$} &
|
||
\bitbox{520}{520-bit $\Proof{B}$} &
|
||
\bitbox{264}{264-bit $\Proof{B}'$} &
|
||
\bitbox{264}{264-bit $\Proof{C}$} &
|
||
\bitbox{264}{264-bit $\Proof{C}'$} &
|
||
\bitbox{264}{264-bit $\Proof{K}$} &
|
||
\bitbox{264}{264-bit $\Proof{H}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
A $\PHGR$ proof is encoded by concatenating the encodings of its elements:
|
||
|
||
\begin{formulae}[leftmargin=0.2em]
|
||
\item $\Justthebox{\phgrbox}$
|
||
\end{formulae}
|
||
|
||
The resulting proof size is 296 bytes.
|
||
|
||
\vspace{0.8ex}
|
||
\introlist
|
||
In addition to the steps to verify a proof given in \cite[Appendix B]{BCTV2015}, the
|
||
verifier \MUST check, for the encoding of each element, that:
|
||
|
||
\begin{itemize}
|
||
\item the lead byte is of the required form;
|
||
\item the remaining bytes encode a big-endian representation of an integer in
|
||
$\range{0}{\ParamS{q}\!-\!1}$ or (in the case of $\Proof{B}$)
|
||
$\range{0}{\ParamSexp{q}{2}\!-\!1}$;
|
||
\item the encoding represents a point in $\GroupGstar{1}$ or (in the case of
|
||
$\Proof{B}$) $\GroupGstar{2}$, including checking that it is of order
|
||
$\ParamG{r}$ in the latter case.
|
||
\end{itemize}
|
||
|
||
|
||
\newsavebox{\grothbox}
|
||
\begin{lrbox}{\grothbox}
|
||
\setsapling
|
||
\begin{bytefield}[bitwidth=0.021em]{1536}
|
||
\bitbox{384}{384-bit $\Proof{A}$} &
|
||
\bitbox{768}{768-bit $\Proof{B}$} &
|
||
\bitbox{384}{384-bit $\Proof{C}$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\sapling{
|
||
\nsubsubsubsection{\GrothProvingSystem} \label{groth}
|
||
|
||
\Sapling uses \zkSNARKs generated by the \bellman library, with the \provingSystem
|
||
described in \cite{Grot2016}.
|
||
|
||
A proof consists of a tuple
|
||
$(\Proof{A} \typecolon \GroupSstar{1},\;
|
||
\Proof{B} \typecolon \GroupSstar{2},\;
|
||
\Proof{C} \typecolon \GroupSstar{1})$.
|
||
It is computed using the parameters above as described in \cite{Grot2016}.
|
||
|
||
\pnote{
|
||
The \quadraticArithmeticPrograms verifying the \spendStatement and
|
||
\outputStatement are described in \crossref{circuitdesign}. However, many
|
||
other details of the \provingSystem are beyond the scope of this protocol
|
||
document. For example, the expressions of the \spendStatement and \outputStatement
|
||
as \rankOneConstraintSystems are not specified in this document.
|
||
In practice it will be necessary to use the specific proving and verification keys
|
||
generated for the \Zcash production \blockchain (see \crossref{saplingparameters}),
|
||
and a \provingSystem implementation that is interoperable with the \bellman
|
||
library used by \Zcash, to ensure compatibility.
|
||
}
|
||
|
||
\introlist
|
||
\subparagraph{\EncodingOfGrothProofs} \vspace{1ex} \label{grothencoding}
|
||
|
||
A $\Groth$ proof is encoded by concatenating the encodings of its elements:
|
||
|
||
\begin{formulae}[leftmargin=0.2em]
|
||
\item $\Justthebox{\grothbox}$
|
||
\end{formulae}
|
||
|
||
The resulting proof size is 192 bytes.
|
||
|
||
\vspace{0.8ex}
|
||
\introlist
|
||
In addition to the steps to verify a proof given in \cite{Grot2016}, the
|
||
verifier \MUST check, for the encoding of each element, that:
|
||
|
||
\begin{itemize}
|
||
\item the leading bitfield is of the required form;
|
||
\item the remaining bits encode a big-endian representation of an integer
|
||
in $\range{0}{\ParamS{q}\!-\!1}$ or (in the case of $\Proof{B}$) two integers in
|
||
that range;
|
||
\item the encoding represents a point in $\GroupSstar{1}$ or (in the case of $\Proof{B}$)
|
||
$\GroupSstar{2}$, including checking that it is of order $\ParamS{r}$
|
||
in the latter case.
|
||
\end{itemize}
|
||
}
|
||
|
||
\nsubsection{\NotePlaintexts{} and \Memos} \label{notept}
|
||
|
||
As explained in \crossref{noteptconcept}, transmitted \notes are stored on
|
||
the \blockchain in encrypted form.
|
||
|
||
% FIXME duplication with {noteptconcept}.
|
||
|
||
The \notePlaintexts in a \joinSplitDescription are encrypted to the
|
||
respective \transmissionKeys $\TransmitPublicNew{\allNew}$.
|
||
Each \notsprout{\Sprout} \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
||
$(\Value, \NoteAddressRand, \NoteCommitRand\changed{, \Memo})$.
|
||
|
||
\saplingonward{
|
||
The \notePlaintext in each \outputDescription is encrypted to the
|
||
\diversifiedTransmissionKey $\DiversifiedTransmitPublic$.
|
||
Each \Sapling \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
||
$(\Diversifier, \Value, \NoteCommitRand, \Memo)$.
|
||
}
|
||
|
||
\changed{$\Memo$ is a 512-byte \memo associated with this \note.
|
||
|
||
\introlist
|
||
The usage of the \memo is by agreement between the sender and recipient of the
|
||
\note. The \memo{} \SHOULD be encoded either as:
|
||
|
||
\begin{itemize}
|
||
\item a UTF-8 human-readable string \cite{Unicode}, padded by appending zero bytes; or
|
||
\item an arbitrary sequence of 512 bytes starting with a byte value of $\hexint{F5}$
|
||
or greater, which is therefore not a valid UTF-8 string.
|
||
\end{itemize}
|
||
|
||
In the former case, wallet software is expected to strip any trailing zero bytes
|
||
and then display the resulting \mbox{UTF-8} string to the recipient user, where applicable.
|
||
Incorrect UTF-8-encoded byte sequences should be displayed as replacement characters
|
||
(\ReplacementCharacter).
|
||
|
||
In the latter case, the contents of the \memo{} \SHOULDNOT be displayed. A start byte
|
||
of $\hexint{F5}$ is reserved for use by automated software by private agreement.
|
||
A start byte of $\hexint{F6}$ followed by $511$ $\hexint{00}$ bytes means ``no memo''.
|
||
A start byte of $\hexint{F6}$ followed by anything else, or a start byte of $\hexint{F7}$
|
||
or greater, are reserved for use in future \Zcash protocol extensions.
|
||
}
|
||
|
||
Other fields are as defined in \crossref{notes}.
|
||
|
||
\introlist
|
||
The encoding of a \SproutOrNothing \notePlaintext consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.029em]{1672}
|
||
\changed{
|
||
\bitbox{180}{$8$-bit $\NotePlaintextLeadByteSprout$}
|
||
&}\bitbox{180}{$64$-bit $\Value$} &
|
||
\bitbox{256}{$256$-bit $\NoteAddressRand$} &
|
||
\bitbox{256}{\changed{$256$}-bit $\NoteCommitRand$} &
|
||
\changed{\bitbox{800}{$\Memo$ ($512$ bytes)}}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\changed{
|
||
\item A byte, $\NotePlaintextLeadByteSprout$, indicating this version of the
|
||
encoding of a \SproutOrNothing \notePlaintext.
|
||
}
|
||
\item $8$ bytes specifying $\Value$.
|
||
\item $32$ bytes specifying $\NoteAddressRand$.
|
||
\item \changed{32} bytes specifying $\NoteCommitRand$.
|
||
\changed{
|
||
\item $512$ bytes specifying $\Memo$.
|
||
}
|
||
\end{itemize}
|
||
|
||
|
||
\sapling{
|
||
\introlist
|
||
The encoding of a \Sapling \notePlaintext consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.029em]{1672}
|
||
\bitbox{180}{$8$-bit $\NotePlaintextLeadByteSapling$}
|
||
\bitbox{240}{$88$-bit $\Diversifier$}
|
||
\bitbox{180}{$64$-bit $\Value$}
|
||
\bitbox{256}{$256$-bit $\NoteCommitRand$}
|
||
\bitbox{800}{$\Memo$ ($512$ bytes)}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item A byte, $\NotePlaintextLeadByteSapling$, indicating this version of the
|
||
encoding of a \Sapling \notePlaintext.
|
||
\item $11$ bytes specifying $\Diversifier$.
|
||
\item $8$ bytes specifying $\Value$.
|
||
\item $32$ bytes specifying $\NoteCommitRand$.
|
||
\item $512$ bytes specifying $\Memo$.
|
||
\end{itemize}
|
||
|
||
\pnote{
|
||
The encoding of $\Diversifier$ and $\DiversifiedTransmitPublic$ is identical
|
||
to the raw encoding of a \Sapling \paymentAddress in \crossref{saplingpaymentaddrencoding}.
|
||
}
|
||
}
|
||
|
||
|
||
\nsubsection{Encodings of Addresses and Keys} \label{addressandkeyencoding}
|
||
|
||
This section describes how \Zcash encodes \paymentAddresses\changed{, \incomingViewingKeys,}
|
||
and \spendingKeys.
|
||
|
||
Addresses and keys can be encoded as a byte sequence; this is called
|
||
the \term{raw encoding}. This byte sequence can then be further encoded using
|
||
Base58Check. The Base58Check layer is the same as for upstream \Bitcoin
|
||
addresses \cite{Bitc-Base58}.
|
||
|
||
\sapling{
|
||
For \Sapling-specific key and address formats, Bech32 \cite{BIP-173} is used
|
||
instead of Base58Check.
|
||
}
|
||
|
||
$\shaCompress$ outputs are always represented as sequences of $32$ bytes.
|
||
|
||
The language consisting of the following encoding possibilities is prefix-free.
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{\TransparentAddresses} \label{transparentaddrencoding}
|
||
|
||
\xTransparentAddresses are either P2SH (Pay to Script Hash) \cite{BIP-13}
|
||
or P2PKH (Pay to Public Key Hash) \cite{Bitc-P2PKH} addresses.
|
||
|
||
\introlist
|
||
The raw encoding of a P2SH address consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.1em]{176}
|
||
\bitbox{80}{$8$-bit $\PtoSHAddressLeadByte$}
|
||
\bitbox{80}{$8$-bit $\PtoSHAddressSecondByte$}
|
||
\bitbox{160}{$160$-bit script hash}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item Two bytes $[\PtoSHAddressLeadByte, \PtoSHAddressSecondByte]$,
|
||
indicating this version of the raw encoding of a P2SH address
|
||
on the production network. (Addresses on the test network use
|
||
$[\PtoSHAddressTestnetLeadByte, \PtoSHAddressTestnetSecondByte]$
|
||
instead.)
|
||
\item $20$ bytes specifying a script hash \cite{Bitc-P2SH}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
The raw encoding of a P2PKH address consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.1em]{176}
|
||
\bitbox{80}{$8$-bit $\PtoPKHAddressLeadByte$}
|
||
\bitbox{80}{$8$-bit $\PtoPKHAddressSecondByte$}
|
||
\bitbox{160}{$160$-bit public key hash}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item Two bytes $[\PtoPKHAddressLeadByte, \PtoPKHAddressSecondByte]$,
|
||
indicating this version of the raw encoding of a P2PKH address
|
||
on the production network. (Addresses on the test network use
|
||
$[\PtoPKHAddressTestnetLeadByte, \PtoPKHAddressTestnetSecondByte]$
|
||
instead.)
|
||
\item $20$ bytes specifying a public key hash, which is a RIPEMD-160
|
||
hash \cite{RIPEMD160} of a SHA-256 hash \cite{NIST2015}
|
||
of an uncompressed ECDSA key encoding.
|
||
\end{itemize}
|
||
|
||
\begin{pnotes}
|
||
\item In \Bitcoin a single byte is used for the version field identifying
|
||
the address type. In \Zcash two bytes are used. For addresses on
|
||
the production network, this and the encoded length cause the first
|
||
two characters of the Base58Check encoding to be fixed as \ascii{t3}
|
||
for P2SH addresses, and as \ascii{t1} for P2PKH addresses. (This does
|
||
\emph{not} imply that a \transparent \Zcash address can be parsed
|
||
identically to a \Bitcoin address just by removing the \ascii{t}.)
|
||
\item \Zcash does not yet support Hierarchical Deterministic Wallet
|
||
addresses \cite{BIP-32}.
|
||
\end{pnotes}
|
||
|
||
|
||
\nsubsubsection{\Transparent{} Private Keys} \label{transparentkeyencoding}
|
||
|
||
These are encoded in the same way as in \Bitcoin \cite{Bitc-Base58},
|
||
for both the production and test networks.
|
||
|
||
|
||
\nsubsubsection{\SproutOrNothing \PaymentAddresses} \label{sproutpaymentaddrencoding}
|
||
|
||
A \SproutOrNothing \paymentAddress consists of $\AuthPublic \typecolon \PRFOutput$
|
||
and $\TransmitPublic \typecolon \KASproutPublic$.
|
||
|
||
$\AuthPublic$ is a $\shaCompress$ output.
|
||
$\TransmitPublic$ is a $\KASproutPublic$ key (see \crossref{concretesproutkeyagreement}),
|
||
for use with the encryption scheme defined in \crossref{inband}. These
|
||
components are derived from a \spendingKey as described in \crossref{sproutkeycomponents}.
|
||
|
||
\introlist
|
||
The raw encoding of a \SproutOrNothing \paymentAddress consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{520}
|
||
\changed{
|
||
\bitbox{80}{$8$-bit $\PaymentAddressLeadByte$}
|
||
\bitbox{80}{$8$-bit $\PaymentAddressSecondByte$}
|
||
&}\bitbox{256}{$256$-bit $\AuthPublic$} &
|
||
\bitbox{256}{\changed{$256$}-bit $\TransmitPublic$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\changed{
|
||
\item Two bytes $[\PaymentAddressLeadByte, \PaymentAddressSecondByte]$,
|
||
indicating this version of the raw encoding of a \SproutOrZcash \paymentAddress
|
||
on the production network. (Addresses on the test network use
|
||
$[\PaymentAddressTestnetLeadByte, \PaymentAddressTestnetSecondByte]$
|
||
instead.)
|
||
}
|
||
\item $32$ bytes specifying $\AuthPublic$.
|
||
\item \changed{$32$ bytes} specifying $\TransmitPublic$, \changed{using the
|
||
normal encoding of a Curve25519 public key \cite{Bern2006}}.
|
||
\end{itemize}
|
||
|
||
\pnote{
|
||
For addresses on the production network, the lead bytes and encoded length
|
||
cause the first two characters of the Base58Check encoding to be fixed as
|
||
\ascii{zc}. For the test network, the first two characters are fixed as
|
||
\ascii{zt}.
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsection{\Sapling \PaymentAddresses} \label{saplingpaymentaddrencoding}
|
||
|
||
A \Sapling \paymentAddress consists of $\Diversifier \typecolon \DiversifierType$
|
||
and $\DiversifiedTransmitPublic \typecolon \KASaplingPublic$.
|
||
|
||
$\Diversifier$ is a sequence of 11 bytes.
|
||
$\DiversifiedTransmitPublic$ is an encoding of a $\KASaplingPublic$ key
|
||
(see \crossref{concretesaplingkeyagreement}),
|
||
for use with the encryption scheme defined in \crossref{inband}.
|
||
These components are derived as described in \crossref{saplingkeycomponents}.
|
||
|
||
\introlist
|
||
The raw encoding of a \Sapling \paymentAddress consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{344}
|
||
\bitbox{88}{$88$-bit $\Diversifier$}
|
||
\bitbox{256}{$256$-bit $\reprJOf{\DiversifiedTransmitPublic}$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item $11$ bytes specifying $\Diversifier$.
|
||
\item $32$ bytes specifying the compressed Edwards encoding of $\DiversifiedTransmitPublic$
|
||
(see \crossref{jubjub}).
|
||
\end{itemize}
|
||
|
||
For addresses on the production network, the \humanReadablePart is \ascii{zs}.
|
||
For addresses on the test network, the \humanReadablePart is \ascii{ztestsapling}.
|
||
}
|
||
|
||
|
||
\nsubsubsection{\SproutOrNothing \IncomingViewingKeys} \label{sproutinviewingkeyencoding}
|
||
|
||
\changed{
|
||
An \incomingViewingKey consists of $\AuthPublic \typecolon \PRFOutput$ and
|
||
$\TransmitPrivate \typecolon \KASproutPrivate$.
|
||
|
||
$\AuthPublic$ is a $\shaCompress$ output.
|
||
$\TransmitPrivate$ is a $\KASproutPrivate$ key (see \crossref{concretesproutkeyagreement}),
|
||
for use with the encryption scheme defined in \crossref{inband}. These
|
||
components are derived from a \spendingKey as described in \crossref{sproutkeycomponents}.
|
||
|
||
\introlist
|
||
The raw encoding of an \incomingViewingKey consists of, in order:
|
||
}
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.062em]{536}
|
||
\changed{
|
||
\bitbox{88}{$8$-bit $\InViewingKeyLeadByte$}
|
||
\bitbox{88}{$8$-bit $\InViewingKeySecondByte$}
|
||
\bitbox{88}{$8$-bit $\InViewingKeyThirdByte$}
|
||
\bitbox{256}{$256$-bit $\AuthPublic$}
|
||
\bitbox{256}{$256$-bit $\TransmitPrivate$}
|
||
}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\changed{
|
||
\begin{itemize}
|
||
\item Three bytes $[\InViewingKeyLeadByte, \InViewingKeySecondByte, \InViewingKeyThirdByte]$,
|
||
indicating this version of the raw encoding of a \Zcash \incomingViewingKey
|
||
on the production network. (Addresses on the test network use
|
||
$[\InViewingKeyTestnetLeadByte, \InViewingKeyTestnetSecondByte, \InViewingKeyTestnetThirdByte]$
|
||
instead.)
|
||
\item $32$ bytes specifying $\AuthPublic$.
|
||
\item $32$ bytes specifying $\TransmitPrivate$, using the normal encoding
|
||
of a Curve25519 private key \cite{Bern2006}.
|
||
\end{itemize}
|
||
|
||
$\TransmitPrivate$ \MUST be ``clamped'' using $\KASproutFormatPrivate$ as specified
|
||
in \crossref{sproutkeycomponents}. That is, a decoded \incomingViewingKey{} \MUST be
|
||
considered invalid if $\TransmitPrivate \neq \KASproutFormatPrivate(\TransmitPrivate)$.
|
||
($\KASproutFormatPrivate$ is defined in \crossref{concretesproutkeyagreement}.)
|
||
|
||
\pnote{
|
||
For addresses on the production network, the lead bytes and encoded length
|
||
cause the first four characters of the Base58Check encoding to be fixed as
|
||
\ascii{ZiVK}. For the test network, the first four characters are fixed as
|
||
\ascii{ZiVt}.
|
||
}
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsection{\Sapling \IncomingViewingKeys} \label{saplinginviewingkeyencoding}
|
||
|
||
A \Sapling \incomingViewingKey consists of $\InViewingKey \typecolon \KASproutPrivate$
|
||
(see \crossref{concretesaplingkeyagreement}).
|
||
|
||
$\InViewingKey$ is a $\KASproutPrivate$ key for use with the encryption scheme
|
||
defined in \crossref{inband}. It is derived as described in \crossref{saplingkeycomponents}.
|
||
|
||
\introlist
|
||
The raw encoding of an \incomingViewingKey consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{256}
|
||
\bitbox{256}{$256$-bit $\InViewingKey$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item $32$ bytes specifying $\InViewingKey$.
|
||
\end{itemize}
|
||
|
||
$\InViewingKey$ \MUST be in the range $\range{0}{\JubjubScalarThreshold-1}$ as specified
|
||
in \crossref{saplingkeycomponents}. That is, a decoded \incomingViewingKey{} \MUST be
|
||
considered invalid if $\InViewingKey$ is not in this range.
|
||
|
||
For \incomingViewingKeys on the production network, the \humanReadablePart is \ascii{zivks}.
|
||
For \incomingViewingKeys on the test network, the \humanReadablePart is \ascii{zivktestsapling}.
|
||
}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsection{\Sapling \FullViewingKeys} \label{saplingfullviewingkeyencoding}
|
||
|
||
A \Sapling \fullViewingKey consists of $\AuthSignPublic \typecolon \GroupJ$
|
||
and $\AuthProvePublic \typecolon \GroupJ$.
|
||
|
||
$\AuthSignPublic$ and $\AuthProvePublic$ are points on the $\JubjubCurve$ curve
|
||
(see \crossref{jubjub}). They are derived as described in \crossref{saplingkeycomponents}.
|
||
|
||
\introlist
|
||
The raw encoding of a \fullViewingKey consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{512}
|
||
\bitbox{256}{$256$-bit $\reprJOf{\AuthSignPublic}$}
|
||
\bitbox{256}{$256$-bit $\reprJOf{\AuthProvePublic}$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item $32$ bytes specifying the compressed Edwards encoding of $\AuthSignPublic$
|
||
(see \crossref{jubjub}).
|
||
\item $32$ bytes specifying the compressed Edwards encoding of $\AuthProvePublic$.
|
||
\end{itemize}
|
||
|
||
When decoding this representation, the key is not valid if $\abstJ$ returns $\bot$
|
||
for either point.
|
||
|
||
For \incomingViewingKeys on the production network, the \humanReadablePart is \ascii{zviews}.
|
||
For \incomingViewingKeys on the test network, the \humanReadablePart is \ascii{zviewtestsapling}.
|
||
}
|
||
|
||
|
||
\nsubsubsection{\SproutOrNothing \SpendingKeys} \label{sproutspendingkeyencoding}
|
||
|
||
A \SproutOrNothing \spendingKey consists of $\AuthPrivate$, which is a sequence of
|
||
\changed{$252$} bits (see \crossref{sproutkeycomponents}).
|
||
|
||
\introlist
|
||
The raw encoding of a \SproutOrNothing \spendingKey consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{264}
|
||
\changed{
|
||
\bitbox{80}{$8$-bit $\SpendingKeyLeadByte$}
|
||
\bitbox{80}{$8$-bit $\SpendingKeySecondByte$}
|
||
\bitbox{32}{$\zeros{4}$} &
|
||
&}\bitbox{252}{\changed{$252$}-bit $\AuthPrivate$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\changed{
|
||
\item Two bytes $[\SpendingKeyLeadByte, \SpendingKeySecondByte]$,
|
||
indicating this version of the raw encoding of a \Zcash \spendingKey
|
||
on the production network. (Addresses on the test network use
|
||
$[\SpendingKeyTestnetLeadByte, \SpendingKeyTestnetSecondByte]$
|
||
instead.)
|
||
}
|
||
\item $32$ bytes: \changed{$4$ zero padding bits and $252$ bits} specifying $\AuthPrivate$.
|
||
\end{itemize}
|
||
|
||
\changed{
|
||
The zero padding occupies the most significant 4 bits of the third byte.
|
||
}
|
||
|
||
\begin{pnotes}
|
||
\changed{
|
||
\item If an implementation represents $\AuthPrivate$ internally as a
|
||
sequence of $32$ bytes with the $4$ bits of zero padding intact,
|
||
it will be in the correct form for use as an input to $\PRFaddr{}$,
|
||
$\PRFnf{}$, and $\PRFpk{}$ without need for bit-shifting.
|
||
Future key representations may make use of these padding bits.
|
||
}
|
||
\item For addresses on the production network, the lead bytes and encoded
|
||
length cause the first two characters of the Base58Check encoding to
|
||
be fixed as \ascii{SK}. For the test network, the first two characters
|
||
are fixed as \ascii{ST}.
|
||
\end{pnotes}
|
||
|
||
|
||
\sapling{
|
||
\nsubsubsection{\Sapling \SpendingKeys} \label{saplingspendingkeyencoding}
|
||
|
||
A \Sapling \spendingKey consists of $\AuthPrivateSeed \typecolon \bitseq{\AuthPrivateSeedLength}$
|
||
(see \crossref{sproutkeycomponents}).
|
||
|
||
\introlist
|
||
The raw encoding of a \Sapling \spendingKey consists of:
|
||
\vspace{2ex}
|
||
\begin{equation*}
|
||
\begin{bytefield}[bitwidth=0.07em]{256}
|
||
\bitbox{256}{$256$-bit $\AuthPrivateSeed$}
|
||
\end{bytefield}
|
||
\end{equation*}
|
||
|
||
\begin{itemize}
|
||
\item $32$ bytes specifying $\AuthPrivateSeed$.
|
||
\end{itemize}
|
||
|
||
For \spendingKeys on the production network, the \humanReadablePart is \ascii{secret-spending-key-main}.
|
||
For \spendingKeys on the test network, the \humanReadablePart is \ascii{secret-spending-key-test}.
|
||
}
|
||
|
||
|
||
\introlist
|
||
\nsubsection{\SproutZKParameters} \label{sproutparameters}
|
||
|
||
For the \Zcash production \blockchain and testnet, the $\SHAFull$ hashes of the
|
||
\provingKey and \verifyingKey for the \SproutOrZcash \joinSplitStatement, encoded in
|
||
\libsnark format, are:
|
||
|
||
\begin{lines}
|
||
\item[] \texttt{8bc20a7f013b2b58970cddd2e7ea028975c88ae7ceb9259a5344a16bc2c0eef7 sprout-proving.key}
|
||
\item[] \texttt{4bd498dae0aacfd8e98dc306338d017d9c08dd0918ead18172bd0aec2fc5df82 sprout-verifying.key}
|
||
\end{lines}
|
||
|
||
These parameters were obtained by a multi-party computation described in
|
||
\cite{GitHub-mpc} and \cite{BGG2016}.
