mirror of https://github.com/zcash/zips.git
3393 lines
143 KiB
TeX
3393 lines
143 KiB
TeX
\documentclass{article}
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\RequirePackage{amsmath}
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\RequirePackage{bytefield}
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\RequirePackage{graphicx}
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\RequirePackage{newtxmath}
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\RequirePackage{mathtools}
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\RequirePackage{xspace}
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\RequirePackage{url}
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\RequirePackage{changepage}
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\RequirePackage[unicode,bookmarksnumbered,bookmarksopen,pdfview=Fit]{hyperref}
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\RequirePackage{cleveref}
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\RequirePackage{nameref}
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\RequirePackage{enumitem}
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\RequirePackage{tabularx}
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\RequirePackage{hhline}
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\RequirePackage[usestackEOL]{stackengine}
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\RequirePackage{comment}
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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
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\RequirePackage{lmodern}
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\RequirePackage{bold-extra}
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\RequirePackage{quattrocento}
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\RequirePackage{dsfont}
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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{\parskip}{1.5ex}
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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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\setlist[itemize]{itemsep=0.5ex,topsep=0.2ex,after=\vspace{1.5ex}}
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\newcommand{\docversion}{Version unavailable (check protocol.ver)}
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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{\coauthors}{Sean Bowe | Taylor Hornby | Nathan Wilcox}
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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\ | \coauthors}
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}
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}
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\renewcommand{\sectionautorefname}{\S\!}
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\renewcommand{\subsectionautorefname}{\S\!}
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\renewcommand{\subsubsectionautorefname}{\S\!}
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\newcommand{\crossref}[1]{\autoref{#1}\, \emph{`\nameref*{#1}\kern -0.05em'} on p.\,\pageref*{#1}}
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\newcommand{\nstrut}{\rule[-.2\baselineskip]{0pt}{\baselineskip}}
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\newcommand{\nsection}[1]{\section{\texorpdfstring{#1\nstrut}{#1}}}
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\newcommand{\nsubsection}[1]{\subsection{\texorpdfstring{#1\nstrut}{#1}}}
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\newcommand{\nsubsubsection}[1]{\subsubsection{\texorpdfstring{#1\nstrut}{#1}}}
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\mathchardef\mhyphen="2D
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% http://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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\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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\newcommand{\hairspace}{~\!}
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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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\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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% terminology
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\newcommand{\term}[1]{\textsl{#1}\kern 0.05em\xspace}
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\newcommand{\titleterm}[1]{#1}
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\newcommand{\termbf}[1]{\textbf{#1}\xspace}
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\newcommand{\conformance}[1]{\textbnx{#1}\xspace}
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\newcommand{\Zcash}{\termbf{Zcash}}
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\newcommand{\Zerocash}{\termbf{Zerocash}}
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\newcommand{\Bitcoin}{\termbf{Bitcoin}}
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\newcommand{\ZEC}{\termbf{ZEC}}
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\newcommand{\zatoshi}{\term{zatoshi}}