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsection{\SaplingZKParameters} \label{saplingparameters}
|
||
|
||
The $\SHAFull$ hashes of the \provingKey and \verifyingKey for the \Sapling
|
||
\spendStatement, encoded in \bellman format, are:
|
||
|
||
\begin{lines}
|
||
\item[] \texttt{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx sapling-spend-proving.key}
|
||
\item[] \texttt{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx sapling-spend-verifying.key}
|
||
\end{lines}
|
||
|
||
The $\SHAFull$ hashes of the \provingKey and \verifyingKey for the \Sapling
|
||
\outputStatement, encoded in \bellman format, are:
|
||
|
||
\begin{lines}
|
||
\item[] \texttt{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx sapling-output-proving.key}
|
||
\item[] \texttt{xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx sapling-output-verifying.key}
|
||
\end{lines}
|
||
|
||
These parameters were obtained by a multi-party computation described in \todo{}.
|
||
}
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsection{\Sapling Transition} \label{saplingtransition}
|
||
|
||
\todo{Separate this out into a ZIP that describes the bilateral
|
||
hard fork strategy generally, and then describe the \Sapling
|
||
transition as an instance of that.}
|
||
|
||
\Zcash launched with a protocol revision that we call \Sprout.
|
||
At the time of writing, two upgrades are planned: \NUZero, and
|
||
\Sapling. This section summarizes the planned strategy for upgrading
|
||
from \Sprout to \NUZero and then \Sapling.
|
||
|
||
The upgrade mechanism is described in \cite{ZIP-200}.
|
||
The content of the \NUZero upgrade is described in \cite{ZIP-201},
|
||
\cite{ZIP-202}, \cite{ZIP-203}, and \cite{ZIP-143}.
|
||
|
||
\NUZero and \Sapling will each be introduced in a
|
||
``bilateral hard fork''. In this kind of fork,
|
||
|
||
\begin{enumerate}
|
||
\item there is a \blockHeight at which the fork takes effect;
|
||
\item \blocks and \transactions that are valid according to
|
||
the post-fork rules are not valid before the forking \block;
|
||
\item \blocks and \transactions that are valid according to
|
||
the pre-fork rules are no longer valid in or after the
|
||
forking \block.
|
||
\end{enumerate}
|
||
|
||
Full support for each upgrade is indicated by a minimum version
|
||
of the peer-to-peer protocol. Before the planned fork \blockHeight,
|
||
nodes that support \Sapling will disconnect from (and will not
|
||
reconnect to) nodes with a protocol version lower than this
|
||
minimum.
|
||
|
||
This ensures that \Sapling-supporting nodes transition cleanly
|
||
from the old protocol to the new protocol. Nodes that do not
|
||
support \Sapling will find themselves, in advance of the fork,
|
||
on a network that uses the old protocol and is fully partitioned
|
||
from the \Sapling-supporting network.
|
||
|
||
This allows us to specify arbitrary protocol changes that
|
||
take effect at a given \blockHeight. Note, however, that a
|
||
\blockchain reorganization across a forking \block is possible.
|
||
In the case of such a reorganization, \blocks at a height
|
||
before the forking \blockHeight will still be created and
|
||
validated according to the pre-\Sapling rules, and
|
||
\Sapling-supporting nodes \MUST allow for this.
|
||
However, once a node has seen 99 valid \blocks on top of a
|
||
forking \block, it may assume that the fork is ``locked in''
|
||
and need not support reorganizations that roll back to before
|
||
that forking \block.
|
||
|
||
For the \Sapling hard fork (but not necessarily for bilateral
|
||
hard forks in general), both the \blockVersionNumber and the
|
||
\transactionVersionNumber are set to a value that was invalid
|
||
according to the pre-\Sapling rules. (They are actually
|
||
decreased, not increased.) The pre-\Sapling \blockVersionNumber
|
||
is not accepted for the forking \block or subsequent blocks,
|
||
and the pre-\Sapling \transactionVersionNumber is not accepted
|
||
for \transactions in such blocks.
|
||
|
||
A new \Sapling \nullifierSet and \noteCommitmentTree are created
|
||
for use by \Sapling \transactions.
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsection{Consensus Changes from \Bitcoin}
|
||
|
||
\nsubsection{Encoding of \Transactions} \label{txnencoding}
|
||
|
||
\nuzero{\pnote{This section has not yet been updated for v3 transactions; see ZIP 202.}}
|
||
|
||
The \Zcash \transaction format is as follows:
|
||
|
||
\begin{center}
|
||
\hbadness=10000
|
||
\begin{tabularx}{0.92\textwidth}{|c|l|p{10.7em}|X|}
|
||
\hline
|
||
Bytes & \heading{Name} & \heading{Data Type} & \heading{Description} \\
|
||
\hhline{|=|=|=|=|}
|
||
|
||
$4$ & $\versionField$ & \type{int32\_t} & Transaction version number; either $1$ or $2$. \\ \hline
|
||
|
||
\Varies & $\txInCount$ & \compactSize & Number of \transparent inputs in this transaction. \\ \hline
|
||
|
||
\Varies & $\txIn$ & $\txIn$ & \xTransparent inputs, encoded as in \Bitcoin. \\ \hline
|
||
|
||
\Varies & $\txOutCount$ & \compactSize & Number of \transparent outputs in this transaction. \\ \hline
|
||
|
||
\Varies & $\txOut$ & $\txOut$ & \xTransparent outputs, encoded as in \Bitcoin. \\ \hline
|
||
|
||
$4$ & $\lockTime$ & \type{uint32\_t} & A Unix epoch time (UTC) or block number, encoded as in \Bitcoin. \\ \hline
|
||
|
||
\Varies\;$\dagger$ & $\nJoinSplit$ & \compactSize & The number of \joinSplitDescriptions
|
||
in $\vJoinSplit$. \\ \hline
|
||
|
||
\Longunderstack{$1802 \mult$ \\ $\nJoinSplit\,\dagger$} & $\vJoinSplit$ & \type{JoinSplitDescription} \type{[$\nJoinSplit$]} &
|
||
A \sequenceOfJoinSplitDescriptions, each encoded as described in \crossref{joinsplitencoding}. \\ \hline
|
||
|
||
$32$ $\ddagger$ & $\joinSplitPubKey$ & \type{char[32]} & An encoding of a $\JoinSplitSig$
|
||
public verification key. \\ \hline
|
||
|
||
$64$ $\ddagger$ & $\joinSplitSig$ & \type{char[64]} & A signature on a prefix of the \transaction encoding,
|
||
to be verified using $\joinSplitPubKey$. \\ \hline
|
||
\end{tabularx}
|
||
\end{center}
|
||
|
||
$\dagger$ The $\nJoinSplit$ and $\vJoinSplit$ fields are present if and only if
|
||
$\versionField > 1$.
|
||
|
||
$\ddagger$ The $\joinSplitPubKey$ and $\joinSplitSig$ fields are present if and only if
|
||
$\versionField > 1$ and $\nJoinSplit > 0$.
|
||
|
||
The encoding of $\joinSplitPubKey$ and the data to be signed are specified in
|
||
\crossref{nonmalleability}.
|
||
|
||
\begin{consensusrules}
|
||
\sproutonlyitem{The \transactionVersionNumber{} \MUST be greater than or equal to $1$.}
|
||
\notsprout{
|
||
\sproutonlyitem{The $\fOverwintered$ flag \MUSTNOT be set.}
|
||
}
|
||
\nuzeroonlyitem{The \transactionVersionNumber{} \MUST be $3$. \todo{is this a consensus rule?}}
|
||
\saplingonwarditem{The \transactionVersionNumber{} \MUST be $3$ or $4$. \todo{is this a consensus rule?}}
|
||
\item If $\versionField = 1$ or $\nJoinSplit = 0$, then \txInCount{} \MUSTNOT be $0$.
|
||
\item A \transaction with one or more inputs from \coinbaseTransactions{} \MUST have no
|
||
\transparent outputs (i.e.\ \txOutCount{} \MUST be $0$).
|
||
\item If $\nJoinSplit > 0$, then \joinSplitSig{} \MUST represent a valid signature
|
||
over $\dataToBeSigned$ as defined in \crossref{nonmalleability}.
|
||
\item If $\nJoinSplit > 0$, then \joinSplitPubKey{} \MUST represent a valid
|
||
$\JoinSplitSigSpecific$ public key encoding as specified in \crossref{concretejssig}.
|
||
\sproutonlyitem{The encoded size of the \transaction{} \MUST be less than or equal to
|
||
$100000$ bytes.}
|
||
\item A \coinbaseTransaction{} \MUSTNOT have any
|
||
\joinSplitDescriptions\sapling{, \spendDescriptions, or \outputDescriptions}.
|
||
\item A \transaction{} \MUSTNOT spend an output of a \coinbaseTransaction
|
||
(necessarily a \transparent output) from a \block less than 100 \blocks prior
|
||
to the spend.
|
||
\item \todo{Other rules inherited from \Bitcoin.}
|
||
\end{consensusrules}
|
||
|
||
\begin{pnotes}
|
||
\item The semantics of \transactions with \transactionVersionNumber not equal to\sprout{
|
||
either $1$ or $2$ is not currently defined. Miners \MUSTNOT create \blocks
|
||
containing such \transactions.
|
||
}\notsprout{
|
||
$1$, $2$, \nuzero{$3$,}\sapling{ or $4$} is not currently defined.
|
||
Miners \MUSTNOT create \blocks before the \NUZero \activationHeight
|
||
containing \transactions with version other than $1$ or $2$.
|
||
}
|
||
\item The exclusion of \transactions with \transactionVersionNumber
|
||
\emph{greater than} $2$ is not a consensus rule\notsprout{ before \NUZero activation}.
|
||
Such \transactions may exist in the \blockchain and \MUST be treated
|
||
identically to version $2$ \transactions.
|
||
\nuzeroonwarditem{Once \NUZero has activated, limits on the maximum
|
||
\transactionVersionNumber are consensus rules.}
|
||
\item Note that a future hard fork might use \emph{any} \transactionVersionNumber.
|
||
It is likely that a hard fork that changes the \transactionVersionNumber
|
||
will also change the \transaction format, and software that parses
|
||
\transactions{} \SHOULD take this into account.
|
||
\sprout{
|
||
\item The $\versionField$ field is a signed integer. (It was incorrectly specified
|
||
as unsigned in a previous version of this specification.) A future hard fork
|
||
might \sprout{use negative values for this field, or otherwise} change its
|
||
interpretation.
|
||
}
|
||
\nuzero{
|
||
\item \todo{Describe interpretation of $\fOverwintered$ and $\versionField$.}
|
||
}
|
||
\item A \transactionVersionNumber of $2$ does not have the same meaning as in
|
||
\Bitcoin, where it is associated with support for \ScriptOP{CHECKSEQUENCEVERIFY}
|
||
as specified in \cite{BIP-68}. \Zcash was forked from \Bitcoin v0.11.2
|
||
and does not currently support BIP 68, or the related BIPs 9, 112 and 113.
|
||
\end{pnotes}
|
||
|
||
\introlist
|
||
The changes relative to \Bitcoin version $1$ \transactions as described in \cite{Bitc-Format} are:
|
||
|
||
\begin{itemize}
|
||
\item \Transactionversion $0$ is not supported.
|
||
\item A version $1$ \transaction is equivalent to a version $2$ \transaction with
|
||
$\nJoinSplit = 0$.
|
||
\item The $\nJoinSplit$, $\vJoinSplit$, $\joinSplitPubKey$, and $\joinSplitSig$ fields
|
||
have been added.
|
||
\item In \Zcash it is permitted for a \transaction to have no \transparent inputs provided
|
||
that $\nJoinSplit > 0$.
|
||
\item A consensus rule limiting \transaction size has been added. In \Bitcoin there is
|
||
a corresponding standard rule but no consensus rule.
|
||
\end{itemize}
|
||
|
||
\sproutonly{
|
||
Software that creates \transactions{} \SHOULD use version $1$ for \transactions with no
|
||
\joinSplitDescriptions.
|
||
}
|
||
|
||
\introsection
|
||
\nsubsection{Encoding of \JoinSplitDescriptions} \label{joinsplitencoding}
|
||
|
||
An abstract \joinSplitDescription, as described in \crossref{joinsplit}, is encoded in
|
||
a \transaction as an instance of a \type{JoinSplitDescription} type as follows:
|
||
|
||
\begin{center}
|
||
\hbadness=2000
|
||
\begin{tabularx}{0.92\textwidth}{|c|l|l|X|}
|
||
\hline
|
||
Bytes & \heading{Name} & \heading{Data Type} & \heading{Description} \\
|
||
\hhline{|=|=|=|=|}
|
||
|
||
\setchanged 8 &\setchanged $\vpubOldField$ &\setchanged \type{uint64\_t} &\mbox{}\setchanged
|
||
A value $\vpubOld$ that the \joinSplitTransfer removes from the \transparentValuePool. \\ \hline
|
||
|
||
$8$ & $\vpubNewField$ & \type{uint64\_t} & A value $\vpubNew$ that the \joinSplitTransfer inserts
|
||
into the \transparentValuePool. \\ \hline
|
||
|
||
$32$ & $\anchorField$ & \type{char[32]} & A merkle root $\rt$ of the \SproutOrNothing
|
||
\noteCommitmentTree at some \blockHeight in the past, or the merkle root produced by a previous
|
||
\joinSplitTransfer in this \transaction. \\ \hline
|
||
|
||
$64$ & $\nullifiersField$ & \type{char[32][$\NOld$]} & A sequence of \nullifiers of the input
|
||
\notes $\nfOld{\allOld}$. \\ \hline
|
||
|
||
$64$ & $\commitments$ & \type{char[32][$\NNew$]} & A sequence of \noteCommitments for the
|
||
output \notes $\cmNew{\allNew}$. \\ \hline
|
||
|
||
\setchanged $32$ &\setchanged $\ephemeralKey$ &\setchanged \type{char[32]} &\mbox{}\setchanged
|
||
A Curve25519 public key $\EphemeralPublic$. \\ \hline
|
||
|
||
\setchanged $32$ &\setchanged $\randomSeed$ &\setchanged \type{char[32]} &\mbox{}\setchanged
|
||
A $256$-bit seed that must be chosen independently at random for each \joinSplitDescription. \\ \hline
|
||
|
||
$64$ & $\vmacs$ & \type{char[32][$\NOld$]} & A sequence of message authentication tags
|
||
$\h{\allOld}$ that bind $\hSig$ to each $\AuthPrivate$ of the
|
||
$\joinSplitDescription$. \\ \hline
|
||
|
||
$296$ & $\zkproof$ & \type{char[296]} & An encoding of the \zeroKnowledgeProof
|
||
$\ProofJoinSplit$ (see \crossref{phgr}). \\ \hline
|
||
|
||
$1202$ & $\encCiphertexts$ & \type{char[601][$\NNew$]} & A sequence of ciphertext
|
||
components for the encrypted output \notes, $\TransmitCiphertext{\allNew}$. \\ \hline
|
||
|
||
\end{tabularx}
|
||
\end{center}
|
||
|
||
The $\vmacs$ field encodes $\h{\allOld}$ which are computed as described in
|
||
\crossref{nonmalleability}.
|
||
|
||
The $\ephemeralKey$ and $\encCiphertexts$ fields together form the \notesCiphertext,
|
||
which is computed as described in \crossref{inband}.
|
||
|
||
Consensus rules applying to a \joinSplitDescription are given in \crossref{joinsplitdesc}.
|
||
|
||
|
||
\sapling{
|
||
\introsection
|
||
\nsubsection{Encoding of \SpendDescriptions} \label{spendencoding}
|
||
|
||
An abstract \spendDescription, as described in \crossref{spendsandoutputs}, is encoded in
|
||
a \transaction as an instance of a \type{SpendDescription} type as follows:
|
||
|
||
\begin{center}
|
||
\hbadness=2000
|
||
\begin{tabularx}{0.92\textwidth}{|c|l|l|X|}
|
||
\hline
|
||
Bytes & \heading{Name} & \heading{Data Type} & \heading{Description} \\
|
||
\hhline{|=|=|=|=|}
|
||
|
||
$32$ & $\cvField$ & \type{char[32]} & A \valueCommitment to the value of the input \note. \\ \hline
|
||
|
||
$32$ & $\anchorField$ & \type{char[32]} & A merkle root $\rt$ of the \Sapling
|
||
\noteCommitmentTree at some \blockHeight in the past. \\ \hline
|
||
|
||
$32$ & $\nullifierField$ & \type{char[32]} & The \nullifier of the input \note, $\nf$. \\ \hline
|
||
|
||
$192$ & $\zkproof$ & \type{char[192]} & An encoding of the \zeroKnowledgeProof
|
||
$\ProofSpend$ (see \crossref{groth}). \\ \hline
|
||
|
||
$64$ & $\spendAuthSig$ & \type{char[64]} & A signature authorizing this spend. \\ \hline
|
||
|
||
\end{tabularx}
|
||
\end{center}
|
||
|
||
Consensus rules applying to a \spendDescription are given in \crossref{spenddesc}.
|
||
|
||
|
||
\introsection
|
||
\nsubsection{Encoding of \OutputDescriptions} \label{outputencoding}
|
||
|
||
An abstract \outputDescription, as described in \crossref{spendsandoutputs}, is encoded in
|
||
a \transaction as an instance of an \type{OutputDescription} type as follows:
|
||
|
||
\begin{center}
|
||
\hbadness=2000
|
||
\begin{tabularx}{0.92\textwidth}{|c|l|l|X|}
|
||
\hline
|
||
Bytes & \heading{Name} & \heading{Data Type} & \heading{Description} \\
|
||
\hhline{|=|=|=|=|}
|
||
|
||
$32$ & $\cvField$ & \type{char[32]} & A \valueCommitment to the value of the output \note. \\ \hline
|
||
|
||
$32$ & $\cmField$ & \type{char[32]} & The \noteCommitment for the output \note, $\cm$. \\ \hline
|
||
|
||
$32$ & $\ephemeralKey$ & \type{char[32]} & A $\JubjubCurve$ public key $\EphemeralPublic$. \\ \hline
|
||
|
||
$580$ & $\encCiphertext$ & \type{char[580]} & A ciphertext component for the
|
||
encrypted output \note, $\TransmitCiphertext{}$. \\ \hline
|
||
|
||
$192$ & $\zkproof$ & \type{char[192]} & An encoding of the \zeroKnowledgeProof
|
||
$\ProofOutput$ (see \crossref{groth}). \\ \hline
|
||
|
||
\end{tabularx}
|
||
\end{center}
|
||
|
||
The $\ephemeralKey$ and $\encCiphertext$ fields together form the \noteCiphertext,
|
||
which is computed as described in \crossref{inband}.
|
||
|
||
Consensus rules applying to an \outputDescription are given in \crossref{outputdesc}.
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsubsection{\BlockHeader} \label{blockheader}
|
||
|
||
The \Zcash \blockHeader format is as follows:
|
||
|
||
\begin{center}
|
||
\hbadness=2500
|
||
\begin{tabularx}{0.92\textwidth}{|c|l|p{8.6em}|X|}
|
||
\hline
|
||
Bytes & \heading{Name} & \heading{Data Type} & \heading{Description} \\
|
||
\hhline{|=|=|=|=|}
|
||
|
||
$4$ & $\nVersion$ & \type{int32\_t} & The \blockVersionNumber indicates which set of
|
||
\block validation rules to follow. The current and only defined \blockVersionNumber
|
||
for \Zcash is $4$. \\ \hline
|
||
|
||
$32$ & $\hashPrevBlock$ & \type{char[32]} & A $\SHAd$ hash in internal byte order of the
|
||
previous \block's \header. This ensures no previous \block can be changed without also
|
||
changing this \block's \header. \\ \hline
|
||
|
||
$32$ & $\hashMerkleRoot$ & \type{char[32]} & A $\SHAd$ hash in internal byte order. The
|
||
merkle root is derived from the hashes of all \transactions included in this \block,
|
||
ensuring that none of those \transactions can be modified without modifying the \header. \\ \hline
|
||
|
||
$32$ & \sprout{$\hashReserved$}
|
||
\notsprout{\Longunderstack[l]{$\hashReserved$ /\\ \sapling{$\hashFinalSaplingRoot$}}} &
|
||
\type{char[32]} &
|
||
\sproutonly{A reserved field which should be ignored.}
|
||
\saplingonward{A \merkleRoot (\todo{specify bit sequence to byte sequence conversion}) of the \Sapling{}
|
||
\noteCommitmentTree corresponding to the final \Sapling{} \treestate of this \block.} \\ \hline
|
||
|
||
$4$ & $\nTimeField$ & \type{uint32\_t} & The \blockTime is a Unix epoch time (UTC) when the miner
|
||
started hashing the \header (according to the miner). \\ \hline
|
||
|
||
$4$ & $\nBitsField$ & \type{uint32\_t} & An encoded version of the \targetThreshold this \block's
|
||
\header hash must be less than or equal to, in the same nBits format used by \Bitcoin.
|
||
\cite{Bitc-nBits} \\ \hline
|
||
|
||
$32$ & $\nNonce$ & \type{char[32]} & An arbitrary field miners change to modify the
|
||
\header hash in order to produce a hash less than or equal to the \targetThreshold. \\ \hline
|
||
|
||
$3$ & $\solutionSize$ & \compactSize & The size of an Equihash solution in bytes (always $1344$). \\ \hline
|
||
|
||
$1344$ & $\solution$ & \type{char[1344]} & The Equihash solution. \\ \hline
|
||
|
||
\end{tabularx}
|
||
\end{center}
|
||
|
||
A \block consists of a \blockHeader and a sequence of \transactions. How transactions
|
||
are encoded in a \block is part of the Zcash peer-to-peer protocol but not part of
|
||
the consensus protocol.
|
||
|
||
Let $\ThresholdBits$ be as defined in \crossref{diffadjustment}, and let $\PoWMedianBlockSpan$
|
||
be the constant defined in \crossref{constants}.
|
||
|
||
\begin{consensusrules}
|
||
\item The \blockVersionNumber{} \MUST be greater than or equal to $4$.
|
||
\item For a \block at \blockHeight $\BlockHeight$, $\nBitsField$ \MUST be equal to
|
||
$\ThresholdBits(\BlockHeight)$.
|
||
\item The \block{} \MUST pass the difficulty filter defined in \crossref{difficulty}.
|
||
\item $\solution$ \MUST represent a valid Equihash solution as defined in \crossref{equihash}.
|
||
\item $\nTimeField$ \MUST be strictly greater than the median time of the previous
|
||
$\PoWMedianBlockSpan$ \blocks.
|
||
\item The size of a \block{} \MUST be less than or equal to $2000000$ bytes.
|
||
\saplingonwarditem{$\hashFinalSaplingRoot$ \MUST be the \merkleRoot of the
|
||
\Sapling{} \noteCommitmentTree for the final \Sapling{} \treestate
|
||
of this \block.}
|
||
\item \todo{Other rules inherited from \Bitcoin.}
|
||
\end{consensusrules}
|
||
|
||
In addition, a \fullValidator{} \MUSTNOT accept \blocks with $\nTimeField$ more than two hours
|
||
in the future according to its clock. This is not strictly a consensus rule because it is
|
||
nondeterministic, and clock time varies between nodes. Also note that a \block that is
|
||
rejected by this rule at a given point in time may later be accepted.
|
||
|
||
\begin{pnotes}
|
||
\item The semantics of blocks with \blockVersionNumber{} not equal to $4$
|
||
is not currently defined. Miners \MUSTNOT create such \blocks, and
|
||
\SHOULDNOT mine other blocks on top of them.
|
||
\item The exclusion of \blocks with \blockVersionNumber{} \emph{greater than} $4$
|
||
is not a consensus rule; such \blocks may exist in the \blockchain
|
||
and \MUST be treated identically to version $4$ \blocks by \fullValidators.
|
||
Note that a future hard fork might use \blockVersionNumber{} either
|
||
greater than or less than $4$. It is likely that such a hard fork will
|
||
change the \block header and/or \transaction format, and software that
|
||
parses \blocks{} \SHOULD take this into account.
|
||
\item The $\nVersion$ field is a signed integer. (It was incorrectly specified
|
||
as unsigned in a previous version of this specification.) A future
|
||
hard fork might use negative values for this field, or otherwise change
|
||
its interpretation.
|
||
\item There is no relation between the values of the $\versionField$ field of a \transaction,
|
||
and the $\nVersion$ field of a \blockHeader.
|
||
\item Like other serialized fields of type $\compactSize$, the $\solutionSize$ field \MUST
|
||
be encoded with the minimum number of bytes ($3$ in this case), and other encodings
|
||
\MUST be rejected. This is necessary to avoid a potential attack in which a miner
|
||
could test several distinct encodings of each Equihash solution against the difficulty
|
||
filter, rather than only the single intended encoding.
|
||
\item As in \Bitcoin, the $\nTimeField$ field \MUST represent a time \emph{strictly greater than}
|
||
the median of the timestamps of the past $\PoWMedianBlockSpan$ \blocks. The
|
||
Bitcoin Developer Reference \cite{Bitc-Block} was previously in error on this point,
|
||
but has now been corrected.
|
||
\nuzero{
|
||
\item There are no changes to the \blockVersionNumber or format for \NUZero.
|
||
}
|
||
\sapling{
|
||
\item Although the \blockVersionNumber does not change for \Sapling,
|
||
the previously reserved (and ignored) field $\hashReserved$ has been
|
||
repurposed for $\hashFinalSaplingRoot$. There are no other format changes.
|
||
}
|
||
\end{pnotes}
|
||
|
||
\introlist
|
||
The changes relative to \Bitcoin version $4$ blocks as described in \cite{Bitc-Block} are:
|
||
|
||
\begin{itemize}
|
||
\item \Blockversions less than $4$ are not supported.
|
||
\item The $\hashReserved$\sapling{ (or $\hashFinalSaplingRoot$)}, $\solutionSize$, and
|
||
$\solution$ fields have been added.
|
||
\item The type of the $\nNonce$ field has changed from \type{uint32\_t} to \type{char[32]}.
|
||
\item The maximum \block size has been doubled to $2000000$ bytes.
|
||
\end{itemize}
|
||
|
||
|
||
\introsection
|
||
\nsubsection{Proof of Work}
|
||
|
||
\Zcash uses Equihash \cite{BK2016} as its Proof of Work. Motivations for
|
||
changing the Proof of Work from \SHAd used by \Bitcoin are described
|
||
in \cite{WG2016}.
|
||
|
||
\introlist
|
||
A \block satisfies the Proof of Work if and only if:
|
||
|
||
\begin{itemize}
|
||
\item The $\solution$ field encodes a \validEquihashSolution according to \crossref{equihash}.
|
||
\item The \blockHeader satisfies the difficulty check according to \crossref{difficulty}.
|
||
\end{itemize}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Equihash} \label{equihash}
|
||
|
||
An instance of the Equihash algorithm is parameterized by positive integers $n$ and $k$,
|
||
such that $n$ is a multiple of $k+1$. We assume $k \geq 3$.
|
||
|
||
The Equihash parameters for the production and test networks are $n = 200, k = 9$.
|
||
|
||
The Generalized Birthday Problem is defined as follows: given a sequence
|
||
$X_\barerange{1}{\mathrm{N}}$ of $n$-bit strings, find $2^k$ distinct $X_{i_j}$ such that
|
||
$\vxor{j=1}{2^k} X_{i_j} = 0$.
|
||
|
||
\introlist
|
||
In Equihash, $\mathrm{N} = 2^{\frac{n}{k+1}+1}$, and the sequence $X_\barerange{1}{\mathrm{N}}$ is
|
||
derived from the \blockHeader and a nonce:
|
||
|
||
\newsavebox{\powheaderbox}
|
||
\begin{lrbox}{\powheaderbox}
|
||
\begin{bytefield}[bitwidth=0.064em]{1152}
|
||
\bitbox{128}{$32$-bit $\nVersion$} &
|
||
\bitbox{256}{$256$-bit $\hashPrevBlock$} &
|
||
\bitbox{256}{$256$-bit $\hashMerkleRoot$} \\
|
||
\bitbox{256}{$256$-bit $\hashReserved$} &
|
||
\bitbox{128}{$32$-bit $\nTimeField$} &
|
||
\bitbox{128}{$32$-bit $\nBitsField$} \\
|
||
\bitbox{256}{$256$-bit $\nNonce$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
Let $\powheader := \Justthebox[-11.5ex]{\powheaderbox}$
|
||
|
||
For $i \in \range{1}{N}$, let $X_i = \EquihashGen{n, k}(\powheader, i)$.
|
||
|
||
$\EquihashGen{}$ is instantiated in \crossref{equihashgen}.
|
||
|
||
Define $\ItoBEBSP{} \typecolon (u \typecolon \Nat) \times \range{0}{2^u\!-\!1} \rightarrow \bitseq{u}$
|
||
as in \crossref{endian}.
|
||
|
||
A \validEquihashSolution is then a sequence $i \typecolon \range{1}{N}^{2^k}$ that
|
||
satisfies the following conditions:
|
||
|
||
\subparagraph{Generalized Birthday condition}
|
||
|
||
$\vxor{j=1}{2^k} X_{i_j} = 0$.
|
||
|
||
\subparagraph{Algorithm Binding conditions}
|
||
|
||
\introlist
|
||
\begin{itemize}
|
||
\item For all $r \in \range{1}{k\!-\!1}$, for all $w \in \range{0}{2^{k-r}\!-\!1}:
|
||
\vxor{j=1}{2^r} X_{i_{w \mult 2^r + j}}$ has $\frac{n \mult r}{k+1}$ leading zeros; and
|
||
\item For all $r \in \range{1}{k}$, for all $w \in \range{0}{2^{k-r}\!-\!1}:
|
||
i_{w \mult 2^r + 1 .. w \mult 2^r + 2^{r-1}} <
|
||
i_{w \mult 2^r + 2^{r-1} + 1 .. w \mult 2^r + 2^r}$ lexicographically.
|
||
\end{itemize}
|
||
|
||
\begin{pnotes}
|
||
\item This does not include a difficulty condition, because here we are
|
||
defining validity of an Equihash solution independent of difficulty.
|
||
\item Previous versions of this specification incorrectly specified the
|
||
range of $r$ to be $\range{1}{k\!-\!1}$ for both parts of the algorithm
|
||
binding condition. The implementation in \zcashd was as intended.
|
||
\end{pnotes}
|
||
|
||
\introlist
|
||
An Equihash solution with $n = 200$ and $k = 9$ is encoded in the $\solution$
|
||
field of a \blockHeader as follows:
|
||
|
||
\newsavebox{\solutionbox}
|
||
\begin{lrbox}{\solutionbox}
|
||
\begin{bytefield}[bitwidth=0.45em]{105}
|
||
\bitbox{21}{$\ItoBEBSP{21}(i_1-1)$} &
|
||
\bitbox{21}{$\ItoBEBSP{21}(i_2-1)$} &
|
||
\bitbox{42}{$\cdots$} &
|
||
\bitbox{21}{$\ItoBEBSP{21}(i_{512}-1)$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\newcommand{\zb}{\bitbox{1}{$0$}}
|
||
\newcommand{\ob}{\bitbox{1}{$1$}}
|
||
\newsavebox{\eqexamplebox}
|
||
\begin{lrbox}{\eqexamplebox}
|
||
\begin{bytefield}[bitwidth=0.75em]{63}
|
||
\bitbox{21}{$\ItoBEBSP{21}(68)$} &
|
||
\bitbox{21}{$\ItoBEBSP{21}(41)$} &
|
||
\bitbox{21}{$\ItoBEBSP{21}(2^{21}-1)$} \\
|
||
\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\ob\zb\zb\zb\ob\zb\zb
|
||
\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\zb\ob\zb\ob\zb\zb\ob
|
||
\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob\ob \\
|
||
\bitbox{8}{8-bit $0$}
|
||
\bitbox{8}{8-bit $2$}
|
||
\bitbox{8}{8-bit $32$}
|
||
\bitbox{8}{8-bit $0$}
|
||
\bitbox{8}{8-bit $10$}
|
||
\bitbox{8}{8-bit $127$}
|
||
\bitbox{8}{8-bit $255$}
|
||
\bitbox{7}{$\cdots$}
|
||
\end{bytefield}
|
||
\end{lrbox}
|
||
|
||
\begin{formulae}
|
||
\item $\Justthebox{\solutionbox}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
Recall from \crossref{boxnotation} that bits in the above diagram are
|
||
ordered from most to least significant in each byte.
|
||
For example, if the first $3$ elements of $i$ are $[69, 42, 2^{21}]$,
|
||
then the corresponding bit array is:
|
||
|
||
\begin{formulae}
|
||
\item $\Justthebox{\eqexamplebox}$
|
||
\end{formulae}
|
||
|
||
and so the first $7$ bytes of $\solution$ would be
|
||
$[0, 2, 32, 0, 10, 127, 255]$.
|
||
|
||
\pnote{
|
||
$\ItoBEBSP{}$ is big-endian, while integer field encodings in $\powheader$
|
||
and in the instantiation of $\EquihashGen{}$ are little-endian.
|
||
The rationale for this is that little-endian serialization of
|
||
\blockHeaders is consistent with \Bitcoin, but using little-endian
|
||
ordering of bits in the solution encoding would require bit-reversal
|
||
(as opposed to only shifting).
|
||
}
|
||
|
||
\nsubsubsection{Difficulty filter} \label{difficulty}
|
||
|
||
Let $\ToTarget$ be as defined in \crossref{nbits}.
|
||
|
||
Difficulty is defined in terms of a \targetThreshold, which is adjusted for each
|
||
\block according to the algorithm defined in \crossref{diffadjustment}.
|
||
|
||
The difficulty filter is unchanged from \Bitcoin, and is calculated using
|
||
\SHAd on the whole \blockHeader (including $\solutionSize$ and $\solution$).
|
||
The result is interpreted as a $256$-bit integer represented in little-endian
|
||
byte order, which \MUST be less than or equal to the \targetThreshold given by
|
||
$\ToTarget(\nBitsField)$.
|
||
|
||
|
||
\nsubsubsection{Difficulty adjustment} \label{diffadjustment}
|
||
|
||
\Zcash uses a difficulty adjustment algorithm based on DigiShield v3/v4 \cite{DigiByte-PoW},
|
||
with simplifications and altered parameters, to adjust difficulty to target
|
||
the desired 2.5-minute block time.
|
||
Unlike \Bitcoin, the difficulty adjustment occurs after every block.
|
||
|
||
The constants $\PoWLimit$, $\PoWAveragingWindow$, $\PoWMaxAdjustDown$, $\PoWMaxAdjustUp$,
|
||
$\PoWDampingFactor$, and $\PoWTargetSpacing$ are instantiated in \crossref{constants}.
|
||
|
||
Let $\ToCompact$ and $\ToTarget$ be as defined in \crossref{nbits}.
|
||
|
||
Let $\nTime(\BlockHeight)$ be the value of the $\nTimeField$ field in the \header of the
|
||
\block at \blockHeight $\BlockHeight$.
|
||
|
||
Let $\nBits(\BlockHeight)$ be the value of the $\nBitsField$ field in the \header of the
|
||
\block at \blockHeight $\BlockHeight$.
|
||
|
||
\Blockheader fields are specified in \crossref{blockheader}.
|
||
|
||
\vspace{1ex}
|
||
\introlist
|
||
Define:
|
||
|
||
\begin{formulae}
|
||
\hfuzz=10pt
|
||
\item $\mean(S) := \left( \vsum{i=1}{\length(S)} S_i \right) \raisebox{-0.4ex}{\scalebox{1.4}{/\,}} \length(S)$.
|
||
\item $\median(S) := \sorted(S)_{\ceiling{\length(S) / 2}}$
|
||
\item $\bound{\Lower}{\Upper}(x) := \maximum(\Lower, \minimum(\Upper, x)))$
|
||
\item $\trunc{x} := \begin{cases}
|
||
\floor{x},&\caseif x \geq 0 \\
|
||
-\floor{-x},&\caseotherwise
|
||
\end{cases}$
|
||
|
||
\item $\AveragingWindowTimespan := \PoWAveragingWindow \mult \PoWTargetSpacing$
|
||
\item $\MinActualTimespan := \floor{\AveragingWindowTimespan \mult (1 - \PoWMaxAdjustUp)}$
|
||
\item $\MaxActualTimespan := \floor{\AveragingWindowTimespan \mult (1 + \PoWMaxAdjustDown)}$
|
||
\item $\MedianTime(\BlockHeight) := \median(\listcomp{\nTime(i) \for i \from
|
||
\maximum(0, \BlockHeight - \PoWMedianBlockSpan) \upto \BlockHeight - 1})$
|
||
\item $\ActualTimespan(\BlockHeight) := \MedianTime(\BlockHeight) - \MedianTime(\BlockHeight - \PoWAveragingWindow)$
|
||
\item $\ActualTimespanDamped(\BlockHeight) := \AveragingWindowTimespan + \trunc{\scalebox{0.98}{\hfrac{\ActualTimespan(\BlockHeight) - \AveragingWindowTimespan}{\PoWDampingFactor}}}$
|
||
\item $\ActualTimespanBounded(\BlockHeight) := \bound{\MinActualTimespan}{\MaxActualTimespan}(\ActualTimespanDamped(\BlockHeight))$
|
||
\item $\MeanTarget(\BlockHeight) := \begin{cases}
|
||
\PoWLimit, \hspace{16em}\text{if } \BlockHeight \leq \PoWAveragingWindow \\
|
||
\mean(\listcomp{\ToTarget(\nBits(i)) \for i \from \BlockHeight - \PoWAveragingWindow \upto \BlockHeight - 1}),\\
|
||
\hspace{20.7em}\text{otherwise}
|
||
\end{cases}$
|
||
\end{formulae}
|
||
|
||
\vspace{1ex}
|
||
\introlist
|
||
The \targetThreshold for a given \blockHeight $\BlockHeight$ is then calculated as:
|
||
|
||
\begin{formulae}
|
||
\item $\Threshold(\BlockHeight) \hspace{0.43em} := \hspace{0.43em} \begin{cases}
|
||
\PoWLimit, \hspace{16em}\text{if } \BlockHeight = 0 \\
|
||
\minimum(\PoWLimit, \floor{\hfrac{\MeanTarget(\BlockHeight)}{\AveragingWindowTimespan}}
|
||
\mult \ActualTimespanBounded(\BlockHeight)),\\
|
||
\hspace{20.7em}\text{otherwise}
|
||
\end{cases}$
|
||
\item $\ThresholdBits(\BlockHeight) := \ToCompact(\Threshold(\BlockHeight))$.
|
||
\end{formulae}
|
||
|
||
\pnote{
|
||
The convention used for the height parameters to $\MedianTime$, $\ActualTimespan$,
|
||
$\ActualTimespanDamped$, $\ActualTimespanBounded$, $\MeanTarget$, $\Threshold$, and
|
||
$\ThresholdBits$ is that these functions use only information from \blocks \emph{preceding}
|
||
the given \blockHeight.
|
||
}
|
||
|
||
\introlist
|
||
\nsubsubsection{nBits conversion} \label{nbits}
|
||
|
||
Deterministic conversions between a \targetThreshold and a ``compact" nBits value are not
|
||
fully defined in the Bitcoin documentation \cite{Bitc-nBits}, and so we define them here:
|
||
|
||
\begin{formulae}[leftmargin=1.5em,label=]
|
||
\item $\size(x) := \ceiling{\hfrac{\bitlength(x)}{8}}$
|
||
\item $\mantissa(x) := \floor{x \mult 256^{3 - \size(x)}}$
|
||
\item $\ToCompact(x) := \begin{cases}
|
||
\mantissa(x) + 2^{24} \smult \size(x),&\caseif \mantissa(x) < 2^{23} \\
|
||
\floor{\hfrac{\mantissa(x)}{256}} + 2^{24} \smult (\size(x)+1),&\caseotherwise
|
||
\end{cases}$
|
||
\item $\ToTarget(x) := \begin{cases}
|
||
0,&\caseif x \band 2^{23} = 2^{23} \\
|
||
(x \band (2^{23}-1)) \mult 256^{\floor{x / 2^{24}} - 3},&\caseotherwise.
|
||
\end{cases}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
\nsubsubsection{Definition of Work} \label{workdef}
|
||
|
||
As explained in \crossref{blockchain}, a node chooses the ``best'' \blockchain
|
||
visible to it by finding the chain of valid \blocks with the greatest total work.
|
||
|
||
Let $\ToTarget$ be as defined in \crossref{nbits}.
|
||
|
||
The work of a \block with value $\nBits$ for the $\nBitsField$ field
|
||
in its \blockHeader is defined as $\floor{\hfrac{2^{256}}{\ToTarget(\nBits) + 1}}$.
|
||
|
||
|
||
\introlist
|
||
\nsubsection{Calculation of Block Subsidy and Founders' Reward} \label{subsidies}
|
||
|
||
\crossref{subsidyconcepts} defines the \blockSubsidy, \minerSubsidy, and \foundersReward.
|
||
Their amounts in \zatoshi are calculated from the \blockHeight using
|
||
the formulae below. The constants $\SlowStartInterval$, $\HalvingInterval$,
|
||
$\MaxBlockSubsidy$, and $\FoundersFraction$ are instantiated in \crossref{constants}.
|
||
|
||
\begin{formulae}
|
||
\item $\SlowStartShift \typecolon \Nat := \hfrac{\SlowStartInterval}{2}$
|
||
\item $\SlowStartRate \typecolon \Nat := \hfrac{\MaxBlockSubsidy}{\SlowStartInterval}$
|
||
\item $\Halving(\BlockHeight) := \floor{\hfrac{\BlockHeight - \SlowStartShift}{\HalvingInterval}}$
|
||
\item $\BlockSubsidy(\BlockHeight) := \begin{cases}
|
||
\SlowStartRate \mult \BlockHeight,&\caseif \BlockHeight < \hfrac{\SlowStartInterval}{2} \\[1.4ex]
|
||
\SlowStartRate \mult (\BlockHeight + 1),&\caseif \hfrac{\SlowStartInterval}{2} \leq \BlockHeight < \SlowStartInterval \\[1.4ex]
|
||
\floor{\hfrac{\MaxBlockSubsidy}{2^{\Halving(\BlockHeight)}}},&\caseotherwise
|
||
\end{cases}$
|
||
|
||
\item $\FoundersReward(\BlockHeight) := \begin{cases}
|
||
\BlockSubsidy(\BlockHeight) \mult \FoundersFraction,&\caseif \BlockHeight < \SlowStartShift + \HalvingInterval \\
|
||
0,&\caseotherwise
|
||
\end{cases}$
|
||
|
||
\item $\MinerSubsidy(\BlockHeight) := \BlockSubsidy(\BlockHeight) - \FoundersReward(\BlockHeight)$.
|
||
\end{formulae}
|
||
|
||
\introsection
|
||
\nsubsection{Payment of Founders' Reward} \label{foundersreward}
|
||
|
||
The \foundersReward is paid by a \transparent output in the \coinbaseTransaction, to
|
||
one of $\NumFounderAddresses$ \transparent addresses, depending on the \blockHeight.