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\newcommand{\MUST}{\conformance{MUST}}
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\newcommand{\MUSTNOT}{\conformance{MUST NOT}}
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\newcommand{\SHOULD}{\conformance{SHOULD}}
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\newcommand{\SHOULDNOT}{\conformance{SHOULD NOT}}
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\newcommand{\note}{\term{note}}
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\newcommand{\notes}{\term{notes}}
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\newcommand{\Note}{\titleterm{Note}}
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\newcommand{\Notes}{\titleterm{Notes}}
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\newcommand{\dummy}{\term{dummy}}
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\newcommand{\dummyNotes}{\term{dummy notes}}
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\newcommand{\DummyNotes}{\titleterm{Dummy Notes}}
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\newcommand{\commitmentScheme}{\term{commitment scheme}}
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\newcommand{\commitmentTrapdoor}{\term{commitment trapdoor}}
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\newcommand{\commitmentTrapdoors}{\term{commitment trapdoors}}
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\newcommand{\trapdoor}{\term{trapdoor}}
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\newcommand{\noteCommitment}{\term{note commitment}}
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\newcommand{\noteCommitments}{\term{note commitments}}
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\newcommand{\NoteCommitment}{\titleterm{Note Commitment}}
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\newcommand{\NoteCommitments}{\titleterm{Note Commitments}}
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\newcommand{\noteCommitmentTree}{\term{note commitment tree}}
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\newcommand{\NoteCommitmentTree}{\titleterm{Note Commitment Tree}}
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\newcommand{\noteTraceabilitySet}{\term{note traceability set}}
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\newcommand{\noteTraceabilitySets}{\term{note traceability sets}}
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\newcommand{\joinSplitDescription}{\term{JoinSplit description}}
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\newcommand{\joinSplitDescriptions}{\term{JoinSplit descriptions}}
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\newcommand{\JoinSplitDescriptions}{\titleterm{JoinSplit Descriptions}}
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\newcommand{\sequenceOfJoinSplitDescriptions}{\changed{sequence of} \joinSplitDescription\changed{\term{s}}\xspace}
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\newcommand{\joinSplitTransfer}{\term{JoinSplit transfer}}
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\newcommand{\joinSplitTransfers}{\term{JoinSplit transfers}}
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\newcommand{\JoinSplitTransfer}{\titleterm{JoinSplit Transfer}}
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\newcommand{\JoinSplitTransfers}{\titleterm{JoinSplit Transfers}}
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\newcommand{\joinSplitSignature}{\term{JoinSplit signature}}
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\newcommand{\joinSplitSignatures}{\term{JoinSplit signatures}}
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\newcommand{\joinSplitSigningKey}{\term{JoinSplit signing key}}
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\newcommand{\joinSplitVerifyingKey}{\term{JoinSplit verifying key}}
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\newcommand{\joinSplitStatement}{\term{JoinSplit statement}}
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\newcommand{\joinSplitStatements}{\term{JoinSplit statements}}
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\newcommand{\JoinSplitStatement}{\titleterm{JoinSplit Statement}}
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\newcommand{\joinSplitProof}{\term{JoinSplit proof}}
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\newcommand{\statement}{\term{statement}}
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\newcommand{\zeroKnowledgeProof}{\term{zero-knowledge proof}}
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\newcommand{\ZeroKnowledgeProofs}{\titleterm{Zero-Knowledge Proofs}}
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\newcommand{\provingSystem}{\term{proving system}}
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\newcommand{\zeroKnowledgeProvingSystem}{\term{zero-knowledge proving system}}
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\newcommand{\ZeroKnowledgeProvingSystem}{\titleterm{Zero-Knowledge Proving System}}
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\newcommand{\ppzkSNARK}{\term{preprocessing zk-SNARK}}
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\newcommand{\provingKey}{\term{proving key}}