|
||
|
||
\renewcommand{\arraystretch}{0.95}
|
||
|
||
For the production network, $\FounderAddressList_\barerange{\mathrm{1}}{\NumFounderAddresses}$ is:
|
||
|
||
\begin{tabular}{@{\hskip 2.5em}l@{\;}l}
|
||
[& \ascii{t3Vz22vK5z2LcKEdg16Yv4FFneEL1zg9ojd}, \ascii{t3cL9AucCajm3HXDhb5jBnJK2vapVoXsop3}, \\
|
||
& \ascii{t3fqvkzrrNaMcamkQMwAyHRjfDdM2xQvDTR}, \ascii{t3TgZ9ZT2CTSK44AnUPi6qeNaHa2eC7pUyF}, \\
|
||
& \ascii{t3SpkcPQPfuRYHsP5vz3Pv86PgKo5m9KVmx}, \ascii{t3Xt4oQMRPagwbpQqkgAViQgtST4VoSWR6S}, \\
|
||
& \ascii{t3ayBkZ4w6kKXynwoHZFUSSgXRKtogTXNgb}, \ascii{t3adJBQuaa21u7NxbR8YMzp3km3TbSZ4MGB}, \\
|
||
& \ascii{t3K4aLYagSSBySdrfAGGeUd5H9z5Qvz88t2}, \ascii{t3RYnsc5nhEvKiva3ZPhfRSk7eyh1CrA6Rk}, \\
|
||
& \ascii{t3Ut4KUq2ZSMTPNE67pBU5LqYCi2q36KpXQ}, \ascii{t3ZnCNAvgu6CSyHm1vWtrx3aiN98dSAGpnD}, \\
|
||
& \ascii{t3fB9cB3eSYim64BS9xfwAHQUKLgQQroBDG}, \ascii{t3cwZfKNNj2vXMAHBQeewm6pXhKFdhk18kD}, \\
|
||
& \ascii{t3YcoujXfspWy7rbNUsGKxFEWZqNstGpeG4}, \ascii{t3bLvCLigc6rbNrUTS5NwkgyVrZcZumTRa4}, \\
|
||
& \ascii{t3VvHWa7r3oy67YtU4LZKGCWa2J6eGHvShi}, \ascii{t3eF9X6X2dSo7MCvTjfZEzwWrVzquxRLNeY}, \\
|
||
& \ascii{t3esCNwwmcyc8i9qQfyTbYhTqmYXZ9AwK3X}, \ascii{t3M4jN7hYE2e27yLsuQPPjuVek81WV3VbBj}, \\
|
||
& \ascii{t3gGWxdC67CYNoBbPjNvrrWLAWxPqZLxrVY}, \ascii{t3LTWeoxeWPbmdkUD3NWBquk4WkazhFBmvU}, \\
|
||
& \ascii{t3P5KKX97gXYFSaSjJPiruQEX84yF5z3Tjq}, \ascii{t3f3T3nCWsEpzmD35VK62JgQfFig74dV8C9}, \\
|
||
& \ascii{t3Rqonuzz7afkF7156ZA4vi4iimRSEn41hj}, \ascii{t3fJZ5jYsyxDtvNrWBeoMbvJaQCj4JJgbgX}, \\
|
||
& \ascii{t3Pnbg7XjP7FGPBUuz75H65aczphHgkpoJW}, \ascii{t3WeKQDxCijL5X7rwFem1MTL9ZwVJkUFhpF}, \\
|
||
& \ascii{t3Y9FNi26J7UtAUC4moaETLbMo8KS1Be6ME}, \ascii{t3aNRLLsL2y8xcjPheZZwFy3Pcv7CsTwBec}, \\
|
||
& \ascii{t3gQDEavk5VzAAHK8TrQu2BWDLxEiF1unBm}, \ascii{t3Rbykhx1TUFrgXrmBYrAJe2STxRKFL7G9r}, \\
|
||
& \ascii{t3aaW4aTdP7a8d1VTE1Bod2yhbeggHgMajR}, \ascii{t3YEiAa6uEjXwFL2v5ztU1fn3yKgzMQqNyo}, \\
|
||
& \ascii{t3g1yUUwt2PbmDvMDevTCPWUcbDatL2iQGP}, \ascii{t3dPWnep6YqGPuY1CecgbeZrY9iUwH8Yd4z}, \\
|
||
& \ascii{t3QRZXHDPh2hwU46iQs2776kRuuWfwFp4dV}, \ascii{t3enhACRxi1ZD7e8ePomVGKn7wp7N9fFJ3r}, \\
|
||
& \ascii{t3PkLgT71TnF112nSwBToXsD77yNbx2gJJY}, \ascii{t3LQtHUDoe7ZhhvddRv4vnaoNAhCr2f4oFN}, \\
|
||
& \ascii{t3fNcdBUbycvbCtsD2n9q3LuxG7jVPvFB8L}, \ascii{t3dKojUU2EMjs28nHV84TvkVEUDu1M1FaEx}, \\
|
||
& \ascii{t3aKH6NiWN1ofGd8c19rZiqgYpkJ3n679ME}, \ascii{t3MEXDF9Wsi63KwpPuQdD6by32Mw2bNTbEa}, \\
|
||
& \ascii{t3WDhPfik343yNmPTqtkZAoQZeqA83K7Y3f}, \ascii{t3PSn5TbMMAEw7Eu36DYctFezRzpX1hzf3M}, \\
|
||
& \ascii{t3R3Y5vnBLrEn8L6wFjPjBLnxSUQsKnmFpv}, \ascii{t3Pcm737EsVkGTbhsu2NekKtJeG92mvYyoN}\, ]
|
||
\end{tabular}
|
||
|
||
\introlist
|
||
For the test network, $\FounderAddressList_\barerange{\mathrm{1}}{\NumFounderAddresses}$ is:
|
||
|
||
\begin{tabular}{@{\hskip 2.5em}l@{\;}l}
|
||
[& \ascii{t2UNzUUx8mWBCRYPRezvA363EYXyEpHokyi}, \ascii{t2N9PH9Wk9xjqYg9iin1Ua3aekJqfAtE543}, \\
|
||
& \ascii{t2NGQjYMQhFndDHguvUw4wZdNdsssA6K7x2}, \ascii{t2ENg7hHVqqs9JwU5cgjvSbxnT2a9USNfhy}, \\
|
||
& \ascii{t2BkYdVCHzvTJJUTx4yZB8qeegD8QsPx8bo}, \ascii{t2J8q1xH1EuigJ52MfExyyjYtN3VgvshKDf}, \\
|
||
& \ascii{t2Crq9mydTm37kZokC68HzT6yez3t2FBnFj}, \ascii{t2EaMPUiQ1kthqcP5UEkF42CAFKJqXCkXC9}, \\
|
||
& \ascii{t2F9dtQc63JDDyrhnfpzvVYTJcr57MkqA12}, \ascii{t2LPirmnfYSZc481GgZBa6xUGcoovfytBnC}, \\
|
||
& \ascii{t26xfxoSw2UV9Pe5o3C8V4YybQD4SESfxtp}, \ascii{t2D3k4fNdErd66YxtvXEdft9xuLoKD7CcVo}, \\
|
||
& \ascii{t2DWYBkxKNivdmsMiivNJzutaQGqmoRjRnL}, \ascii{t2C3kFF9iQRxfc4B9zgbWo4dQLLqzqjpuGQ}, \\
|
||
& \ascii{t2MnT5tzu9HSKcppRyUNwoTp8MUueuSGNaB}, \ascii{t2AREsWdoW1F8EQYsScsjkgqobmgrkKeUkK}, \\
|
||
& \ascii{t2Vf4wKcJ3ZFtLj4jezUUKkwYR92BLHn5UT}, \ascii{t2K3fdViH6R5tRuXLphKyoYXyZhyWGghDNY}, \\
|
||
& \ascii{t2VEn3KiKyHSGyzd3nDw6ESWtaCQHwuv9WC}, \ascii{t2F8XouqdNMq6zzEvxQXHV1TjwZRHwRg8gC}, \\
|
||
& \ascii{t2BS7Mrbaef3fA4xrmkvDisFVXVrRBnZ6Qj}, \ascii{t2FuSwoLCdBVPwdZuYoHrEzxAb9qy4qjbnL}, \\
|
||
& \ascii{t2SX3U8NtrT6gz5Db1AtQCSGjrpptr8JC6h}, \ascii{t2V51gZNSoJ5kRL74bf9YTtbZuv8Fcqx2FH}, \\
|
||
& \ascii{t2FyTsLjjdm4jeVwir4xzj7FAkUidbr1b4R}, \ascii{t2EYbGLekmpqHyn8UBF6kqpahrYm7D6N1Le}, \\
|
||
& \ascii{t2NQTrStZHtJECNFT3dUBLYA9AErxPCmkka}, \ascii{t2GSWZZJzoesYxfPTWXkFn5UaxjiYxGBU2a}, \\
|
||
& \ascii{t2RpffkzyLRevGM3w9aWdqMX6bd8uuAK3vn}, \ascii{t2JzjoQqnuXtTGSN7k7yk5keURBGvYofh1d}, \\
|
||
& \ascii{t2AEefc72ieTnsXKmgK2bZNckiwvZe3oPNL}, \ascii{t2NNs3ZGZFsNj2wvmVd8BSwSfvETgiLrD8J}, \\
|
||
& \ascii{t2ECCQPVcxUCSSQopdNquguEPE14HsVfcUn}, \ascii{t2JabDUkG8TaqVKYfqDJ3rqkVdHKp6hwXvG}, \\
|
||
& \ascii{t2FGzW5Zdc8Cy98ZKmRygsVGi6oKcmYir9n}, \ascii{t2DUD8a21FtEFn42oVLp5NGbogY13uyjy9t}, \\
|
||
& \ascii{t2UjVSd3zheHPgAkuX8WQW2CiC9xHQ8EvWp}, \ascii{t2TBUAhELyHUn8i6SXYsXz5Lmy7kDzA1uT5}, \\
|
||
& \ascii{t2Tz3uCyhP6eizUWDc3bGH7XUC9GQsEyQNc}, \ascii{t2NysJSZtLwMLWEJ6MH3BsxRh6h27mNcsSy}, \\
|
||
& \ascii{t2KXJVVyyrjVxxSeazbY9ksGyft4qsXUNm9}, \ascii{t2J9YYtH31cveiLZzjaE4AcuwVho6qjTNzp}, \\
|
||
& \ascii{t2QgvW4sP9zaGpPMH1GRzy7cpydmuRfB4AZ}, \ascii{t2NDTJP9MosKpyFPHJmfjc5pGCvAU58XGa4}, \\
|
||
& \ascii{t29pHDBWq7qN4EjwSEHg8wEqYe9pkmVrtRP}, \ascii{t2Ez9KM8VJLuArcxuEkNRAkhNvidKkzXcjJ}, \\
|
||
& \ascii{t2D5y7J5fpXajLbGrMBQkFg2mFN8fo3n8cX}, \ascii{t2UV2wr1PTaUiybpkV3FdSdGxUJeZdZztyt}\, ]
|
||
\end{tabular}
|
||
|
||
\renewcommand{\arraystretch}{1}
|
||
|
||
\pnote{For the test network only, the addresses from index 4 onward have been changed from
|
||
what was implemented at launch. This reflects a hard fork on the test network, starting
|
||
from \blockHeight 53127. \cite{ZcashIssue-2113}}
|
||
|
||
Each address representation in $\FounderAddressList$ denotes a \transparent
|
||
P2SH multisig address.
|
||
|
||
\introlist
|
||
Let $\SlowStartShift$ be defined as in the previous section.
|
||
|
||
Define:
|
||
|
||
\begin{formulae}
|
||
\item $\FounderAddressChangeInterval := \ceiling{\hfrac{\SlowStartShift + \HalvingInterval}{\NumFounderAddresses}}$
|
||
\item $\FounderAddressIndex(\BlockHeight) := 1 + \floor{\hfrac{\BlockHeight}{\FounderAddressChangeInterval}}$.
|
||
\end{formulae}
|
||
|
||
Let $\RedeemScriptHash(\BlockHeight)$ be the standard redeem script hash, as defined in
|
||
\cite{Bitc-Multisig}, for the P2SH multisig address with Base58Check representation
|
||
given by $\FounderAddressList_{\,\FounderAddressIndex(\BlockHeight)}$.
|
||
|
||
\consensusrule{
|
||
A \coinbaseTransaction for \blockHeight $\BlockHeight \in \range{1}{\SlowStartShift + \HalvingInterval - 1}$
|
||
\MUST include at least one output that pays exactly $\FoundersReward(\BlockHeight)$ \zatoshi
|
||
with a standard P2SH script of the form \ScriptOP{HASH160} \;$\RedeemScriptHash(\BlockHeight)$\; \ScriptOP{EQUAL}
|
||
as its $\scriptPubKey$.
|
||
}
|
||
|
||
\begin{pnotes}
|
||
\item No \foundersReward is required to be paid for $\BlockHeight \geq \SlowStartShift + \HalvingInterval$
|
||
(i.e.\ after the first halving), or for $\BlockHeight = 0$ (i.e.\ the \genesisBlock).
|
||
\item The \foundersReward addresses are not treated specially in any other way, and
|
||
there can be other outputs to them, in \coinbaseTransactions or otherwise.
|
||
In particular, it is valid for a \coinbaseTransaction with
|
||
$\BlockHeight \in \range{1}{\SlowStartShift + \HalvingInterval - 1}$ to have
|
||
other outputs, possibly to the same address, that do not meet the criterion
|
||
in the above consensus rule, as long as at least one output meets it.
|
||
\end{pnotes}
|
||
|
||
|
||
\nsubsection{Changes to the Script System} \label{scripts}
|
||
|
||
The \ScriptOP{CODESEPARATOR} opcode has been disabled. This opcode also no longer
|
||
affects the calculation of signature hashes.
|
||
|
||
|
||
\nsubsection{Bitcoin Improvement Proposals} \label{bips}
|
||
|
||
In general, Bitcoin Improvement Proposals (BIPs) do not apply to \Zcash unless
|
||
otherwise specified in this section.
|
||
|
||
All of the BIPs referenced below should be interpreted by replacing
|
||
``BTC'', or ``bitcoin'' used as a currency unit, with ``ZEC''; and
|
||
``satoshi'' with ``zatoshi''.
|
||
|
||
The following BIPs apply, otherwise unchanged, to \Zcash:
|
||
\cite{BIP-11},
|
||
\cite{BIP-14},
|
||
\cite{BIP-31},
|
||
\cite{BIP-35},
|
||
\cite{BIP-37},
|
||
\cite{BIP-61}.
|
||
|
||
The following BIPs apply starting from the \Zcash \genesisBlock, i.e.\ any activation
|
||
rules or exceptions for particular \blocks in the \Bitcoin \blockchain are to
|
||
be ignored:
|
||
\cite{BIP-16},
|
||
\cite{BIP-30},
|
||
\cite{BIP-65},
|
||
\cite{BIP-66}.
|
||
|
||
\cite{BIP-34} applies to all blocks other than the \Zcash \genesisBlock
|
||
(for which the ``height in coinbase'' was inadvertently omitted).
|
||
|
||
\cite{BIP-13} applies with the changes to address version bytes described
|
||
in \crossref{transparentaddrencoding}.
|
||
|
||
\begin{comment}
|
||
\cite{BIP-22} and \cite{BIP-23} apply with some protocol changes, which are
|
||
to be specified in a Zcash Improvement Proposal.
|
||
|
||
The following BIPs can be used unchanged, but do not define consensus rules:
|
||
\cite{BIP-69},
|
||
\cite{BIP-126}.
|
||
|
||
The following BIPs can be used by replacing the URI scheme \ascii{bitcoin:}
|
||
with \ascii{zcash:}, and the MIME types starting with \ascii{bitcoin-} with
|
||
corresponding types starting with \ascii{zcash-}:
|
||
\cite{BIP-21},
|
||
\cite{BIP-70},
|
||
\cite{BIP-71},
|
||
\cite{BIP-72},
|
||
\cite{BIP-73}.
|
||
(Note that this URI scheme and these MIME types are not formally allocated,
|
||
and would require an RFC in order to do so.)
|
||
\end{comment}
|
||
|
||
|
||
\introsection
|
||
\nsection{Differences from the Zerocash paper} \label{differences}
|
||
|
||
\nsubsection{Transaction Structure} \label{trstructure}
|
||
|
||
\Zerocash introduces two new operations, which are described in
|
||
the paper as new transaction types, in addition to the original
|
||
transaction type of the cryptocurrency on which it is based
|
||
(e.g.\ \Bitcoin).
|
||
|
||
In \Zcash, there is only the original \Bitcoin transaction type,
|
||
which is extended to contain a sequence of zero or more
|
||
\Zcash-specific operations.
|
||
|
||
This allows for the possibility of chaining transfers of \shielded
|
||
value in a single \Zcash \transaction, e.g.\ to spend a \shieldedNote
|
||
that has just been created. (In \Zcash, we refer to value stored in
|
||
UTXOs as \transparent, and value stored in \joinSplitTransfer output
|
||
\notes as \shielded.)
|
||
This was not possible in the \Zerocash design without using multiple
|
||
transactions. It also allows \transparent and \shielded transfers to
|
||
happen atomically --- possibly under the control of nontrivial script
|
||
conditions, at some cost in distinguishability.
|
||
|
||
\todo{Describe changes to signing.}
|
||
|
||
|
||
\nsubsection{\Memos}
|
||
|
||
\Zcash adds a \memo sent from the creator of a \joinSplitDescription to
|
||
the recipient of each output \note. This feature is described in
|
||
more detail in \crossref{notept}.
|
||
|
||
|
||
\introlist
|
||
\nsubsection{Unification of Mints and Pours}
|
||
|
||
In the original \Zerocash protocol, there were two kinds of transaction
|
||
relating to \shieldedNotes:
|
||
|
||
\begin{itemize}
|
||
\item a ``Mint'' transaction takes value from \transparent UTXOs as
|
||
input and produces a new \shieldedNote as output.
|
||
\item a ``Pour'' transaction takes up to $\NOld$ \shieldedNotes
|
||
as input, and produces up to $\NNew$ \shieldedNotes and a
|
||
\transparent UTXO as output.
|
||
\end{itemize}
|
||
|
||
Only ``Pour'' transactions included a \zkSNARK proof.
|
||
|
||
\sproutonly{
|
||
In \Zcash, the sequence of operations added to a \transaction
|
||
(see \crossref{trstructure}) consists only of \joinSplitTransfers.
|
||
A \joinSplitTransfer is a Pour operation generalized to take a \transparent
|
||
UTXO as input, allowing \joinSplitTransfers to subsume the functionality of
|
||
Mints. An advantage of this is that a \Zcash \transaction that takes
|
||
input from an UTXO can produce up to $\NNew$ output \notes, improving
|
||
the indistinguishability properties of the protocol. A related change
|
||
conceals the input arity of the \joinSplitTransfer: an unused (zero-value)
|
||
input is indistinguishable from an input that takes value from a \note.
|
||
}
|
||
|
||
This unification also simplifies the fix to the Faerie Gold attack
|
||
described below, since no special case is needed for Mints.
|
||
|
||
\saplingonward{
|
||
In \Sapling, there are still no ``Mint'' transactions. Instead of
|
||
\joinSplitTransfers, there are \spendTransfers and \outputTransfers.
|
||
These make use of \xPedersenValueCommitments to represent the shielded
|
||
values that are transferred. Because these commitments are additively
|
||
homomorphic (using elliptic curve addition), it is possible to check
|
||
that all \spendTransfers and \outputTransfers balance; see \crossref{saplingbalance}
|
||
for detail. This reduces the granularity of the circuit, allowing
|
||
a substantial performance improvement (orthogonal to other \Sapling
|
||
circuit improvements) when the numbers of \shielded inputs and outputs
|
||
are significantly different. This comes at the cost of revealing the
|
||
exact number of \shielded inputs and outputs, but dummy (zero-valued)
|
||
outputs are still possible.
|
||
}
|
||
|
||
\nsubsection{Faerie Gold attack and fix} \label{faeriegold}
|
||
|
||
When a \shieldedNote is created in \Zerocash, the creator is
|
||
supposed to choose a new $\NoteAddressRand$ value at random.
|
||
The \nullifier of the \note is derived from its \spendingKey
|
||
($\AuthPrivate$) and $\NoteAddressRand$. The \noteCommitment
|
||
is derived from the recipient address component $\AuthPublic$,
|
||
the value $\Value$, and the commitment trapdoor $\NoteCommitRand$,
|
||
as well as $\NoteAddressRand$. However nothing prevents creating
|
||
multiple \notes with different $\Value$ and $\NoteCommitRand$
|
||
(hence different \noteCommitments) but the same $\NoteAddressRand$.
|
||
|
||
An adversary can use this to mislead a \note recipient, by sending
|
||
two \notes both of which are verified as valid by $\Receive$ (as
|
||
defined in \cite[Figure 2]{BCG+2014}), but only one of
|
||
which can be spent.
|
||
|
||
We call this a ``Faerie Gold'' attack --- referring to various Celtic
|
||
legends in which faeries pay mortals in what appears to be gold,
|
||
but which soon after reveals itself to be leaves, gorse blossoms,
|
||
gingerbread cakes, or other less valuable things \cite{LG2004}.
|
||
|
||
\introlist
|
||
This attack does not violate the security definitions given in
|
||
\cite{BCG+2014}. The issue could be framed as a problem
|
||
either with the definition of Completeness, or the definition of
|
||
Balance:
|
||
|
||
\begin{itemize}
|
||
\item The Completeness property asserts that a validly received
|
||
\note can be spent provided that its \nullifier does not appear
|
||
on the ledger. This does not take into account the possibility
|
||
that distinct \notes, which are validly received, could have the
|
||
same \nullifier. That is, the security definition depends on
|
||
a protocol detail --\nullifiers-- that is not part of the
|
||
intended abstract security property, and that could be implemented
|
||
incorrectly.
|
||
\item The Balance property only asserts that an adversary cannot
|
||
obtain \emph{more} funds than they have minted or received via
|
||
payments. It does not prevent an adversary from causing others'
|
||
funds to decrease. In a Faerie Gold attack, an adversary can cause
|
||
spending of a \note to reduce (to zero) the effective value of another
|
||
\note for which the attacker does not know the \spendingKey, which
|
||
violates an intuitive conception of global balance.
|
||
\end{itemize}
|
||
|
||
These problems with the security definitions need to be repaired,
|
||
but doing so is outside the scope of this specification. Here we
|
||
only describe how \Zcash addresses the immediate attack.
|
||
|
||
It would be possible to address the attack by requiring that a
|
||
recipient remember all of the $\NoteAddressRand$ values for all
|
||
\notes they have ever received, and reject duplicates (as proposed
|
||
in \cite{GGM2016}). However, this requirement would interfere
|
||
with the intended \Zcash feature that a holder of a \spendingKey
|
||
can recover access to (and be sure that they are able to spend) all
|
||
of their funds, even if they have forgotten everything but the
|
||
\spendingKey.
|
||
|
||
\sproutspecific{
|
||
Instead, \Zcash enforces that an adversary must choose distinct values
|
||
for each $\NoteAddressRand$, by making use of the fact that all of the
|
||
\nullifiers in \joinSplitDescriptions that appear in a \validBlockchain
|
||
must be distinct. This is true regardless of whether the \nullifiers
|
||
corresponded to real or dummy notes (see \crossref{dummynotes}).
|
||
The \nullifiers are used as input to $\hSigCRH$ to derive a public value
|
||
$\hSig$ which uniquely identifies the transaction, as described in
|
||
\crossref{joinsplitdesc}. ($\hSig$ was already used in \Zerocash
|
||
in a way that requires it to be unique in order to maintain
|
||
indistinguishability of \joinSplitDescriptions; adding the \nullifiers
|
||
to the input of the hash used to calculate it has the effect of making
|
||
this uniqueness property robust even if the \transaction creator is an
|
||
adversary.)
|
||
}
|
||
|
||
\sproutspecific{
|
||
The $\NoteAddressRand$ value for each output \note is then derived from
|
||
a random private seed $\NoteAddressPreRand$ and $\hSig$ using
|
||
$\PRFrho{\NoteAddressPreRand}$. The correct construction of
|
||
$\NoteAddressRand$ for each output \note is enforced by
|
||
\crossref{sproutuniquerho} in the \joinSplitStatement.
|
||
}
|
||
|
||
\sproutspecific{
|
||
Now even if the creator of a \joinSplitDescription does not choose
|
||
$\NoteAddressPreRand$ randomly, uniqueness of \nullifiers and
|
||
collision resistance of both $\hSigCRH$ and $\PRFrho{}$ will ensure
|
||
that the derived $\NoteAddressRand$ values are unique, at least for
|
||
any two \joinSplitDescriptions that get into a \validBlockchain.
|
||
This is sufficient to prevent the Faerie Gold attack.
|
||
}
|
||
|
||
A variation on the attack attempts to cause the \nullifier of a sent
|
||
\note to be repeated, without repeating $\NoteAddressRand$.
|
||
However, since the \nullifier is computed as
|
||
$\PRFnf{\AuthPrivate}(\NoteAddressRand)$, this is only possible if
|
||
the adversary finds a collision (across both inputs) on $\PRFnf{}$,
|
||
which is assumed to be infeasible --- see \crossref{abstractprfs}.
|
||
|
||
\sproutspecific{
|
||
Crucially, ``\nullifier integrity'' (\crossref{sproutnullifierintegrity})
|
||
is enforced whether or not the $\EnforceMerklePath{i}$ flag is set
|
||
for an input \note. If this were not the case then an adversary could
|
||
perform the attack by creating a zero-valued \note with a repeated
|
||
\nullifier, since the \nullifier does not depend on the value.
|
||
}
|
||
|
||
\sproutspecific{
|
||
\xNullifier{} integrity also prevents a ``roadblock attack'' in which the
|
||
attacker sees a victim's \transaction, and is able to publish another
|
||
\transaction that is mined first and blocks the victim's \transaction.
|
||
This attack would be possible if the public value(s) used to
|
||
enforce uniqueness of $\NoteAddressRand$ could be chosen arbitrarily
|
||
by the \transaction creator: the victim's \transaction, rather than
|
||
the attacker's, would be considered to be repeating these values.
|
||
In the chosen solution that uses \nullifiers for these public values,
|
||
they are enforced to be dependent on \spendingKeys controlled by the
|
||
original \transaction creator (whether or not each input note is a
|
||
dummy), and so a roadblock attack cannot be performed by another party
|
||
who does not know these keys.
|
||
}
|
||
|
||
\saplingonward{
|
||
In \Sapling, uniqueness of $\NoteAddressRand$ is ensured by making it
|
||
dependent on the position of the \noteCommitment in the \Sapling{}
|
||
\noteCommitmentTree. Specifically,
|
||
$\NoteAddressRand = \cm + \scalarmult{\NotePosition}{\NotePositionBase}$,
|
||
where $\NotePositionBase$ is a generator independent of the generators
|
||
used in $\NoteCommitSaplingAlg$. Therefore, $\NoteAddressRand$ commits uniquely
|
||
to the \note and its position, and this commitment is collision-resistant
|
||
by the same argument used to prove collision resistance of \xPedersenHashes.
|
||
Note that it is possible for two distinct \Sapling \positionedNotes (having
|
||
different $\NoteAddressRand$ values and \nullifiers, but different
|
||
\notePositions) to have the same \noteCommitment, but this causes no security
|
||
problem. Roadblock attacks are not possible because a given \notePosition
|
||
does not repeat for outputs of different \transactions in the same \blockchain.
|
||
}
|
||
|
||
|
||
\nsubsection{Internal hash collision attack and fix} \label{internalh}
|
||
|
||
The \Zerocash security proof requires that the composition of
|
||
$\Commit{\NoteCommitRand}$ and $\Commit{\NoteCommitS}$ is a
|
||
computationally binding commitment to its inputs $\AuthPublic$,
|
||
$\Value$, and $\NoteAddressRand$. However, the instantiation of
|
||
$\Commit{\NoteCommitRand}$ and $\Commit{\NoteCommitS}$ in
|
||
section 5.1 of the paper did not meet the definition of a binding
|
||
commitment at a $128$-bit security level. Specifically, the internal
|
||
hash of $\AuthPublic$ and $\NoteAddressRand$ is truncated to $128$ bits
|
||
(motivated by providing statistical hiding security). This allows an
|
||
attacker, with a work factor on the order of $2^{64}$, to find distinct
|
||
pairs $(\AuthPublic, \NoteAddressRand)$ and $(\AuthPublic\!', \NoteAddressRand')$
|
||
with colliding outputs of the truncated hash, and therefore the same
|
||
\noteCommitment. This would have allowed such an attacker to break the
|
||
Balance property by double-spending \notes, potentially creating arbitrary
|
||
amounts of currency for themself \cite{HW2016}.
|
||
|
||
\Zcash uses a simpler construction with a single
|
||
\notsprout{hash evaluation for the commitment:
|
||
$\SHAFull$ for \Sprout\sapling{, and $\PedersenHash$ for \Sapling}.}
|
||
\sprout{$\SHAFull$ evaluation for the commitment.}
|
||
The motivation for the nested construction in \Zerocash
|
||
was to allow Mint transactions to be publically verified without requiring
|
||
a \zeroKnowledgeProof (as described under step 3 in
|
||
\cite[section 1.3]{BCG+2014}). Since \Zcash combines ``Mint'' and ``Pour''
|
||
transactions into generalized
|
||
\notsprout{\joinSplitTransfers (for \Sprout), \sapling{or \spendTransfers and
|
||
\outputTransfers (for \Sapling)}, and each transfer always uses a \zeroKnowledgeProof\!\!,
|
||
\Zcash does not require the nesting.}
|
||
\sprout{\joinSplitTransfers, and each transfer always uses a \zeroKnowledgeProof\!\!,
|
||
it does not require the nesting.}
|
||
A side benefit is that this reduces the cost of computing the
|
||
\noteCommitments: \notsprout{for \Sprout} it reduces the number of $\SHACompress$
|
||
evaluations needed to compute each \noteCommitment from three to two,
|
||
saving a total of four $\SHACompress$ evaluations in the \joinSplitStatement.
|
||
|
||
\sproutspecificpnote{
|
||
\notsprout{\Sprout \noteCommitments are not statistically hiding, so for \Sprout notes,}
|
||
\sprout{\Zcash \noteCommitments are not statistically hiding, so}
|
||
\Zcash does not support the ``everlasting anonymity'' property
|
||
described in \cite[section 8.1]{BCG+2014},
|
||
even when used as described in that section. While it is possible to
|
||
define a statistically hiding, computationally binding commitment scheme
|
||
for this use at a 128-bit security level, the overhead of doing so
|
||
within the \joinSplitStatement was not considered to justify the benefits.
|
||
}
|
||
|
||
\saplingonward{
|
||
In \Sapling, \xPedersenCommitments are used instead of $\SHACompress$.
|
||
These commitments are statistically hiding, and so ``everlasting anonymity''
|
||
is supported for \Sapling notes under the same conditions as in \Zerocash
|
||
(by the protocol, not necessarily by \zcashd).
|
||
}
|
||
|
||
\nsubsection{Changes to PRF inputs and truncation} \label{truncation}
|
||
|
||
The format of inputs to the PRFs instantiated in \crossref{concreteprfs}
|
||
has changed relative to \Zerocash. There is also a requirement for another PRF,
|
||
$\PRFrho{}$, which must be domain-separated from the others.
|
||
|
||
In the \Zerocash protocol, $\NoteAddressRandOld{i}$ is truncated from $256$
|
||
to $254$ bits in the input to $\PRFsn{}$ (which corresponds to $\PRFnf{}$ in \Zcash).
|
||
Also, $\hSig$ is truncated from $256$ to $253$ bits in the input to $\PRFpk{}$.
|
||
These truncations are not taken into account in the security proofs.
|
||
|
||
Both truncations affect the validity of the proof sketch for Lemma D.2 in
|
||
the proof of Ledger Indistinguishability in \cite[Appendix D]{BCG+2014}.
|
||
|
||
\introlist
|
||
In more detail:
|
||
|
||
\begin{itemize}
|
||
\item In the argument relating $\mathbf{H}$ and $\Game_2$, it is stated that in $\Game_2$,
|
||
``for each $i \in \setof{1, 2}, \mathsf{sn}_i := \PRFsn{\AuthPrivate}(\NoteAddressRand)$
|
||
for a random (and not previously used) $\NoteAddressRand$''. It is also
|
||
argued that ``the calls to $\PRFsn{\AuthPrivate}$ are each by definition unique''.
|
||
The latter assertion depends on the fact that $\NoteAddressRand$
|
||
is ``not previously used''. However, the argument is incorrect
|
||
because the truncated input to $\PRFsn{\AuthPrivate}$, i.e.
|
||
$[\NoteAddressRand]_{254}$, may repeat even if $\NoteAddressRand$ does not.
|
||
\item In the same argument, it is stated that ``with overwhelming probability,
|
||
$\hSig$ is unique''. In fact what is required to be unique is the
|
||
truncated input to $\PRFpk{}$, i.e.\ $[\hSig]_{253} = [\CRH(\pksig)]_{253}$.
|
||
In practice this value will be unique under a plausible assumption on
|
||
$\CRH$ provided that $\pksig$ is chosen randomly, but no formal argument
|
||
for this is presented.
|
||
\end{itemize}
|
||
|
||
Note that $\NoteAddressRand$ is truncated in the input to $\PRFsn{}$
|
||
but not in the input to $\Commit{\NoteCommitRand}$, which further
|
||
complicates the analysis.
|
||
|
||
As further evidence that it is essential for the proofs to explicitly take any
|
||
such truncations into account, consider a slightly modified protocol in which
|
||
$\NoteAddressRand$ is truncated in the input to $\Commit{\NoteCommitRand}$
|
||
but not in the input to $\PRFsn{}$. In that case, it would be possible to
|
||
violate balance by creating two \notes for which $\NoteAddressRand$ differs
|
||
only in the truncated bits. These \notes would have the same \noteCommitment
|
||
but different \nullifiers, so it would be possible to spend the same value
|
||
twice.
|
||
|
||
\sproutspecific{
|
||
For resistance to Faerie Gold attacks as described in
|
||
\crossref{faeriegold}, \Zcash depends on collision resistance of
|
||
$\hSigCRH$ (instantiated using $\BlakeTwob{256}$) and $\PRFrho{}$
|
||
(instantiated using $\SHACompress$). Collision resistance of a truncated hash
|
||
does not follow from collision resistance of the original hash, even if the
|
||
truncation is only by one bit. This motivated avoiding truncation along any
|
||
path from the inputs to the computation of $\hSig$ to the uses of
|
||
$\NoteAddressRand$.
|
||
}
|
||
|
||
\sproutspecific{
|
||
Since the PRFs are instantiated using $\SHACompress$ which has an input block
|
||
size of $512$ bits (of which $256$ bits are used for the PRF input and $4$ bits
|
||
are used for domain separation), it was necessary to reduce the size of the
|
||
PRF key to $252$ bits. The key is set to $\AuthPrivate$ in the case of
|
||
$\PRFaddr{}$, $\PRFnf{}$, and $\PRFpk{}$, and to $\NoteAddressPreRand$ (which
|
||
does not exist in \Zerocash) for $\PRFrho{}$, and so those values have been
|
||
reduced to $252$ bits. This is preferable to requiring reasoning about truncation,
|
||
and $252$ bits is quite sufficient for security of these cryptovalues.
|
||
}
|
||
|
||
\sapling{
|
||
\Sapling uses \xPedersenHashes and $\BlakeTwosGeneric$ where \Sprout used $\SHACompress$.
|
||
\xPedersenHashes can be efficiently instantiated for arbitrary input lengths.
|
||
$\BlakeTwosGeneric$ has an input block size of $512$ bits, and uses a finalization flag
|
||
rather than padding of the last input block; it also supports domain separation
|
||
via a personalization parameter distinct from the input. Therefore, there is
|
||
no need for truncation in the inputs to any of these hashes.
|
||
\todo{check, especially $\CRHivk$ which has truncated output.}
|
||
}
|
||
|
||
\nsubsection{In-band secret distribution} \label{inbandrationale}
|
||
|
||
\Zerocash specified ECIES (referencing Certicom's SEC 1 standard) as the
|
||
encryption scheme used for the in-band secret distribution. This has been
|
||
changed to a key agreement scheme based on
|
||
\sprout{Curve25519,}
|
||
\notsprout{Curve25519 (for \Sprout) \sapling{or $\JubjubCurve$ (for \Sapling)}}
|
||
and the authenticated encryption algorithm $\SymSpecific$. This scheme is
|
||
still loosely based on ECIES, and on the $\CryptoBoxSeal$ scheme defined in
|
||
libsodium \cite{libsodium-Seal}.
|
||
|
||
\introlist
|
||
The motivations for this change were as follows:
|
||
|
||
\begin{itemize}
|
||
\item The \Zerocash paper did not specify the curve to be used.
|
||
We believe that Curve25519 has significant side-channel resistance,
|
||
performance, implementation complexity, and robustness advantages
|
||
over most other available curve choices, as explained in \cite{Bern2006}.
|
||
\sapling{For \Sapling, the $\JubjubCurve$ curve was designed according to a
|
||
similar design process following the ``Safe curves'' criteria
|
||
\cite{BL-SafeCurves} \cite{GitHub-jubjub}.
|
||
This retains Curve25519's advantages while keeping \paymentAddress sizes
|
||
short, because the same public key material supports both encryption and
|
||
spend authentication.}
|
||
\item ECIES permits many options, which were not specified. There are at least
|
||
--counting conservatively-- 576 possible combinations of options and
|
||
algorithms over the four standards (ANSI X9.63, IEEE Std 1363a-2004,
|
||
ISO/IEC 18033-2, and SEC 1) that define ECIES variants \cite{MAEA2010}.
|
||
\item Although the \Zerocash paper states that ECIES satisfies key privacy
|
||
(as defined in \cite{BBDP2001}), it is not clear that this holds for
|
||
all curve parameters and key distributions. For example, if a group of
|
||
non-prime order is used, the distribution of ciphertexts could be
|
||
distinguishable depending on the order of the points representing the
|
||
ephemeral and recipient public keys. Public key validity is also a concern.
|
||
Curve25519 \sapling{(and $\JubjubCurve$)} key agreement is defined in a way that
|
||
avoids these concerns due to the curve structure and the ``clamping'' of
|
||
private keys\sapling{ (or explicit cofactor multiplication and point
|
||
validation for \Sapling)}.
|
||
\item Unlike the DHAES/DHIES proposal on which it is based \cite{ABR1999}, ECIES
|
||
does not require a representation of the sender's ephemeral public key
|
||
to be included in the input to the KDF, which may impair the security
|
||
properties of the scheme. (The Std 1363a-2004 version of ECIES \cite{IEEE2004}
|
||
has a ``DHAES mode'' that allows this, but the representation of the key
|
||
input is underspecified, leading to incompatible implementations.)
|
||
The scheme we use has both the ephemeral and recipient public key
|
||
encodings --which are unambiguous for Curve25519-- and also $\hSig$ and
|
||
a nonce as described below, as input to the KDF. Note that being able to
|
||
break the Elliptic Curve Diffie-Hellman Problem on Curve25519 (without breaking
|
||
$\SymSpecific$ as an authenticated encryption scheme or $\BlakeTwob{256}$ as
|
||
a KDF) would not help to decrypt the \notesCiphertext unless
|
||
$\TransmitPublic$ is known or guessed.
|
||
\item \sproutspecific{The KDF also takes a public seed $\hSig$ as input.
|
||
This can be modeled as using a different ``randomness extractor'' for each
|
||
\joinSplitTransfer, which limits degradation of security with the number of
|
||
\joinSplitTransfers.
|
||
This facilitates security analysis as explained in \cite{DGKM2011} --- see
|
||
section 7 of that paper for a security proof that can be applied to this
|
||
construction under the assumption that single-block $\BlakeTwob{256}$ is a
|
||
``weak PRF''.
|
||
Note that $\hSig$ is authenticated, by the \zkSNARKProof\!\!, as having been chosen
|
||
with knowledge of $\AuthPrivateOld{\allOld}$, so an adversary cannot
|
||
modify it in a ciphertext from someone else's transaction for use in a
|
||
chosen-ciphertext attack without detection.}
|
||
\sapling{In \Sapling, there is no equivalent to $\hSig$. \todo{Explain why this is ok.}}
|
||
\item \sproutspecific{The scheme used by \SproutOrZcash includes an optimization that reuses
|
||
the same ephemeral key (with different nonces) for the two ciphertexts
|
||
encrypted in each \joinSplitDescription.}
|
||
\end{itemize}
|
||
|
||
The security proofs of \cite{ABR1999} can be adapted straightforwardly to the
|
||
resulting scheme. Although DHAES as defined in that paper does not pass the
|
||
recipient public key or a public seed to the \hashFunction $H$, this does not
|
||
impair the proof because we can consider $H$ to be the specialization of our
|
||
KDF to a given recipient key and seed. \sproutspecific{It is necessary to adapt the
|
||
``HDH independence'' assumptions and the proof slightly to take into account
|
||
that the ephemeral key is reused for two encryptions.}
|
||
|
||
Note that the $256$-bit key for $\SymSpecific$ maintains a high concrete security
|
||
level even under attacks using parallel hardware \cite{Bern2005} in the multi-user
|
||
setting \cite{Zave2012}. This is especially necessary because the privacy of
|
||
\Zcash transactions may need to be maintained far into the future, and upgrading
|
||
the encryption algorithm would not prevent a future adversary from attempting
|
||
to decrypt ciphertexts encrypted before the upgrade. Other cryptovalues that
|
||
could be attacked to break the privacy of transactions are also sufficiently long
|
||
to resist parallel brute force in the multi-user setting: \notsprout{for \Sprout,}
|
||
$\AuthPrivate$ is $252$ bits, and $\TransmitPrivate$ is no shorter than $\AuthPrivate$.
|
||
|
||
|
||
\nsubsection{Omission in \Zerocash security proof} \label{crprf}
|
||
|
||
The abstract \Zerocash protocol requires $\PRFaddr{}$ only to be a PRF;
|
||
it is not specified to be collision-resistant. This reveals a flaw in
|
||
the proof of the Balance property.
|
||
|
||
Suppose that an adversary finds a collision on $\PRFaddr{}$ such that
|
||
$\AuthPrivateSup{1}$ and $\AuthPrivateSup{2}$ are distinct \spendingKeys for
|
||
the same $\AuthPublic$. Because the \noteCommitment is to $\AuthPublic$,
|
||
but the \nullifier is computed from $\AuthPrivate$ (and $\NoteAddressRand$),
|
||
the adversary is able to double-spend the note, once with each $\AuthPrivate$.
|
||
This is not detected because each spend reveals a different \nullifier.
|
||
The \joinSplitStatements are still valid because they can only
|
||
check that the $\AuthPrivate$ in the witness is \emph{some} preimage of
|
||
the $\AuthPublic$ used in the \noteCommitment.
|
||
|
||
\introlist
|
||
The error is in the proof of Balance in \cite[Appendix D.3]{BCG+2014}.
|
||
For the ``$\Adversary$ violates Condition I'' case, the proof says:
|
||
|
||
\begin{itemize}
|
||
\item[``(i)] If $\cmOld{1} = \cmOld{2}$, then the fact that
|
||
$\snOld{1} \neq \snOld{2}$ implies that the witness $a$ contains
|
||
two distinct openings of $\cmOld{1}$ (the first opening contains
|
||
$(\AuthPrivateOldX{1}, \NoteAddressRandOld{1})$, while the second
|
||
opening contains $(\AuthPrivateOldX{2}, \NoteAddressRandOld{2})$).
|
||
This violates the binding property of the commitment scheme $\CommitAlg$."
|
||
\end{itemize}
|
||
|
||
In fact the openings do not contain $\AuthPrivateOld{i}$; they contain
|
||
$\AuthEmphPublicOld{i}$. (In \SproutOrZcash $\cmOld{i}$ opens directly to
|
||
$(\AuthEmphPublicOld{i}, \ValueOld{i}, \NoteAddressRandOld{i})$, and
|
||
in \Zerocash it opens to $(\ValueOld{i},
|
||
\Commit{\NoteCommitS}(\AuthEmphPublicOld{i}, \NoteAddressRandOld{i})$.)
|
||
|
||
A similar error occurs in the argument for the ``$\Adversary$ violates
|
||
Condition II'' case.
|
||
|
||
The flaw is not exploitable for the actual instantiations of $\PRFaddr{}$
|
||
in \Zerocash and \SproutOrZcash, which \emph{are} collision-resistant assuming
|
||
that $\SHACompress$ is.
|
||
|
||
The proof can be straightforwardly repaired. The intuition is that we can rely
|
||
on collision resistance of $\PRFaddr{}$ (on both its arguments) to argue that
|
||
distinctness of $\AuthPrivateOldX{1}$ and $\AuthPrivateOldX{2}$, together with
|
||
constraint 1(b) of the \joinSplitStatement (see \crossref{sproutspendauthority}),
|
||
implies distinctness of $\AuthPublicOldX{1}$ and $\AuthPublicOldX{2}$, therefore
|
||
distinct openings of the \noteCommitment when Condition I or II is violated.
|
||
|
||
\nsubsection{Miscellaneous}
|
||
|
||
\begin{itemize}
|
||
\item The paper defines a \note as $((\AuthPublic, \TransmitPublic), \Value,
|
||
\NoteAddressRand, \NoteCommitRand, \NoteCommitS, \cm)$, whereas this
|
||
specification defines \sprout{it}\notsprout{a \Sprout \note} as
|
||
$(\AuthPublic, \Value, \NoteAddressRand, \NoteCommitRand)$.
|
||
The instantiation of $\Commit{\NoteCommitS}$ in section 5.1 of the paper
|
||
did not actually use $\NoteCommitS$, and neither does the new
|
||
instantiation of $\NoteCommitSprout{}$ in \SproutOrZcash. $\TransmitPublic$ is also
|
||
not needed as part of a \note: it is not an input to $\NoteCommitSprout{}$ nor
|
||
is it constrained by the \Zerocash \POUR{} \statement or the
|
||
\Zcash \joinSplitStatement. $\cm$ can be computed from the other fields.