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\newcommand{\zkProvingKeys}{\term{zero-knowledge proving keys}}
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\newcommand{\verifyingKey}{\term{verifying key}}
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\newcommand{\zkVerifyingKeys}{\term{zero-knowledge verifying keys}}
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\newcommand{\joinSplitParameters}{\term{JoinSplit parameters}}
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\newcommand{\JoinSplitParameters}{\titleterm{JoinSplit Parameters}}
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\newcommand{\arithmeticCircuit}{\term{arithmetic circuit}}
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\newcommand{\rankOneConstraintSystem}{\term{Rank 1 Constraint System}}
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\newcommand{\primary}{\term{primary}}
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\newcommand{\primaryInput}{\term{primary input}}
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\newcommand{\primaryInputs}{\term{primary inputs}}
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\newcommand{\auxiliaryInput}{\term{auxiliary input}}
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\newcommand{\auxiliaryInputs}{\term{auxiliary inputs}}
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\newcommand{\fullnode}{\term{full node}}
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\newcommand{\fullnodes}{\term{full nodes}}
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\newcommand{\anchor}{\term{anchor}}
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\newcommand{\anchors}{\term{anchors}}
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\newcommand{\block}{\term{block}}
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\newcommand{\blocks}{\term{blocks}}
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\newcommand{\blockHeader}{\term{block header}}
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\newcommand{\blockHeaders}{\term{block headers}}
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\newcommand{\BlockHeaders}{\titleterm{Block Headers}}
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\newcommand{\blockVersionNumber}{\term{block version number}}
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\newcommand{\blockTime}{\term{block time}}
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\newcommand{\transaction}{\term{transaction}}
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\newcommand{\transactions}{\term{transactions}}
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\newcommand{\Transactions}{\titleterm{Transactions}}
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\newcommand{\transactionVersionNumber}{\term{transaction version number}}
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\newcommand{\coinbaseTransaction}{\term{coinbase transaction}}
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\newcommand{\coinbaseTransactions}{\term{coinbase transactions}}
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\newcommand{\transparent}{\term{transparent}}
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\newcommand{\transparentValuePool}{\term{transparent value pool}}
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\newcommand{\xprotected}{\term{protected}}
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\newcommand{\protectedNote}{\term{protected note}}
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\newcommand{\protectedNotes}{\term{protected notes}}
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\newcommand{\xProtected}{\term{Protected}}
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\newcommand{\blockchainview}{\term{block chain view}}
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\newcommand{\blockchain}{\term{block chain}}
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\newcommand{\mempool}{\term{mempool}}
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\newcommand{\treestate}{\term{treestate}}
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\newcommand{\treestates}{\term{treestates}}
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\newcommand{\nullifier}{\term{nullifier}}
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\newcommand{\nullifiers}{\term{nullifiers}}
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\newcommand{\Nullifier}{\titleterm{Nullifier}}
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\newcommand{\Nullifiers}{\titleterm{Nullifiers}}
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\newcommand{\nullifierSet}{\term{nullifier set}}
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\newcommand{\NullifierSet}{\titleterm{Nullifier Set}}
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% Daira: This doesn't adequately distinguish between zk stuff and transparent stuff
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\newcommand{\paymentAddress}{\term{payment address}}
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\newcommand{\paymentAddresses}{\term{payment addresses}}
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\newcommand{\viewingKey}{\term{viewing key}}