|
||
\sapling{(The definition of \notes for \Sapling is different again.)}
|
||
\item The length of proof encodings given in the paper is $288$ bytes.
|
||
\sproutspecific{This differs from the $296$ bytes specified in \crossref{phgr},
|
||
because both the $x$-coordinate and compressed $y$-coordinate of each
|
||
point need to be represented. Although it is possible to encode a proof
|
||
in $288$ bytes by making use of the fact that elements of $\GF{q}$ can
|
||
be represented in $254$ bits, we prefer to use the standard formats for points
|
||
defined in \cite{IEEE2004}. The fork of \libsnark used by \Zcash uses
|
||
this standard encoding rather than the less efficient (uncompressed) one
|
||
used by upstream \libsnark.}
|
||
\item The range of monetary values differs. In \Zcash, this range is
|
||
$\range{0}{\MAXMONEY}$; in \Zerocash it is $\range{0}{2^{64}-1}$.
|
||
(The \joinSplitStatement still only directly enforces that the sum
|
||
of amounts in a given \joinSplitTransfer is in the latter range;
|
||
this enforcement is technically redundant given that the Balance
|
||
property holds.)
|
||
\end{itemize}
|
||
|
||
|
||
\nsection{Acknowledgements}
|
||
|
||
The inventors of \Zerocash are Eli Ben-Sasson, Alessandro Chiesa,
|
||
Christina Garman, Matthew Green, Ian Miers, Eran Tromer, and Madars
|
||
Virza.
|
||
|
||
The authors would like to thank everyone with whom they have discussed
|
||
the \Zerocash protocol design; in addition to the inventors, this includes
|
||
Mike Perry, Isis Lovecruft, Leif Ryge, Andrew Miller, Zooko Wilcox,
|
||
Samantha Hulsey, Jack Grigg, Simon Liu, Ariel Gabizon, jl777, Ben Blaxill,
|
||
Alex Balducci, Jake Tarren, Solar Designer, Ling Ren, Alison Stevenson,
|
||
John Tromp, Paige Peterson, Maureen Walsh, Jay Graber, Jack Gavigan,
|
||
Filippo Valsorda, Zaki Manian, George Tankersley, Tracy Hu,
|
||
and no doubt others.
|
||
|
||
\Zcash has benefited from security audits performed by NCC Group and
|
||
Coinspect.
|
||
|
||
The Faerie Gold attack was found by Zooko Wilcox; subsequent analysis
|
||
of variations on the attack was performed by Daira Hopwood and Sean Bowe.
|
||
The internal hash collision attack was found by Taylor Hornby.
|
||
The error in the \Zerocash proof of Balance relating to collision-resistance
|
||
of $\PRFaddr{}$ was found by Daira Hopwood.
|
||
The errors in the proof of Ledger Indistinguishability mentioned in
|
||
\crossref{truncation} were also found by Daira Hopwood.
|
||
|
||
\sapling{
|
||
The design of \Sapling is primarily due to Matthew Green, Ian Miers,
|
||
Daira Hopwood, Sean Bowe, and Jack Grigg.
|
||
}
|
||
|
||
|
||
\introsection
|
||
\nsection{Change History}
|
||
|
||
\subparagraph{2018.0-beta-12}
|
||
|
||
\begin{itemize}
|
||
\item No changes to \Sprout.
|
||
\nuzero{
|
||
\item Add references to \NUZero ZIPs and update the section on
|
||
\NUZero/\Sapling transitions.
|
||
}
|
||
\sapling{
|
||
\item Add a section on re-randomizable signatures.
|
||
\item Add definition of $\PRFnr{}$.
|
||
\item Work-in-progress on \Sapling statements.
|
||
\item Rename \quotedterm{raw} to \quotedterm{homomorphic} \xPedersenCommitments.
|
||
\item Add packing modulo the field size and range checks to Appendix A.
|
||
\item Update the algorithm for variable-base scalar multiplication to
|
||
what is implemented by sapling-crypto.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-11}
|
||
|
||
\begin{itemize}
|
||
\item No changes to \Sprout.
|
||
\sapling{
|
||
\item Add sections on \spendDescriptions and \outputDescriptions.
|
||
\item Swap order of $\cv$ and $\rt$ in a \spendDescription for consistency.
|
||
\item Fix off-by-one error in the range of $\InViewingKey$.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-10}
|
||
|
||
\begin{itemize}
|
||
\item Split the descriptions of $\SHAFull$ and $\SHACompress$\sapling{, and of $\BlakeTwoGeneric$,}
|
||
into their own sections. Specify $\SHACompress$ more precisely.
|
||
\item Add Tracy Hu to acknowledgements\sapling{ (for the idea of explicitly
|
||
encoding the root of the \Sapling \noteCommitmentTree in \blockHeaders)}.
|
||
\sapling{
|
||
\item Refer to \NUZero and \Sapling just as ``upgrades'' in the abstract, not as
|
||
the next ``minor version'' and ``major version''.
|
||
\item $\PRFnr{}$ must be collision-resistant.
|
||
\item Correct an error in the \xPedersenHash specification.
|
||
\item Use a named variable, $c$, for chunks per segment in the \xPedersenHash
|
||
specification, and change its value from $61$ to $63$. Add a proof
|
||
justifying this value of $c$.
|
||
\item Specify \xPedersenCommitments.
|
||
\item Notation changes.
|
||
\item Generalize the \distinctXCriterion (\theoremref{thmdistinctxcriterion})
|
||
to allow negative indices.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-9}
|
||
|
||
\begin{itemize}
|
||
\item Specify the coinbase maturity rule, and the rule that \coinbaseTransactions
|
||
cannot contain \joinSplitDescriptions\sapling{, \spendDescriptions, or
|
||
\outputDescriptions}.
|
||
\nuzero{
|
||
\item Delay lifting the 100000-byte \transaction size limit from \NUZero to
|
||
\Sapling.
|
||
}
|
||
\sapling{
|
||
\item Improve presentation of the proof of injectivity for $\ExtractJ$.
|
||
\item Specify $\GroupJHash{}$.
|
||
\item Specify \xPedersenHashes.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-8}
|
||
|
||
\begin{itemize}
|
||
\item No changes to \Sprout.
|
||
\sapling{
|
||
\item Add instantiation of $\CRHivk$.
|
||
\item Add instantiation of a hash extractor for \Jubjub.
|
||
\item Make the background lighter and the \Sapling green darker, for contrast.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-7}
|
||
|
||
\begin{itemize}
|
||
\item Specify the $100000$-byte limit on \transaction size.
|
||
(The implementation in \zcashd was as intended.)
|
||
\item Specify that $\hexint{F6}$ followed by $511$ zero bytes encodes an
|
||
empty \memo.
|
||
\item Reference security definitions for
|
||
\pseudoRandomFunctions\sapling{ and \pseudoRandomGenerators}.
|
||
\item Rename $\mathsf{clamp}$ to $\mathsf{bound}$ and
|
||
$\mathsf{ActualTimespanClamped}$ to $\ActualTimespanBounded$
|
||
in the difficulty adjustment algorithm, to avoid a name
|
||
collision with Curve25519 scalar ``clamping''.
|
||
\item Change uses of the term \term{full node} to \fullValidator.
|
||
A \term{full node} by definition participates in the
|
||
peer-to-peer network, whereas a \fullValidator just needs a copy
|
||
of the \blockchain from somewhere. The latter is what was meant.
|
||
\sapling{
|
||
\item Add an explanation of how \Sapling prevents Faerie Gold and
|
||
roadblock attacks.
|
||
\item \Sapling work in progress.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-6}
|
||
|
||
\begin{itemize}
|
||
\item No changes to \Sprout.
|
||
\sapling{
|
||
\item \Sapling work in progress, mainly on \crossref{circuitdesign}.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-5}
|
||
|
||
\begin{itemize}
|
||
\item Specify more precisely the requirements on $\JoinSplitSigSpecific$
|
||
public keys and signatures.
|
||
\sapling{
|
||
\item \Sapling work in progress.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-4}
|
||
|
||
\begin{itemize}
|
||
\item No changes to \Sprout.
|
||
\sapling{
|
||
\item Update key components diagram for \Sapling.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2018.0-beta-3}
|
||
|
||
\begin{itemize}
|
||
\item Explain how the chosen fix to Faerie Gold avoids a potential
|
||
``roadblock'' attack.
|
||
\sapling{
|
||
\item Update some explanations of changes from \Zerocash for \Sapling.
|
||
\item Add a description of the $\JubjubCurve$ curve.
|
||
\item Add an acknowledgement to George Tankersley.
|
||
\item Add an appendix on the design of the \Sapling circuits at the
|
||
\quadraticArithmeticProgram level.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.9}
|
||
|
||
\begin{itemize}
|
||
\item Refer to $\TransmitPrivate$ as a \receivingKey rather than as a
|
||
viewing key.
|
||
\item Updates for \incomingViewingKey support.
|
||
\nuzero{
|
||
\item Refer to Network Upgrade 0 as \NUZero.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.8}
|
||
|
||
\begin{itemize}
|
||
\item Correct the non-normative note describing how to check the order
|
||
of $\Proof{B}$.
|
||
\sapling{
|
||
\item Initial version of draft \Sapling protocol specification.
|
||
}
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.7}
|
||
|
||
\begin{itemize}
|
||
\item Fix an off-by-one error in the specification of the Equihash algorithm
|
||
binding condition. (The implementation in \zcashd was as intended.)
|
||
\item Correct the types and consensus rules for \transactionVersionNumbers
|
||
and \blockVersionNumbers. (Again, the implementation in \zcashd was as
|
||
intended.)
|
||
\item Clarify the computation of $\h{i}$ in a \joinSplitStatement.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.6}
|
||
|
||
\begin{itemize}
|
||
\item Be more precise when talking about curve points and pairing groups.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.5}
|
||
|
||
\begin{itemize}
|
||
\item Clarify the consensus rule preventing double-spends.
|
||
\item Clarify what a \noteCommitment opens to in \crossref{crprf}.
|
||
\item Correct the order of arguments to $\CommitAlg$ in \crossref{concretesproutcommit}.
|
||
\item Correct a statement about indistinguishability of \joinSplitDescriptions.
|
||
\item Change the \foundersReward addresses, for the test network only, to
|
||
reflect the hard fork described in \cite{ZcashIssue-2113}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.4}
|
||
|
||
\begin{itemize}
|
||
\item Explain a variation on the Faerie Gold attack and why it is prevented.
|
||
\item Generalize the description of the InternalH attack to include finding
|
||
collisions on $(\AuthPublic, \NoteAddressRand)$ rather than just on
|
||
$\NoteAddressRand$.
|
||
\item Rename $\mathsf{enforce}_i$ to $\EnforceMerklePath{i}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.3}
|
||
|
||
\begin{itemize}
|
||
\item Specify the security requirements on the $\shaCompress$ function in order
|
||
for the scheme in \crossref{concretesproutcommit} to be a secure commitment.
|
||
\item Specify $\GroupG{2}$ more precisely.
|
||
\item Explain the use of interstitial \treestates in chained \joinSplitTransfers.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.2}
|
||
|
||
\begin{itemize}
|
||
\item Give definitions of computational binding and computational hiding
|
||
for commitment schemes.
|
||
\item Give a definition of statistical zero knowledge.
|
||
\item Reference the white paper on MPC parameter generation \cite{BGG2016}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2.1}
|
||
|
||
\begin{itemize}
|
||
\item $\MerkleHashLength$ is a bit length, not a byte length.
|
||
\item Specify the maximum \block size.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2017.0-beta-2}
|
||
|
||
\begin{itemize}
|
||
\item Add abstract and keywords.
|
||
\item Fix a typo in the definition of \nullifier integrity.
|
||
\item Make the description of \blockchains more consistent with
|
||
upstream \Bitcoin documentation (referring to ``best`` chains
|
||
rather than using the concept of a \term{block chain view}).
|
||
\item Define how nodes select a best chain.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.13}
|
||
|
||
\begin{itemize}
|
||
\item Specify the difficulty adjustment algorithm.
|
||
\item Clarify some definitions of fields in a \blockHeader.
|
||
\item Define $\PRFaddr{}$ in \crossref{sproutkeycomponents}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.12}
|
||
|
||
\begin{itemize}
|
||
\item Update the hashes of proving and verifying keys for the final Sprout parameters.
|
||
\item Add cross references from \paymentAddress and \spendingKey encoding
|
||
sections to where the key components are specified.
|
||
\item Add acknowledgements for Filippo Valsorda and Zaki Manian.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.11}
|
||
|
||
\begin{itemize}
|
||
\item Specify a check on the order of $\Proof{B}$ in a \zeroKnowledgeProof.
|
||
\item Note that due to an oversight, the \Zcash \genesisBlock does not
|
||
follow \cite{BIP-34}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.10}
|
||
|
||
\begin{itemize}
|
||
\item Update reference to the Equihash paper \cite{BK2016}. (The newer version
|
||
has no algorithmic changes, but the section discussing potential ASIC
|
||
implementations is substantially expanded.)
|
||
\item Clarify the discussion of proof size in ``Differences from the \Zerocash paper''.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.9}
|
||
|
||
\begin{itemize}
|
||
\item Add \foundersReward addresses for the production network.
|
||
\item Change \quotedterm{protected} terminology to \quotedterm{shielded}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.8}
|
||
|
||
\begin{itemize}
|
||
\item Revise the lead bytes for \transparent P2SH and P2PKH addresses,
|
||
and reencode the testnet \foundersReward addresses.
|
||
\item Add a section on which BIPs apply to \Zcash.
|
||
\item Specify that \ScriptOP{CODESEPARATOR} has been disabled, and
|
||
no longer affects signature hashes.
|
||
\item Change the representation type of $\vpubOldField$ and $\vpubNewField$
|
||
to \type{uint64\_t}. (This is not a consensus change because the type of
|
||
$\vpubOld$ and $\vpubNew$ was already specified to be $\range{0}{\MAXMONEY}$;
|
||
it just better reflects the implementation.)
|
||
\item Correct the representation type of the \block $\nVersion$ field to
|
||
\type{uint32\_t}.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.7}
|
||
|
||
\begin{itemize}
|
||
\item Clarify the consensus rule for payment of the \foundersReward, in
|
||
response to an issue raised by the NCC audit.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.6}
|
||
|
||
\begin{itemize}
|
||
\item Fix an error in the definition of the sortedness condition for Equihash:
|
||
it is the sequences of indices that are sorted, not the sequences of
|
||
hashes.
|
||
\item Correct the number of bytes in the encoding of $\solutionSize$.
|
||
\item Update the section on encoding of \transparent addresses.
|
||
(The precise prefixes are not decided yet.)
|
||
\item Clarify why $\BlakeTwob{\ell}$ is different from truncated $\BlakeTwob{512}$.
|
||
\item Clarify a note about SU-CMA security for signatures.
|
||
\item Add a note about $\PRFnf{}$ corresponding to $\PRFsn{}$ in \Zerocash.
|
||
\item Add a paragraph about key length in \crossref{inbandrationale}.
|
||
\item Add acknowledgements for John Tromp, Paige Peterson, Maureen Walsh,
|
||
Jay Graber, and Jack Gavigan.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.5}
|
||
|
||
\begin{itemize}
|
||
\item Update the \foundersReward address list.
|
||
\item Add some clarifications based on Eli Ben-Sasson's review.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.4}
|
||
|
||
\begin{itemize}
|
||
\item Specify the \blockSubsidy, \minerSubsidy, and the \foundersReward.
|
||
\item Specify \coinbaseTransaction outputs to \foundersReward addresses.
|
||
\item Improve notation (for example ``$\mult$'' for multiplication and
|
||
``$\typeexp{T}{\ell}$'' for sequence types) to avoid ambiguity.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.3}
|
||
|
||
\begin{itemize}
|
||
\item Correct the omission of $\solutionSize$ from the \blockHeader format.
|
||
\item Document that \compactSize{} encodings must be canonical.
|
||
\item Add a note about conformance language in the introduction.
|
||
\item Add acknowledgements for Solar Designer, Ling Ren and Alison Stevenson,
|
||
and for the NCC Group and Coinspect security audits.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.2}
|
||
|
||
\begin{itemize}
|
||
\item Remove $\mathsf{GeneralCRH}$ in favour of specifying $\hSigCRH$ and
|
||
$\EquihashGen{}$ directly in terms of $\BlakeTwob{\ell}$.
|
||
\item Correct the security requirement for $\EquihashGen{}$.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1.1}
|
||
|
||
\begin{itemize}
|
||
\item Add a specification of abstract signatures.
|
||
\item Clarify what is signed in the ``Sending Notes'' section.
|
||
\item Specify ZK parameter generation as a randomized algorithm, rather
|
||
than as a distribution of parameters.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-beta-1}
|
||
|
||
\begin{itemize}
|
||
\item Major reorganization to separate the abstract cryptographic protocol
|
||
from the algorithm instantiations.
|
||
\item Add type declarations.
|
||
\item Add a ``High-level Overview'' section.
|
||
\item Add a section specifying the \zeroKnowledgeProvingSystem and the
|
||
encoding of proofs. Change the encoding of points in proofs to follow
|
||
IEEE Std 1363[a].
|
||
\item Add a section on consensus changes from \Bitcoin, and the specification
|
||
of Equihash.
|
||
\item Complete the ``Differences from the \Zerocash paper'' section.
|
||
\item Correct the Merkle tree depth to 29.
|
||
\item Change the length of \memos to 512 bytes.
|
||
\item Switch the \joinSplitSignature scheme to Ed25519, with consequent
|
||
changes to the computation of $\hSig$.
|
||
\item Fix the lead bytes in \paymentAddress and \spendingKey encodings to
|
||
match the implemented protocol.
|
||
\item Add a consensus rule about the ranges of $\vpubOld$ and $\vpubNew$.
|
||
\item Clarify cryptographic security requirements and added definitions
|
||
relating to the in-band secret distribution.
|
||
\item Add various citations: the ``Fixing Vulnerabilities in the Zcash
|
||
Protocol'' and ``Why Equihash?'' blog posts, several crypto papers
|
||
for security definitions, the \Bitcoin whitepaper, the \CryptoNote
|
||
whitepaper, and several references to \Bitcoin documentation.
|
||
\item Reference the extended version of the \Zerocash paper rather than the
|
||
Oakland proceedings version.
|
||
\item Add \joinSplitTransfers to the Concepts section.
|
||
\item Add a section on Coinbase Transactions.
|
||
\item Add acknowledgements for Jack Grigg, Simon Liu, Ariel Gabizon, jl777,
|
||
Ben Blaxill, Alex Balducci, and Jake Tarren.
|
||
\item Fix a \texttt{Makefile} compatibility problem with the escaping behaviour
|
||
of \texttt{echo}.
|
||
\item Switch to \texttt{biber} for the bibliography generation, and add
|
||
backreferences.
|
||
\item Make the date format in references more consistent.
|
||
\item Add visited dates to all URLs in references.
|
||
\item Terminology changes.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-alpha-3.1}
|
||
|
||
\begin{itemize}
|
||
\item Change main font to Quattrocento.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2016.0-alpha-3}
|
||
|
||
\begin{itemize}
|
||
\item Change version numbering convention (no other changes).
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2.0-alpha-3}
|
||
|
||
\begin{itemize}
|
||
\item Allow anchoring to any previous output \treestate in the same \transaction,
|
||
rather than just the immediately preceding output \treestate.
|
||
\item Add change history.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2.0-alpha-2}
|
||
|
||
\begin{itemize}
|
||
\item Change from truncated $\BlakeTwob{512}$ to $\BlakeTwob{256}$.
|
||
\item Clarify endianness, and that uses of $\BlakeTwobGeneric$ are unkeyed.
|
||
\item Minor correction to what \sighashTypes cover.
|
||
\item Add ``as intended for the \Zcash release of summer 2016" to title page.
|
||
\item Require $\PRFaddr{}$ to be collision-resistant (see \crossref{crprf}).
|
||
\item Add specification of path computation for the \incrementalMerkleTree.
|
||
\item Add a note in \crossref{sproutmerklepathvalidity} about how this condition
|
||
corresponds to conditions in the \Zerocash paper.
|
||
\item Changes to terminology around keys.
|
||
\end{itemize}
|
||
|
||
\introlist
|
||
\subparagraph{2.0-alpha-1}
|
||
|
||
\begin{itemize}
|
||
\item First version intended for public review.
|
||
\end{itemize}
|
||
|
||
|
||
\nsection{References}
|
||
|
||
\begingroup
|
||
\hfuzz=2pt
|
||
\renewcommand{\section}[2]{}
|
||
\renewcommand{\emph}[1]{\textit{#1}}
|
||
\printbibliography
|
||
\endgroup
|
||
|
||
\notsprout{
|
||
|
||
\introsection
|
||
\vspace{20ex}
|
||
\appendix
|
||
\phantomsection
|
||
\addcontentsline{toc}{section}{\larger{\nstrut{Appendices}}}
|
||
{\Larger{\textbf{Appendices}}}
|
||
|
||
\nsection{Circuit Design} \label{circuitdesign}
|
||
|
||
\nsubsection{\QuadraticArithmeticPrograms}
|
||
|
||
\Sapling defines two circuits, Spend and Output, each implementing an abstract
|
||
statement described in \crossref{spendstatement} and \crossref{outputstatement}
|
||
respectively.
|
||
At the next lower level, each circuit is defined in terms of a
|
||
\quadraticArithmeticProgram, detailed in this section. The description
|
||
given here is necessary to compute witness elements for the circuit.
|
||
|
||
\vspace{1.5ex}
|
||
Let $\GF{\ParamS{r}}$ be the finite field over which $\JubjubCurve$ is defined, as
|
||
given in \crossref{jubjub}.
|
||
|
||
\introlist
|
||
A \quadraticArithmeticProgram consists of a set of constraints over
|
||
variables in $\GF{\ParamS{r}}$, each of the form:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{A}{B}{C}$
|
||
\end{formulae}
|
||
\vspace{-2ex}
|
||
where $\lincomb{A}$, $\lincomb{B}$, and $\lincomb{C}$ are \linearCombinations
|
||
of variables and constants in $\GF{\ParamS{r}}$.
|
||
|
||
Here $\times$ and $\mult$ both represent multiplication in the field $\GF{\ParamS{r}}$,
|
||
but we use $\times$ for multiplications corresponding to gates of the circuit,
|
||
and $\mult$ for multiplications by constants in the terms of a \linearCombination.
|
||
|
||
\nsubsection{Elliptic curve background} \label{ecbackground}
|
||
|
||
The circuit makes use of a twisted Edwards curve, $\JubjubCurve$, and also a
|
||
Montgomery curve that is birationally equivalent to $\JubjubCurve$.
|
||
From here on we omit ``twisted'' when referring to the Edwards $\JubjubCurve$
|
||
curve or coordinates. By convention we use $(u, \varv)$ for affine coordinates
|
||
on the Edwards curve, and $(x, y)$ for affine coordinates on the Montgomery curve.
|
||
|
||
\introlist
|
||
The Montgomery curve has parameters $\ParamM{A} = 40962$ and $\ParamM{B} = 1$.
|
||
We use an affine representation of this curve with the formula:
|
||
|
||
\begin{formulae}
|
||
\item $\ParamM{B} \smult y^2 = x^3 + \ParamM{A} \smult x^2 + x$
|
||
\end{formulae}
|
||
|
||
Usually, elliptic curve arithmetic over prime fields is implemented using
|
||
some form of projective coordinates, in order to reduce the number of expensive
|
||
inversions required. In the circuit, it turns out that a division can be
|
||
implemented at the same cost as a multiplication, i.e.\ one constraint.
|
||
Therefore it is beneficial to use affine coordinates for both curves.
|
||
|
||
\introlist
|
||
We define the following types representing affine Edwards and Montgomery
|
||
coordinates respectively:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l@{\;}l}
|
||
$\AffineEdwardsJubjub$ &$:= (u \typecolon \GF{\ParamS{r}}) \times (\hspace{0.04em}\varv\hspace{0.04em} \typecolon \GF{\ParamS{r}})$
|
||
&$: \ParamJ{a} \smult u^2 + \varv^2 = 1 + \ParamJ{d} \smult u^2 \smult \varv^2$ \\
|
||
$\AffineMontJubjub$ &$:= (x \typecolon \GF{\ParamS{r}}) \times (y \typecolon \GF{\ParamS{r}})$
|
||
&$: \ParamM{B} \smult y^2 = x^3 + \ParamM{A} \smult x^2 + x$
|
||
\end{tabular}
|
||
|
||
\introlist
|
||
We also define a type representing compressed, \emph{not necessarily valid},
|
||
Edwards coordinates:
|
||
|
||
\begin{formulae}
|
||
\item $\CompressedEdwardsJubjub := (\tilde{u} \typecolon \bit) \times (\varv \typecolon \GF{\ParamS{r}})$
|
||
\end{formulae}
|
||
\vspace{-1.5ex}
|
||
See \crossref{jubjub} for how this type is represented as a byte sequence in
|
||
external encodings.
|
||
|
||
\vspace{2ex}
|
||
We use affine Montgomery arithmetic in parts of the circuit because it is
|
||
more efficient, in terms of the number of constraints, than affine Edwards
|
||
arithmetic.
|
||
|
||
An important consideration when using Montgomery arithmetic is that the
|
||
addition formula is not complete, that is, there are cases where it produces
|
||
the wrong answer. We must ensure that these cases do not arise.
|
||
|
||
\introlist
|
||
We will need the theorem below about $y$-coordinates of points on
|
||
Montgomery curves.
|
||
|
||
\fact{$\ParamM{A}^2 - 4$ is a nonsquare in $\GF{\ParamJ{r}}$.}
|
||
|
||
\begin{theorem} \label{thmmontynotzero}
|
||
Let $P = (x, y)$ be a point other than $(0, 0)$ on a Montgomery curve
|
||
over $\GF{r}$ with parameter $A$, such that $A^2 - 4$ is a nonsquare in $\GF{r}$.
|
||
Then $y \neq 0$.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
Substituting $y = 0$ into the Montgomery curve equation gives
|
||
$0 = x^3 + A \mult x^2 + x = x \mult (x^2 + A \mult x + 1)$.
|
||
So either $x = 0$ or $x^2 + A \mult x + 1 = 0$.
|
||
Since $P \neq (0, 0)$, the case $x = 0$ is excluded.
|
||
In the other case, complete the square for $x^2 + A \mult x + 1 = 0$
|
||
to give the equivalent $(2 \mult x + A)^2 = A^2 - 4$.
|
||
The left-hand side is a square, so if the right-hand side is a nonsquare,
|
||
then there are no solutions for $x$.
|
||
\end{proof}
|
||
|
||
|
||
\introsection
|
||
\nsubsection{Circuit Components}
|
||
|
||
Each of the following sections describes how to implement a particular
|
||
component of the circuit, and counts the number of constraints required.
|
||
Some components make use of others; the order of presentation is ``bottom-up''.
|
||
|
||
It is important for security to ensure that variables intended to be of
|
||
boolean type are boolean-constrained; and for efficiency that they are
|
||
boolean-constrained only once. We follow the convention that components
|
||
typically boolean-constrain their inputs when needed, but not their outputs.
|
||
Exceptions to this convention are explicitly noted.
|
||
|
||
In this section, variables have type $\GF{\ParamS{r}}$ unless otherwise specified.
|
||
In contrast to most of this document, we use zero-based indexing in order
|
||
to more closely match the implementation.
|
||
|
||
\introlist
|
||
\nsubsubsection{Boolean constraints} \label{cctboolean}
|
||
|
||
A boolean constraint $b \in \bit$ can be implemented as:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{1 - b}{b}{0}$
|
||
\end{formulae}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Selection constraints} \label{cctselection}
|
||
|
||
A selection constraint $b \bchoose x : y = z$, where $b \in \bit$, can be implemented as:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{b}{y - x}{y - z}$
|
||
\end{formulae}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Nonzero constraints} \label{cctnonzero}
|
||
|
||
Since only nonzero elements of $\GF{\ParamS{r}}$ have a multiplicative inverse, the
|
||
assertion $a \neq 0$ can be implemented by witnessing the inverse,
|
||
$\ainv = a^{-1} \pmod{\ParamS{r}}$:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{\ainv}{a}{1}$
|
||
\end{formulae}
|
||
|
||
A global optimization allows to use a single inverse computation outside
|
||
the circuit for any number of nonzero constraints. Suppose that we have
|
||
$n$ variables (or \linearCombinations) that are supposed to be nonzero:
|
||
$a_\barerange{0}{n-1}$. Multiply these together to give $a = \vproduct{i=0}{n-1} a_i$;
|
||
then, constrain $a$ to be nonzero. This works because the product $a$ is nonzero
|
||
if and only if all of $a_\barerange{0}{n-1}$ are nonzero.
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Not-all-one constraints} \label{cctnotallone}
|
||
|
||
Given a sequence $b_\barerange{0}{n-1}$ of variables that have already been
|
||
boolean-constrained, we can assert that they are not all one by letting
|
||
$a = -n + \vsum{i=0}{n-1} b_i$, and asserting $a \neq 0$ as in the previous
|
||
section:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{\ainv}{-n + \vsum{i=0}{n-1} b_i}{1}$
|
||
\end{formulae}
|
||
|
||
(This assumes $n < \ParamS{r}$ which is in practice always the case.)
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Unpacking} \label{cctunpack}
|
||
|
||
A field element $a$ may need to be ``unpacked'' to a sequence of boolean
|
||
variables $b_\barerange{0}{n-1} \typecolon \bitseq{n}$, so that
|
||
$a = \vsum{i=0}{n-1} b_i \mult 2^i$.
|
||
|
||
\introlist
|
||
This costs $n$ constraints to boolean-constrain $b_\barerange{0}{n-1}$
|
||
as in \crossref{cctboolean}, and one constraint that equates the sum with
|
||
$a$:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{\vsum{i=0}{n-1} b_i \mult 2^i}{1}{a}$
|
||
\end{formulae}
|
||
|
||
\pnote{
|
||
Since the last constraint has only a trivial multiplication, it is
|
||
possible to eliminate it by expressing one of the output bits as
|
||
a linear combination of the others and $a$. However, this optimization
|
||
requires substitutions that would interfere with the modularity of the
|
||
circuit implementation (for a saving of only one constraint per unpacking
|
||
operation), and so we do not use it for the \Sapling circuit.
|
||
\todo{Do we want to use it internally to the BLAKE2s implementation where
|
||
modularity is not significantly affected?}
|
||
}
|
||
|
||
In the case $n = 255$, for $a < 2^{255} - \ParamS{r}$ there are two possible
|
||
representations of $a \typecolon \GF{\ParamS{r}}$ as a sequence of $255$ bits,
|
||
corresponding to $\ItoLEBSP{255}(a)$ and $\ItoLEBSP{255}(a + \ParamS{r})$.
|
||
This is a potential hazard, but it may or may not be necessary to force use
|
||
of the canonical representation $\ItoLEBSP{255}(a)$, depending on the context
|
||
in which the unpacking operation is used. We therefore do not consider this
|
||
to be part of the unpacking operation itself.
|
||
|
||
\todo{Check where canonical $255$-bit unpackings are needed. They are not
|
||
needed for the Merkle path check.}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Packing modulo \rS} \label{cctmodpack}
|
||
|
||
Let $a = \vsum{i=0}{n-1} b_i \mult 2^i$.
|
||
|
||
Then, $a \bmod \ParamS{r} = \left(\vsum{i=0}{n-1} b_i \mult (2^i \bmod \ParamS{r})\!\right) \bmod \ParamS{r}$.
|
||
|
||
The bit length $n$ is not limited by the field element size.
|
||
|
||
This operation costs one constraint; it is used in the definition of
|
||
$\PRFnr{}$ in \crossref{concreteprfs}.
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Range check} \label{cctrange}
|
||
|
||
Let $a = \vsum{i=0}{n-1} a_i \mult 2^i$, and suppose we want to constrain
|
||
$a \leq c$ for some \emph{constant} $c = \vsum{i=0}{n-1} c_i \mult 2^i$.
|
||
|
||
Without loss of generality we can assume that $c_{n-1} = 1$, because if it
|
||
were not then we would reduce $n$.
|
||
|
||
Note that since $a$ and $c$ are provided in binary representation, their
|
||
bit length $n$ is not limited by the field element size. We \emph{do not} assume
|
||
that the bits $a_\barerange{0}{n-1}$ are already boolean-constrained.
|
||
|
||
Suppose $c$ has $k$ bits set to $1$, and let $j_\barerange{0}{k-1}$ be the
|
||
indices of those bits in ascending order. Let $t$ be the minimum of $k-1$ and
|
||
the number of trailing $1$ bits in $c$.
|
||
|
||
Let $\Pi_{j_{k-1}} = a_{j_{k-1}}$. For $z \in \range{t}{k-2}$, constrain:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{\Pi_{j_{z+1}}}{a_{j_z}}{\Pi_{j_z}}$
|
||
\end{formulae}
|
||
|
||
For $i \in \range{0}{n-1}$:
|
||
\begin{itemize}
|
||
\item if $c_i = 0$, constrain $\constraint{1 - \Pi_{j_z} - a_i}{a_i}{0}$ where $j_z$ is the least element of $j$ greater than $i$;
|
||
\item if $c_i = 1$, boolean-constrain $a_i$ as in \crossref{cctboolean}.
|
||
\end{itemize}
|
||
|
||
Note that the constraints corresponding to zero bits of $c$ are \emph{in place of}
|
||
boolean constraints on bits of $a_i$.
|
||
|
||
This costs $n + k - 1 - t$ constraints.
|
||
|
||
\todo{Explain why this works (see \url{https://github.com/zcash/zcash/issues/2234\#issuecomment-338930637}).}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Checking that affine Edwards coordinates are on the curve} \label{cctedvalidate}
|
||
|
||
To check that $(u, \varv)$ is a point on the Edwards curve, use:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{u}{u}{uu}$
|
||
\item $\constraint{\varv}{\varv}{\varvv}$
|
||
\item $\constraint{\ParamJ{d} \smult uu}{\varvv}{\ParamJ{a} \smult uu + \varvv - 1}$
|
||
\end{formulae}
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Edwards decompression and validation} \label{ccteddecompressvalidate}
|
||
|
||
Define $\DecompressValidate \typecolon \CompressedEdwardsJubjub \rightarrow \AffineEdwardsJubjub$
|
||
as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\DecompressValidate(\tilde{u}, \varv) = ...$
|
||
\end{formulae}
|
||
|
||
This can be implemented by:
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Edwards \lrarrow\ Montgomery conversion} \label{cctconversion}
|
||
|
||
Define $\EdwardsToMont \typecolon \AffineEdwardsJubjub \rightarrow \AffineMontJubjub$
|
||
as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\EdwardsToMont(u, \varv) = \left(\hfrac{1 + \varv}{1 - \varv},
|
||
\scalebox{0.8}{$\ssqrt{-40964}$} \mult \hfrac{1 + \varv}{(1 - \varv) \mult u}\right)
|
||
\sidecondition{1 - \varv \neq 0 \tand u \neq 0}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
Define $\MontToEdwards \typecolon \AffineMontJubjub \rightarrow \AffineEdwardsJubjub$
|
||
as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\MontToEdwards(x, y) = \left(\scalebox{0.8}{$\ssqrt{-40964}$} \mult \hfrac{x}{y},
|
||
\hfrac{x - 1}{x + 1}\right)
|
||
\sidecondition{x + 1 \neq 0 \tand y \neq 0}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
Either of these conversions can be implemented by the same \quadraticArithmeticProgram:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{y}{u}{\ssqrt{-40964} \mult x}$
|
||
\item $\constraint{x + 1}{\varv}{x - 1}$
|
||
\end{formulae}
|
||
|
||
The above conversions should only be used if the input is guaranteed to be
|
||
a point on the relevant curve. If that is the case, the theorems below
|
||
enumerate all exceptional inputs that may violate the side-conditions.
|
||
|
||
\vspace{1ex}
|
||
\begin{theorem} \label{thmconversiontomontnoexcept}
|
||
Let $(u, \varv)$ be an affine point on a complete twisted Edwards curve.
|
||
Then the only points with $u \neq 0$ or $\varv \neq 0$
|
||
are $(0, 1) = \ZeroJ$; $(0, -1)$ of order $2$; and
|
||
$\left(\pm\, 1/\!\ssqrt{\ParamJ{a}}, 0\right)$ of order $4$.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
Straightforward from the curve equation. (The fact that the points
|
||
$\left(\pm\, 1/\!\ssqrt{\ParamJ{a}}, 0\right)$ are of order $4$
|
||
can be inferred by applying the doubling formula.)
|
||
\end{proof}
|
||
|
||
\vspace{0.5ex}
|
||
\begin{theorem} \label{thmconversiontoedwardsnoexcept}
|
||
Let $(x, y)$ be an affine point on a Montgomery curve over $\GF{r}$
|
||
with parameter $A$ such that $A^2 - 4$ is a nonsquare in $\GF{r}$,
|
||
that is birationally equivalent to a complete twisted Edwards curve.
|
||
Then $x + 1 \neq 0$, and the only point $(x, y)$ with $y = 0$ is
|
||
$(0, 0)$ of order 2.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
That the only point with $y = 0$ is $(0, 0)$ is proven by \theoremref{thmmontynotzero}.
|
||
|
||
If $x + 1 = 0$, then subtituting $x = -1$ into the Montgomery curve equation gives
|
||
$\ParamM{B} \mult y^2 = x^3 + \ParamM{A}.x^2 + x = \ParamM{A} - 2$.
|
||
So in that case $y^2 = (\ParamM{A} - 2)/\ParamM{B}$. The right-hand-side is equal
|
||
to the parameter $d$ of a particular complete twisted Edwards curve birationally
|
||
equivalent to the Montgomery curve (see \cite[section 4.3.5]{BL2017}).
|
||
For all complete twisted Edwards curves, $d$ is nonsquare, so this equation
|
||
has no solutions for $y$, hence $x + 1 \neq 0$.
|
||
\end{proof}
|
||
|
||
(The complete twisted Edwards curve referred to in the proof is an
|
||
isomorphic $y$-coordinate rescaling of the $\JubjubCurve$ curve.)
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Affine-Montgomery arithmetic} \label{cctmontarithmetic}
|
||
|
||
The incomplete affine-Montgomery addition formulae given in
|
||
\cite[section 4.3.2]{BL2017} are:
|
||
|
||
\begin{formulae}
|
||
\item $x_3 = \ParamM{B} \smult \lambda^2 - \ParamM{A} - x_1 - x_2$
|
||
\item $y_3 = (x_1 - x_3) \smult \lambda^2 - y_1$
|
||
\item where $\lambda = \begin{cases}
|
||
\hfrac{3 \smult x_1^2 + 2 \smult \ParamM{A} \smult x_1 + 1}{2 \smult \ParamM{B} \smult y_1},
|
||
&\caseif x_1 = x_2 \\[1.4ex]
|
||
\hfrac{y_2 - y_1}{x_2 - x_1}, &\caseotherwise.
|
||
\end{cases}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
The following theorem helps to determine when these incomplete addition formulae
|
||
can be safely used:
|
||
|
||
\newcommand{\halfs}{\frac{s-1}{2}}
|
||
|
||
\begin{theorem} \label{thmdistinctxcriterion}
|
||
Let $Q$ be a point of odd-prime order $s$ on a Montgomery curve $E_{\ParamM{A},\ParamM{B}} / \GF{\ParamS{r}}$.
|
||
Let $k_\barerange{1}{2}$ be integers in $\rangenozero{-\halfs}{\halfs}$.
|
||
Let $P_i = \scalarmult{k_i}{Q} = (x_i, y_i)$ for $i \in \range{1}{2}$, with
|
||
$k_1 \neq \pm k_2$. Then the non-unified addition constraints
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{x_2 - x_1}{\lambda}{y_2 - y_1}$
|
||
\item $\constraint{\ParamM{B} \smult \lambda}{\lambda}{\ParamM{A} + x_1 + x_2 + x_3}$
|
||
\item $\constraint{x_1 - x_3}{\lambda}{y_3 + y_1}$
|
||
\end{formulae}
|
||
|
||
implement the affine-Montgomery addition $P_1 + P_2 = (x_3, y_3)$ in all cases.
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
The given constraints are equivalent to the Montgomery addition formulae
|
||
under the side condition $x_1 \neq x_2$. (Note that neither $P_i$ can be
|
||
the zero point since $k_\barerange{1}{2} \neq 0 \pmod s$.)
|
||
Assume for a contradiction that $x_1 = x_2$. For any
|
||
$P_1 = \scalarmult{k_1}{Q}$, there can be only one other point $-P_1$ with
|
||
the same $x$-coordinate. (This follows from the fact that the curve equation
|
||
determines $\pm y$ as a function of $x$.)
|
||
But $-P_1 = \scalarmult{-1}{\scalarmult{k_1}{Q}} = \scalarmult{-k_1}{Q}$.
|
||
Since $\fun{k \typecolon \range{-\halfs}{\halfs}}{\scalarmult{k}{Q} \typecolon \GroupJ}$
|
||
is injective and $k_\barerange{1}{2}$ are in $\range{-\halfs}{\halfs}$,
|
||
then $k_2 = \pm k_1$ (contradiction).
|
||
\end{proof}
|
||
|
||
The conditions of this theorem are called the \distinctXCriterion.
|
||
|
||
In particular, if $k_\barerange{1}{2}$ are integers in $\range{1}{\halfs}$
|
||
then it is sufficient to require $k_1 \neq k_2$, since that implies
|
||
$k_1 \neq \pm k_2$.
|
||
|
||
\vspace{2ex}
|
||
\introlist
|
||
Affine-Montgomery doubling can be implemented as:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{x}{x}{xx}$
|
||
\item $\constraint{2 \smult \ParamM{B} \smult y}{\lambda}{3 \smult xx + 2 \smult \ParamM{A} \smult x + 1}$
|
||
\item $\constraint{\ParamM{B} \smult \lambda}{\lambda}{\ParamM{A} + 2 \smult x + x_3}$
|
||
\item $\constraint{x - x_3}{\lambda}{y_3 + y}$
|
||
\end{formulae}
|
||
|
||
This doubling formula is valid when $y \neq 0$, which is the case when $(x, y)$
|
||
is not the point $(0, 0)$ (the only point of order $2$), as proven in
|
||
\theoremref{thmmontynotzero}.
|
||
|
||
|
||
\introlist
|
||
\nsubsubsection{Affine-Edwards arithmetic} \label{cctedarithmetic}
|
||
|
||
Formulae for affine-Edwards addition are given in \cite[section 6]{BBJLP2008}.
|
||
With a change of variable names to match our convention, the formulae for
|
||
$(u_1, \varv_1) + (u_2, \varv_2) = (u_3, \varv_3)$ are:
|
||
|
||
\begin{formulae}
|
||
\item $u_3 = \cfrac{u_1 \smult \varv_2 + \varv_1 \smult u_2}{1 + \ParamJ{d} \smult u_1 \smult u_2 \smult \varv_1 \smult \varv_2}$
|
||
\item $\varv_3 = \cfrac{\varv_1 \smult \varv_2 - \ParamJ{a} \smult u_1 \smult u_2}{1 - \ParamJ{d} \smult u_1 \smult u_2 \smult \varv_1 \smult \varv_2}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
We use an optimized implementation found by Daira Hopwood making use of an
|
||
observation by Bernstein and Lange in \cite[last paragraph of section 4.5.2]{BL2017}:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{u_1 + \varv_1}{\varv_2 - \ParamJ{a} \smult u_2}{T}$
|
||
\item $\constraint{u_1}{\varv_2}{A}$
|
||
\item $\constraint{\varv_1}{u_2}{B}$
|
||
\item $\constraint{\ParamJ{d} \smult A}{B}{C}$
|
||
\item $\constraint{1 + C}{u_3}{A + B}$
|
||
\item $\constraint{1 - C}{\varv_3}{T - A + \ParamJ{a} \smult B}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
The correctness of this implementation can be seen by expanding $T - A + \ParamJ{a} \smult B$:
|
||
|
||
\begin{tabular}{@{\hskip 2em}r@{\;}l}
|
||
$T - A + \ParamJ{a} \smult B$
|
||
& $= (u_1 + \varv_1) \mult (\varv_2 - \ParamJ{a} \smult u_2) - u_1 \smult \varv_2 + \ParamJ{a} \smult \varv_1 \smult u_2$ \\
|
||
& $= \varv_1 \smult \varv_2 - \ParamJ{a} \smult u_1 \smult u_2 + u_1 \smult \varv_2 - \ParamJ{a} \smult \varv_1 \smult u_2
|
||
- u_1 \smult \varv_2 + \ParamJ{a} \smult \varv_1 \smult u_2$ \\
|
||
& $= \varv_1 \smult \varv_2 - \ParamJ{a} \smult u_1 \smult u_2$
|
||
\end{tabular}
|
||
|
||
\vspace{2ex}
|
||
\introlist
|
||
The above addition formulae are ``unified'', that is, they can also be
|
||
used for doubling. Affine-Edwards doubling $\scalarmult{2}{(u, \varv)} = (u_3, \varv_3)$
|
||
can also be implemented slightly more efficiently as:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{u + \varv}{\varv - \ParamJ{a} \smult u}{T}$
|
||
\item $\constraint{u}{\varv}{A}$
|
||
\item $\constraint{\ParamJ{d} \smult A}{A}{C}$
|
||
\item $\constraint{1 + C}{u_3}{2 \smult A}$
|
||
\item $\constraint{1 - C}{\varv_3}{T + (\ParamJ{a} - 1) \smult A}$
|
||
\end{formulae}
|
||
|
||
This implementation is obtained by specializing the addition formulae to
|
||
$(u, \varv) = (u_1, \varv_1) = (u_2, \varv_2)$ and observing that $u \mult \varv = A = B$.