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\newcommand{\viewingKeys}{\term{viewing keys}}
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\newcommand{\spendingKey}{\term{spending key}}
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\newcommand{\spendingKeys}{\term{spending keys}}
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\newcommand{\payingKey}{\term{paying key}}
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\newcommand{\transmissionKey}{\term{transmission key}}
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\newcommand{\transmissionKeys}{\term{transmission keys}}
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\newcommand{\keyTuple}{\term{key tuple}}
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\newcommand{\notePlaintext}{\term{note plaintext}}
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\newcommand{\notePlaintexts}{\term{note plaintexts}}
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\newcommand{\NotePlaintexts}{\titleterm{Note Plaintexts}}
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\newcommand{\notesCiphertext}{\term{transmitted notes ciphertext}}
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\newcommand{\incrementalMerkleTree}{\term{incremental Merkle tree}}
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\newcommand{\merkleRoot}{\term{root}}
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\newcommand{\merkleNode}{\term{node}}
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\newcommand{\merkleNodes}{\term{nodes}}
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\newcommand{\merkleHash}{\term{hash value}}
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\newcommand{\merkleHashes}{\term{hash values}}
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\newcommand{\merkleLeafNode}{\term{leaf node}}
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\newcommand{\merkleLeafNodes}{\term{leaf nodes}}
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\newcommand{\merkleInternalNode}{\term{internal node}}
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\newcommand{\merkleInternalNodes}{\term{internal nodes}}
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\newcommand{\MerkleInternalNodes}{\term{Internal nodes}}
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\newcommand{\merklePath}{\term{path}}
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\newcommand{\merkleLayer}{\term{layer}}
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\newcommand{\merkleLayers}{\term{layers}}
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\newcommand{\merkleIndex}{\term{index}}
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\newcommand{\merkleIndices}{\term{indices}}
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\newcommand{\zkSNARK}{\term{zk-SNARK}}
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\newcommand{\zkSNARKs}{\term{zk-SNARKs}}
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\newcommand{\libsnark}{\term{libsnark}}
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\newcommand{\memo}{\term{memo field}}
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\newcommand{\memos}{\term{memo fields}}
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\newcommand{\Memos}{\titleterm{Memo Fields}}
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\newcommand{\keyAgreementScheme}{\term{key agreement scheme}}
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\newcommand{\KeyAgreement}{\titleterm{Key Agreement}}
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\newcommand{\keyDerivationFunction}{\term{Key Derivation Function}}
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\newcommand{\KeyDerivation}{\titleterm{Key Derivation}}
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\newcommand{\encryptionScheme}{\term{encryption scheme}}
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\newcommand{\symmetricEncryptionScheme}{\term{authenticated one-time symmetric encryption scheme}}
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\newcommand{\SymmetricEncryption}{\titleterm{Authenticated One-Time Symmetric Encryption}}
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\newcommand{\signatureScheme}{\term{signature scheme}}
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\newcommand{\pseudoRandomFunction}{\term{Pseudo Random Function}}
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\newcommand{\pseudoRandomFunctions}{\term{Pseudo Random Functions}}
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\newcommand{\PseudoRandomFunctions}{\titleterm{Pseudo Random Functions}}
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% conventions
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\newcommand{\bytes}[1]{\underline{\raisebox{-0.22ex}{}\smash{#1}}}
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\newcommand{\zeros}[1]{[0]^{#1}}
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\newcommand{\bit}{\mathds{B}}
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\newcommand{\bitseq}[1]{\bit^{#1}}
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\newcommand{\byteseqs}{\bit^{8\star}}
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\newcommand{\concatbits}{\mathsf{concat}_\bit}
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\newcommand{\hexint}[1]{\mathbf{0x{#1}}}
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\newcommand{\dontcare}{\kern -0.06em\raisebox{0.1ex}{\footnotesize{$\times$}}}
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\newcommand{\ascii}[1]{\textbf{``\texttt{#1}"}}
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\newcommand{\Justthebox}[2][-1.3ex]{\;\raisebox{#1}{\usebox{#2}}\;}
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\newcommand{\hSigCRH}{\mathsf{hSigCRH}}
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\newcommand{\hSigLength}{\mathsf{\ell_{hSig}}}
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\newcommand{\hSigType}{\bitseq{\hSigLength}}
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\newcommand{\GeneralCRH}[1]{\mathsf{GeneralCRH}_{#1}}
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\newcommand{\EquihashGen}[1]{\mathsf{EquihashGen}_{#1}}
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\newcommand{\CRH}{\mathsf{CRH}}
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\newcommand{\CRHbox}[1]{\SHA\left(\Justthebox{#1}\right)}
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\newcommand{\SHA}{\mathtt{SHA256Compress}}
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\newcommand{\SHAName}{\term{SHA-256 compression}}
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\newcommand{\FullHash}{\mathtt{SHA256}}
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\newcommand{\FullHashName}{\mathsf{SHA\mhyphen256}}
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\newcommand{\Blake}[1]{\mathsf{BLAKE2b\kern 0.05em\mhyphen{#1}}}
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\newcommand{\BlakeGeneric}{\mathsf{BLAKE2b}}
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\newcommand{\FullHashbox}[1]{\FullHash\left(\Justthebox{#1}\right)}
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\newcommand{\setof}[1]{\{{#1}\}}
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\newcommand{\range}[2]{\{{#1}\,..\,{#2}\}}
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\newcommand{\Nat}{\mathbb{N}}
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\newcommand{\PosInt}{\mathbb{N}^+}
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\newcommand{\minimum}{\mathsf{min}}
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\newcommand{\floor}[1]{\mathsf{floor}\!\left({#1}\right)}
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\newcommand{\ceiling}[1]{\mathsf{ceiling}\!\left({#1}\right)}
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\newcommand{\vsum}[2]{\smashoperator[r]{\sum_{#1}^{#2}}}
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\newcommand{\vxor}[2]{\smashoperator[r]{\bigoplus_{#1}^{#2}}}
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\newcommand{\xor}{\oplus}
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\newcommand{\rightarrowR}{\buildrel{\scriptstyle\mathrm{R}}\over\rightarrow}
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% key pairs:
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\newcommand{\PaymentAddress}{\mathsf{addr_{pk}}}
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\newcommand{\PaymentAddressLeadByte}{\hexint{16}}
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\newcommand{\PaymentAddressSecondByte}{\hexint{9A}}
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\newcommand{\SpendingKeyLeadByte}{\hexint{AB}}
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\newcommand{\SpendingKeySecondByte}{\hexint{36}}
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\newcommand{\PaymentAddressTestnetLeadByte}{\hexint{14}}
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\newcommand{\PaymentAddressTestnetSecondByte}{\hexint{51}}
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\newcommand{\SpendingKeyTestnetLeadByte}{\hexint{B1}}
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\newcommand{\SpendingKeyTestnetSecondByte}{\hexint{EB}}
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\newcommand{\NotePlaintextLeadByte}{\hexint{00}}
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\newcommand{\AuthPublic}{\mathsf{a_{pk}}}
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\newcommand{\AuthPrivate}{\mathsf{a_{sk}}}
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\newcommand{\AuthPublicX}[1]{\mathsf{a^\mathrm{#1}_{pk}}}
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\newcommand{\AuthPrivateX}[1]{\mathsf{a^\mathrm{#1}_{sk}}}
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\newcommand{\AuthPrivateLength}{\mathsf{\ell_{\AuthPrivate}}}
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\newcommand{\AuthPublicOld}[1]{\mathsf{a^{old}_{pk,\mathnormal{#1}}}}
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\newcommand{\AuthPrivateOld}[1]{\mathsf{a^{old}_{sk,\mathnormal{#1}}}}
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\newcommand{\AuthPublicOldX}[1]{\mathsf{a^{old}_{pk,\mathrm{#1}}}}
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\newcommand{\AuthPrivateOldX}[1]{\mathsf{a^{old}_{sk,\mathrm{#1}}}}