|
||
|
||
|
||
\nsubsubsection{Affine-Edwards cofactor multiplication and nonsmall-order check} \label{cctcofactormult}
|
||
|
||
Cofactor multiplication is used to ensure that that the resulting point is of
|
||
order $\ParamJ{r}$, which avoids certain small-subgroup attacks.
|
||
|
||
\introlist
|
||
The cofactor for the Jubjub curve is $8$. A cofactor multiplication can therefore
|
||
be implemented by doubling three times, using the affine-Edwards doubling implementation
|
||
in \crossref{cctedarithmetic}:
|
||
|
||
\begin{formulae}
|
||
\item $(u, \varv) = \scalarmult{2}{\scalarmult{2}{\scalarmult{2}{(u_0, \varv_0)}}}$
|
||
\end{formulae}
|
||
|
||
We can ensure that the original point $(u_0, \varv_0)$ was not of small order (and that
|
||
the resulting point is not $\ZeroJ$) by asserting that $u \neq 0$.
|
||
On a twisted Edwards curve, only the zero point $\ZeroJ$, and the unique point
|
||
of order $2$ at $(0, -1)$ have zero $u$-coordinate.
|
||
|
||
The assertion $u \neq 0$ can be implemented in a single constraint as described
|
||
in \crossref{cctnonzero}.
|
||
|
||
The cost is therefore $3 \mult 5$ constraints for the doublings and $1$ constraint for
|
||
the check on $u$, for a total of $16$ constraints.
|
||
|
||
In the case where we \emph{only} need to reject points of small order (less than $\ParamJ{r}$)
|
||
and the result of cofactor multiplication is not needed, it is sufficient to double twice
|
||
and check $u \neq 0$. This works because the check on $u$ rejects both the zero point
|
||
and the point of order $2$, and no other points. The second doubling does not need to
|
||
compute $T$ or the $\varv$-coordinate of the result, so the total cost of this
|
||
nonsmall-order check is $5 + 3 + 1 = 9$ constraints.
|
||
|
||
|
||
\nsubsubsection{Fixed-base affine-Edwards scalar multiplication} \label{cctfixedscalarmult}
|
||
|
||
If the base point $B$ is fixed for a given scalar multiplication $\scalarmult{k}{B}$,
|
||
we can fully precompute window tables for each window position.
|
||
|
||
It is most efficient to use $3$-bit fixed windows. Since the length of
|
||
$\ParamJ{r}$ is $252$ bits, we need $84$ windows.
|
||
|
||
Express $k$ in base $8$, i.e.\ $k = \vsum{i=0}{83} k_i \smult 8^i$.
|
||
|
||
Then $\scalarmult{k}{B} = \vsum{i=0}{83} w_{(B,\,i,\,k_i)}$, where
|
||
$w_{(B,\,i,\,k_i)} = \scalarmult{k_i \smult 8^i}{B}$.
|
||
|
||
We precompute all of $w_{(B,\,i,\,s)}$ for $i \in \range{0}{83}, s \in \range{0}{7}$.
|
||
|
||
\introlist
|
||
To look up a given window entry $w_{(B,\,i,\,s)} = (u_s, \varv_s)$, where
|
||
$s = 4 \smult s_2 + 2 \smult s_1 + s_0$, we use:
|
||
|
||
\begin{formulae}
|
||
\item $\lincomb{s_1} \times \lincomb{s_0} = \lincomb{s\suband}$
|
||
\item $\lincomb{s_2} \times \big(\!- u_0 \smult s\suband \plus u_0 \smult s_1 \plus u_0 \smult s_0 - u_0 \plus u_1 \smult s\suband
|
||
- u_1 \smult s_0 \plus u_2 \smult s\suband - u_2 \smult s_1 - u_3 \smult s\suband \\
|
||
\mhspace{3.28em} \plus u_4 \smult s\suband - u_4 \smult s_1 - u_4 \smult s_0 \plus u_4 - u_5 \smult s\suband
|
||
\plus u_5 \smult s_0 - u_6 \smult s\suband \plus u_6 \smult s_1 \plus u_7 \smult s\suband\big) = \\
|
||
\mhspace{1.68em} \lincomb{u_s - u_0 \smult s\suband \plus u_0 \smult s_1 \plus u_0 \smult s_0 - u_0 \plus u_1 \smult s\suband
|
||
- u_1 \smult s_0 \plus u_2 \smult s\suband - u_2 \smult s_1 - u_3 \smult s\suband}$
|
||
\item $\lincomb{s_2} \times \big(\!- \vv_0 \smult s\suband \plus \vv_0 \smult s_1 \plus \vv_0 \smult s_0 - \vv_0 \plus \vv_1 \smult s\suband
|
||
- \vv_1 \smult s_0 \plus \vv_2 \smult s\suband - \vv_2 \smult s_1 - \vv_3 \smult s\suband \\
|
||
\mhspace{3.27em} \plus \vv_4 \smult s\suband - \vv_4 \smult s_1 - \vv_4 \smult s_0 \plus \vv_4 - \vv_5 \smult s\suband
|
||
\plus \vv_5 \smult s_0 - \vv_6 \smult s\suband \plus \vv_6 \smult s_1 \plus \vv_7 \smult s\suband\big) = \\
|
||
\mhspace{1.66em} \lincomb{\vv_s - \vv_0 \smult s\suband \plus \vv_0 \smult s_1 \plus \vv_0 \smult s_0 - \vv_0 \plus \vv_1 \smult s\suband
|
||
- \vv_1 \smult s_0 \plus \vv_2 \smult s\suband - \vv_2 \smult s_1 - \vv_3 \smult s\suband}$
|
||
\end{formulae}
|
||
|
||
This costs $3$ constraints for each of $84$ window lookups, plus $6$ constraints for
|
||
each of $83$ Edwards additions (as in \crossref{cctedarithmetic}), for a total of
|
||
$750$ constraints.
|
||
|
||
\pnote{
|
||
It would be more efficient to use arithmetic on the Montgomery curve, as in
|
||
\crossref{cctpedersenhash}. However since there are only three instances of
|
||
fixed-base scalar multiplication in the \spendCircuit and two in the \outputCircuit
|
||
\footnote{A Pedersen commitment uses fixed-base scalar multiplication as a subcomponent.},
|
||
the additional complexity was not considered justified for \Sapling.
|
||
}
|
||
|
||
|
||
\nsubsubsection{Variable-base affine-Edwards scalar multiplication} \label{cctvarscalarmult}
|
||
|
||
When the base point $B$ is not fixed, the method in the preceding section
|
||
cannot be used. Instead we use a naïve double-and-add method.
|
||
|
||
\introlist
|
||
Given $k = \vsum{i=0}{250} k_i \smult 2^i$, we calculate $R = \scalarmult{k}{B}$ using:
|
||
|
||
\begin{formulae}
|
||
\item // $\Base^i = \scalarmult{2^i}{B}$
|
||
\item let $\Base^0_u = B_u$
|
||
\item let $\Base^0_{\vv}\hairspace = B_{\vv}$
|
||
\item let $\Acc^0_u = k_0 \bchoose B_u : 0$
|
||
\item let $\Acc^0_{\vv}\hairspace = k_0 \bchoose B_{\vv} : 1$
|
||
\vspace{1ex}
|
||
\item for $i$ from $1$ up to $250$:
|
||
\item \tab let $\Base^i = \scalarmult{2}{\Base^{i-1}}$
|
||
\vspace{1ex}
|
||
\item \tab // select $\Base^i$ or $\ZeroJ$ depending on the bit $k_i$
|
||
\item \tab let $\Addend^i_u = k_i \bchoose \Base^i_u : 0$
|
||
\item \tab let $\Addend^i_{\vv}\hairspace = k_i \bchoose \Base^i_{\vv} : 1$
|
||
\item \tab let $\Acc^i = \Acc^{i-1} + \Addend^i$
|
||
\item let $R = \Acc^{250}$.
|
||
\end{formulae}
|
||
|
||
This costs $5$ constraints for each of $250$ Edwards doublings, $6$ constraints for each
|
||
of $250$ Edwards additions, and $2$ constraints for each of $251$ point selections,
|
||
for a total of $3252$ constraints.
|
||
|
||
\pnote{
|
||
It would be more efficient to use $2$-bit fixed windows, and/or to use arithmetic
|
||
on the Montgomery curve in a similar way to \crossref{cctpedersenhash}. However
|
||
since there are only two instances of variable-base scalar multiplication in the
|
||
\spendCircuit and one in the \outputCircuit, the additional complexity was not
|
||
considered justified for \Sapling.
|
||
}
|
||
|
||
\nsubsubsection{Pedersen hash} \label{cctpedersenhash}
|
||
|
||
The specification of the \xPedersenHashes used in \Sapling is given in
|
||
\crossref{concretepedersenhash}. It is based on the scheme from
|
||
\cite[section 5.2]{CvHP1991} --for which a tighter security reduction to
|
||
the Discrete Logarithm Problem was given in \cite{BGG1995}-- but tailored
|
||
to allow several optimizations in the circuit implementation.
|
||
|
||
\xPedersenHashes are the single most commonly used primitive in the
|
||
\Sapling circuits. $\MerkleDepthSapling$ \xPedersenHash instances are used
|
||
in the \spendCircuit to check a Merkle path to the \noteCommitment of the
|
||
\note being spent. We also reuse the \xPedersenHash implementation to
|
||
construct the \commitmentScheme $\NoteCommitSaplingAlg$.
|
||
|
||
This motivates considerable attention to optimizing this circuit
|
||
implementation of this primitive, even at the cost of complexity.
|
||
|
||
First, we use a windowed scalar multiplication algorithm with signed digits.
|
||
Each $3$-bit message chunk corresponds to a window; the chunk is encoded
|
||
as an integer from the set $\Digits = \rangenozero{-4}{4}$.
|
||
This allows a more efficient lookup of the window entry for each chunk than
|
||
if the set $\range{1}{8}$ had been used, because a point can be conditionally
|
||
negated using only a single constraint.
|
||
|
||
Next, we optimize the cost of point addition by allowing as many additions
|
||
as possible to be performed on the Montgomery curve. An incomplete
|
||
Montgomery addition costs $3$ constraints, in comparison with an
|
||
Edwards addition which costs $6$ constraints.
|
||
|
||
\introlist
|
||
However, we cannot do all additions on the Montgomery curve because the
|
||
Montgomery addition is incomplete. In order to be able to prove that
|
||
exceptional cases do not occur, we need to ensure that the \distinctXCriterion
|
||
from \crossref{cctmontarithmetic} is met. This requires splitting the
|
||
input into segments (each using an independent generator), calculating
|
||
an intermediate result for each segment, and then converting to the
|
||
Edwards curve and summing the intermediate results using Edwards addition.
|
||
If the resulting point is $R$, then (abstracting away the changes of curve)
|
||
this calculation can be written as:
|
||
|
||
\begin{formulae}
|
||
\item $\PedersenHashToPoint(D, M) = \vsum{j=1}{N} \scalarmult{\PedersenEncode{M_j}}{\PedersenGen{D}{j}}$
|
||
\end{formulae}
|
||
|
||
where $\PedersenEncode{\paramdot}$ and $\PedersenGen{D}{j}$
|
||
are defined as in \crossref{concretepedersenhash}.
|
||
|
||
\introlist
|
||
We have to prove that:
|
||
\begin{itemize}
|
||
\item the \distinctXCriterion is met for all Montgomery additions within
|
||
a segment;
|
||
\item the Montgomery-to-Edwards conversions can be implemented without
|
||
exceptional cases.
|
||
\end{itemize}
|
||
|
||
The proof of \theoremref{thmpedersenencodeinjective} showed that
|
||
all indices of addition inputs are in the range
|
||
$\rangenozero{-\hfrac{\ParamJ{r}-1}{2}}{\hfrac{\ParamJ{r}-1}{2}}$.
|
||
|
||
Because the $\PedersenGen{D}{j}$ (which are outputs of $\GroupJHash{}$)
|
||
are all of prime order, and $\PedersenEncode{M_j} \neq 0 \pmod{\ParamJ{r}}$,
|
||
it is guaranteed that all of the terms
|
||
$\scalarmult{\PedersenEncode{M_j}}{\PedersenGen{D}{j}}$
|
||
to be converted to Edwards form are of prime order.
|
||
From \theoremref{thmconversiontoedwardsnoexcept}, we can infer that
|
||
the conversions will not encounter exceptional cases.
|
||
|
||
We also need to show that the indices of addition inputs are
|
||
all distinct disregarding sign.
|
||
|
||
\begin{theorem} \label{thmpedersendistinctabsindices}
|
||
For all disjoint nonempty subsets $S$ and $S'$ of $\range{1}{c}$, and for all
|
||
$m \in \typeexp{\bitseq{3}}{c}$,
|
||
|
||
\begin{formulae}
|
||
\item $\vsum{j \in S\vphantom{S'}}{} \enc(m_j) \mult 2^{4 \mult (j-1)}
|
||
\neq \pm\!\!\vsum{j' \in S'}{} \enc(m_{\kern -0.1em j'}) \mult 2^{4 \mult (j'-1)}$
|
||
\end{formulae}
|
||
\end{theorem}
|
||
|
||
\begin{proof}
|
||
\todo{...}
|
||
%Since $\PedersenEncode{\paramdot}$ is injective, the given condition is
|
||
%equivalent to:
|
||
|
||
%\begin{formulae}
|
||
% \item for all disjoint subsets $S$ and $S'$ of $\range{1}{c}$, and for all
|
||
% $M \in \bitseq{3 \mult c},\; \PedersenEncodeSub{S}{M} \neq \pm \PedersenEncodeSub{S'}{M}$
|
||
%\end{formulae}
|
||
|
||
%where $\PedersenEncodeSub{S}{M} = \vsum{j \in S}{} \enc(m_j) \mult 2^{4 \mult (j-1)}$.
|
||
|
||
%This is in turn equivalent to:
|
||
|
||
%\begin{formulae}
|
||
% \item for all disjoint subsets $S$ and $S'$ of $\range{1}{c}$, and for all
|
||
% $M \in \bitseq{3 \mult c},\;
|
||
% \PedersenEncodeSub{S}{M \band \Mask} \neq \PedersenEncodeSub{S'}{M \band \Mask}$
|
||
%\end{formulae}
|
||
|
||
%where $\Mask = \vsum{j=1}{c} 3 \mult 2^{4 \mult (j-1)}$.
|
||
|
||
%(This masks off the bit controlling the sign of each digit, which effectively
|
||
%takes the absolute value of each digit.)
|
||
|
||
%Since $S$ and $S'$ are disjoint and each term of the RHS is separated,
|
||
%it follows that $\Mask_S \band \Mask_{S'} = 0$ and so ...
|
||
|
||
%Suppose this were not
|
||
%the case, then there would exist disjoint subsets of windows $S$ and $S'$
|
||
%such that ..., the space of indices spanned by ...
|
||
%does not overlap the space spanned by $S'$.
|
||
|
||
%is met because all of the terms in the Montgomery addition, as well as any
|
||
%intermediate result formed from adding a subset of terms, have distinct indices
|
||
|
||
%(this bound makes no assumption about the order of additions; the actual
|
||
%maximum will be smaller).
|
||
\end{proof}
|
||
|
||
When these hashes are used in the circuit, the first two windows of the input
|
||
are fixed and can be optimized (for example, in the Merkle tree hashes they
|
||
represent the layer number).
|
||
This is done by precomputing the sum of the relevant two points, and adding them
|
||
to the intermediate result for the remainder of the first segment.
|
||
This requires 3 constraints for a single Montgomery addition rather than
|
||
.. constraints for 2 window lookups and 2 additions.
|
||
|
||
Taking into account this optimization, the cost of a Pedersen hash over
|
||
$\ell$ bits, with the first 6 bits fixed, is ... constraints. In particular,
|
||
for the Merkle tree hashes $\ell = 516$, so the cost is ... constraints.
|
||
|
||
|
||
\nsubsubsection{Mixing Pedersen hash} \label{cctmixinghash}
|
||
|
||
A mixing \xPedersenHash is used to compute $\NoteAddressRand$ from
|
||
$\cm$ and $\NotePosition$ in \crossref{commitmentsandnullifiers}. It takes as
|
||
input a \xPedersenCommitment $P$, and hashes it with another input $x$.
|
||
|
||
\introlist
|
||
We define $\MixingPedersenHash{D} \typecolon \byteseq{8} \times \range{0}{\ParamJ{r}-1}
|
||
\times \GroupJ \rightarrow \GroupJ$ by:
|
||
|
||
\begin{formulae}
|
||
\item $\MixingPedersenHash(D, P, x) := P + \scalarmult{x}{\FindGroupJHashOf{D, \ascii{}}}$.
|
||
\end{formulae}
|
||
|
||
This costs \todo{...} for the scalar multiplication, and $6$ constraints for the
|
||
Edwards addition, for a total of \todo{...} constraints.
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{Merkle path check} \label{cctmerklepath}
|
||
|
||
Checking a Merkle authentication path, as described in \crossref{merklepath},
|
||
requires to:
|
||
|
||
\begin{itemize}
|
||
\item boolean-constrain the path bit specifying whether the previous node
|
||
is a left or right child;
|
||
\item conditionally swap the previous-layer and sibling hashes
|
||
(as $\GF{r}$ elements) depending on the path bit;
|
||
\item unpack the previous-layer and sibling hashes to $255$-bit sequences;
|
||
\item compute the Merkle hash.
|
||
\end{itemize}
|
||
|
||
The unpacking need not be canonical in the sense discussed in \crossref{cctunpack};
|
||
that is, it is \emph{not} necessary to ensure that the previous-layer or sibling
|
||
bit-sequence inputs represent integers in the range $\range{0}{\ParamS{r}-1}$.
|
||
Since the root of the Merkle tree is calculated outside the circuit using the
|
||
canonical representations, and since the \xPedersenHashes are collision-resistant
|
||
on arbitrary bit-sequence inputs, an attempt by an adversarial prover to use a
|
||
non-canonical input would result in the wrong root being calculated, and the
|
||
overall path check would fail.
|
||
|
||
Note that the leaf node input of the authentication path is given as a bit sequence,
|
||
not as a field element.
|
||
|
||
For each layer, the cost is $1 + 2 \smult 255$ boolean constraints,
|
||
$2$ constraints for the conditional swap (implemented as two selection
|
||
constraints), and todo{...} for the Merkle hash, for a total of \todo{...}
|
||
constraints.
|
||
|
||
\pnote{The conditional swap $(a_0, a_1) \mapsto (c_0, c_1)$ could be implemented
|
||
in only one constraint by substituting $c_1 = a_0 + a_1 - c_0$ into the
|
||
uses of $c_1$. The \Sapling circuit does not use this optimization.}
|
||
|
||
|
||
\introsection
|
||
\nsubsubsection{\WindowedPedersenCommitment} \label{cctwindowedcommit}
|
||
|
||
We construct \windowedPedersenCommitments by reusing the Pedersen hash
|
||
implementation, and adding a randomized point:
|
||
|
||
\begin{formulae}
|
||
\item $\WindowedPedersenCommit{r}(D, s) =
|
||
\PedersenHashToPoint(D, s) + \scalarmult{r}{\FindGroupJHashOf{D, \ascii{}}}$
|
||
\end{formulae}
|
||
|
||
\introlist
|
||
This can be implemented in:
|
||
\begin{itemize}
|
||
\item $... \smult \ell + ...$ constraints for the Pedersen hash on
|
||
$\ell = \length(s)$ bits (again assuming that the first $6$ bits are fixed);
|
||
\item $750$ constraints for the fixed-base scalar multiplication;
|
||
\item $6$ constraints for the final Edwards addition
|
||
\end{itemize}
|
||
for a total of $... \smult \ell + 756$ constraints.
|
||
|
||
|
||
\nsubsubsection{\HomomorphicPedersenCommitment} \label{ccthomomorphiccommit}
|
||
|
||
The \windowedPedersenCommitments defined in the preceding section are
|
||
highly efficient, but they do not support the homomorphic property we
|
||
need when instantiating $\ValueCommit{}$ (see \crossref{saplingbalance}
|
||
and \crossref{spendsandoutputs}).
|
||
|
||
\introlist
|
||
In order to support this property, we also define \homomorphicPedersenCommitments
|
||
as follows:
|
||
|
||
\begin{formulae}
|
||
\item $\HomomorphicPedersenCommit{\ValueCommitRand}(D, \Value) =
|
||
\scalarmult{\Value}{\FindGroupJHashOf{D, \ascii{v}}} + \scalarmult{\ValueCommitRand}{\FindGroupJHashOf{D, \ascii{}}}$
|
||
\end{formulae}
|
||
|
||
In the case that we need for $\ValueCommit{}$, $\Value$ has $64$ bits
|
||
\footnote{It would be sufficient to use $51$ bits, which accomodates the range
|
||
$\range{0}{\MAXMONEY}$, but the \Sapling circuit uses $64$.}.
|
||
This can be straightforwardly implemented in ... constraints.
|
||
|
||
|
||
\nsubsubsection{BLAKE2s hashes} \label{cctblake2s}
|
||
|
||
$\BlakeTwosGeneric$ is defined in \cite{ANWW2013}. Its main subcomponent is a
|
||
``$G$ function'', defined as follows:
|
||
|
||
\begin{formulae}
|
||
\item $G \typecolon ... \rightarrow ...$
|
||
\item $G(...) = ...$
|
||
\end{formulae}
|
||
|
||
A 32-bit exclusive-or can be implemented in $32$ constraints, one for each bit position
|
||
$a \xor b = c$:
|
||
|
||
\begin{formulae}
|
||
\item $\constraint{2 \smult a}{b}{a + b - c}$
|
||
\end{formulae}
|
||
|
||
Additions not involving a message word require $33$ constraints:
|
||
|
||
...
|
||
|
||
Additions of message words require one extra constraint each, i.e.\ $a + b + m = c$
|
||
is implemented by declaring a 34-bit boolean array, and ...
|
||
|
||
There are $10 \smult 4 \smult 2$ such message word additions.
|
||
|
||
Each $G$ evaluation requires 260 constraints. There are $10 \smult 8$ instances
|
||
of $G$:
|
||
|
||
$...$
|
||
|
||
There are also 8 output exclusive-ors.
|
||
|
||
The total cost is 21136 constraints. This includes boolean-constraining the hash
|
||
output bits, but not the input bits.
|
||
|
||
\pnote{
|
||
It should be clear that $\BlakeTwosGeneric$ is very expensive in the circuit compared
|
||
to elliptic curve operations. This is primarily because it is inefficient to
|
||
use $\GF{\ParamS{r}}$ elements to represent single bits.
|
||
However Pedersen hashes do not have the necessary cryptographic
|
||
properties for the two cases where the \spendCircuit uses $\BlakeTwosGeneric$.
|
||
While it might be possible to use variants of functions with low circuit cost
|
||
such as MiMC \cite{AGRRT2017}, it was felt that they had not yet received sufficient
|
||
cryptanalytic attention to confidently use them for \Sapling.
|
||
}
|
||
|
||
|
||
\nsubsection{The SaplingSpend circuit} \label{cctsaplingspend}
|
||
|
||
|
||
|
||
\nsubsection{The SaplingOutput circuit} \label{cctsaplingoutput}
|
||
|
||
} %notsprout
|
||
|
||
\end{document}
|