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\newcommand{\AuthPublicNew}[1]{\mathsf{a^{new}_{pk,\mathnormal{#1}}}}
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\newcommand{\AuthPrivateNew}[1]{\mathsf{a^{new}_{sk,\mathnormal{#1}}}}
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\newcommand{\AddressPublicNew}[1]{\mathsf{addr^{new}_{pk,\mathnormal{#1}}}}
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\newcommand{\enc}{\mathsf{enc}}
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\newcommand{\DHSecret}[1]{\mathsf{sharedSecret}_{#1}}
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\newcommand{\EphemeralPublic}{\mathsf{epk}}
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\newcommand{\EphemeralPrivate}{\mathsf{esk}}
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\newcommand{\TransmitPublic}{\mathsf{pk_{enc}}}
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\newcommand{\TransmitPublicSup}[1]{\mathsf{pk}^{#1}_\mathsf{enc}}
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\newcommand{\TransmitPublicNew}[1]{\mathsf{pk^{new}_{\enc,\mathnormal{#1}}}}
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\newcommand{\TransmitPrivate}{\mathsf{sk_{enc}}}
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\newcommand{\TransmitPrivateSup}[1]{\mathsf{sk}^{#1}_\mathsf{enc}}
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% PRFs
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\newcommand{\PRF}[2]{\mathsf{{PRF}^{#2}_\mathnormal{#1}}}
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\newcommand{\PRFaddr}[1]{\PRF{#1}{addr}}
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\newcommand{\PRFnf}[1]{\PRF{#1}{\nf}}
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\newcommand{\PRFsn}[1]{\PRF{#1}{sn}}
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\newcommand{\PRFpk}[1]{\PRF{#1}{pk}}
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\newcommand{\PRFrho}[1]{\PRF{#1}{\NoteAddressRand}}
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\newcommand{\PRFOutputLength}{\mathsf{\ell_{PRF}}}
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\newcommand{\PRFOutput}{\bitseq{\PRFOutputLength}}
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% Commitments
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\newcommand{\Commit}[1]{\mathsf{COMM}_{#1}}
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\newcommand{\CommitTrapdoor}{\mathsf{COMM.Trapdoor}}
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\newcommand{\CommitInput}{\mathsf{COMM.Input}}
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\newcommand{\CommitOutput}{\mathsf{COMM.Output}}
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\newcommand{\NoteCommit}{\mathtt{NoteCommitment}}
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\newcommand{\Uncommitted}{\mathsf{Uncommitted}}
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% Symmetric encryption
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\newcommand{\Sym}{\mathsf{Sym}}
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\newcommand{\SymEncrypt}[1]{\mathsf{Sym.}\mathtt{Encrypt}_\mathsf{#1}}
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\newcommand{\SymDecrypt}[1]{\mathsf{Sym.}\mathtt{Decrypt}_\mathsf{#1}}
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\newcommand{\SymSpecific}{\mathsf{AEAD\_CHACHA20\_POLY1305}}
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\newcommand{\SymCipher}{\mathsf{ChaCha20}}
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\newcommand{\SymAuth}{\mathsf{Poly1305}}
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\newcommand{\Ptext}{\mathsf{P}}
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\newcommand{\Plaintext}{\mathsf{Sym.}\mathbf{P}}
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\newcommand{\Ctext}{\mathsf{C}}
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\newcommand{\Ciphertext}{\mathsf{Sym.}\mathbf{C}}
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\newcommand{\Key}{\mathsf{K}}
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\newcommand{\Keyspace}{\mathsf{Sym.}\mathbf{K}}
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\newcommand{\TransmitPlaintext}[1]{\Ptext^\enc_{#1}}
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\newcommand{\TransmitCiphertext}[1]{\Ctext^\enc_{#1}}
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\newcommand{\TransmitKey}[1]{\Key^\enc_{#1}}
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\newcommand{\Adversary}{\mathcal{A}}
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\newcommand{\CryptoBoxSeal}{\mathsf{crypto\_box\_seal}}
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% Key agreement
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\newcommand{\KA}{\mathsf{KA}}
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\newcommand{\KAPublic}{\mathsf{KA.Public}}
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\newcommand{\KAPrivate}{\mathsf{KA.Private}}
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\newcommand{\KASharedSecret}{\mathsf{KA.SharedSecret}}
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\newcommand{\KAFormatPrivate}{\mathsf{KA.}\mathtt{FormatPrivate}}
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\newcommand{\KADerivePublic}{\mathsf{KA.}\mathtt{DerivePublic}}
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\newcommand{\KAAgree}{\mathsf{KA.}\mathtt{Agree}}
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\newcommand{\CurveMultiply}{\mathsf{Curve25519}}
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|
\newcommand{\CurveBase}{\bytes{9}}
|
|
\newcommand{\Clamp}{\mathsf{clamp_{Curve25519}}}
|
|
|
|
% KDF
|
|
\newcommand{\KDF}{\mathsf{KDF}}
|
|
\newcommand{\kdftag}{\mathsf{kdftag}}
|
|
\newcommand{\kdfinput}{\mathsf{kdfinput}}
|
|
|
|
% Notes
|
|
\newcommand{\Value}{\mathsf{v}}
|
|
\newcommand{\ValueNew}[1]{\mathsf{v^{new}_\mathnormal{#1}}}
|
|
\newcommand{\MAXMONEY}{\mathsf{MAX\_MONEY}}
|
|
\newcommand{\NoteTuple}[1]{\mathbf{n}_{#1}}
|
|
\newcommand{\NoteType}{\mathsf{Note}}
|
|
\newcommand{\NotePlaintext}[1]{\mathbf{np}_{#1}}
|
|
\newcommand{\NoteCommitRand}{\mathsf{r}}
|
|
\newcommand{\NoteCommitRandLength}{\mathsf{\ell_{\NoteCommitRand}}}
|
|
\newcommand{\NoteCommitRandOld}[1]{\mathsf{r^{old}_\mathnormal{#1}}}
|
|
\newcommand{\NoteCommitRandNew}[1]{\mathsf{r^{new}_\mathnormal{#1}}}
|
|
\newcommand{\NoteAddressRand}{\mathsf{\uprho}}
|
|
\newcommand{\NoteAddressRandOld}[1]{\mathsf{\uprho^{old}_\mathnormal{#1}}}
|
|
\newcommand{\NoteAddressRandOldX}[1]{\mathsf{\uprho^{old}_\mathrm{#1}}}
|
|
\newcommand{\NoteAddressRandNew}[1]{\mathsf{\uprho^{new}_\mathnormal{#1}}}
|
|
\newcommand{\NoteAddressPreRand}{\mathsf{\upvarphi}}
|
|
\newcommand{\NoteAddressPreRandLength}{\mathsf{\ell_{\NoteAddressPreRand}}}
|
|
\newcommand{\NoteCommitS}{\mathsf{s}}
|
|
\newcommand{\cm}{\mathsf{cm}}
|
|
\newcommand{\cmOldX}[1]{\mathsf{{cm}^{old}_\mathrm{#1}}}
|
|
\newcommand{\cmNew}[1]{\mathsf{{cm}^{new}_\mathnormal{#1}}}
|
|
\newcommand{\snOldX}[1]{\mathsf{{sn}^{old}_\mathrm{#1}}}
|
|
\newcommand{\nf}{\mathsf{nf}}
|
|
\newcommand{\nfOld}[1]{\nf^\mathsf{old}_\mathnormal{#1}}
|
|
\newcommand{\Memo}{\mathsf{memo}}
|
|
\newcommand{\DecryptNote}{\mathtt{DecryptNote}}
|
|
\newcommand{\ReplacementCharacter}{\textsf{U+FFFD}}
|
|
|
|
% Signatures
|
|
\newcommand{\Sig}{\mathsf{Sig}}
|
|
\newcommand{\SigPublic}{\mathsf{Sig.Public}}
|
|
\newcommand{\SigPrivate}{\mathsf{Sig.Private}}
|
|
\newcommand{\SigMessage}{\mathsf{Sig.Message}}
|
|
\newcommand{\SigSignature}{\mathsf{Sig.Signature}}
|
|
\newcommand{\SigGen}{\mathsf{Sig.Gen}}
|
|
\newcommand{\SigSign}[1]{\mathsf{Sig.Sign}_{#1}}
|
|
\newcommand{\SigVerify}[1]{\mathsf{Sig.Verify}_{#1}}
|
|
\newcommand{\JoinSplitSig}{\mathsf{JoinSplitSig}}
|
|
\newcommand{\JoinSplitSigPublic}{\mathsf{JoinSplitSig.Public}}
|
|
\newcommand{\JoinSplitSigPrivate}{\mathsf{JoinSplitSig.Private}}
|
|
\newcommand{\JoinSplitSigMessage}{\mathsf{JoinSplitSig.Message}}
|
|
\newcommand{\JoinSplitSigSignature}{\mathsf{JoinSplitSig.Signature}}
|
|
\newcommand{\JoinSplitSigGen}{\mathsf{JoinSplitSig.Gen}}
|
|
\newcommand{\JoinSplitSigSign}[1]{\mathsf{JoinSplitSig.Sign}_{#1}}
|
|
\newcommand{\JoinSplitSigVerify}[1]{\mathsf{JoinSplitSig.Verify}_{#1}}
|
|
\newcommand{\JoinSplitSigSpecific}{\mathsf{Ed25519}}
|
|
\newcommand{\JoinSplitSigHashName}{\mathsf{SHA\mhyphen512}}
|
|
\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{\dataToBeSigned}{\mathsf{dataToBeSigned}}
|
|
|
|
% Merkle tree
|
|
\newcommand{\MerkleDepth}{\mathsf{d}}
|
|
\newcommand{\MerkleNode}[2]{\mathsf{M}^{#1}_{#2}}
|
|
\newcommand{\MerkleSibling}{\mathsf{sibling}}
|
|
\newcommand{\MerkleCRH}{\mathsf{MerkleCRH}}
|
|
\newcommand{\MerkleHashLength}{\mathsf{\ell_{Merkle}}}
|
|
\newcommand{\MerkleHash}{\bitseq{\MerkleHashLength}}
|
|
|
|
% Transactions
|
|
\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{\nullifiersField}{\mathtt{nullifiers}}
|
|
\newcommand{\commitments}{\mathtt{commitments}}
|
|
\newcommand{\ephemeralKey}{\mathtt{ephemeralKey}}
|
|
\newcommand{\encCiphertexts}{\mathtt{encCiphertexts}}
|
|
\newcommand{\randomSeed}{\mathtt{randomSeed}}
|
|
\newcommand{\Varies}{\textit{Varies}}
|
|
\newcommand{\heading}[1]{\multicolumn{1}{c|}{#1}}
|
|
\newcommand{\type}[1]{\texttt{#1}}
|
|
|
|
\newcommand{\sighashType}{\term{SIGHASH type}}
|
|
\newcommand{\sighashTypes}{\term{SIGHASH types}}
|
|
\newcommand{\SIGHASHALL}{\mathsf{SIGHASH\_ALL}}
|
|
\newcommand{\scriptSig}{\mathtt{scriptSig}}
|
|
\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{\nTime}{\mathtt{nTime}}
|
|
\newcommand{\nBits}{\mathtt{nBits}}
|
|
\newcommand{\nNonce}{\mathtt{nNonce}}
|
|
\newcommand{\nSolution}{\mathtt{nSolution}}
|
|
\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{\JoinSplit}{\text{\footnotesize\texttt{JoinSplit}}}
|
|
\newcommand{\ZKJoinSplit}{\mathsf{ZK}_{\JoinSplit}}
|
|
\newcommand{\ZKJoinSplitVerify}{\ZKJoinSplit\mathsf{.Verify}}
|
|
\newcommand{\ZKJoinSplitProve}{\ZKJoinSplit\mathsf{.Prove}}
|
|
\newcommand{\Proof}{\pi}
|
|
\newcommand{\JoinSplitProof}{\Proof_{\JoinSplit}}
|
|
\newcommand{\zkproof}{\mathtt{zkproof}}
|
|
\newcommand{\POUR}{\texttt{POUR}}
|
|
|
|
% 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{\GroupG}[1]{\mathbb{G}_{#1}}
|
|
\newcommand{\PointP}[1]{\mathcal{P}_{#1}}
|
|
\newcommand{\GF}[1]{\mathbb{F}_{#1}}
|
|
\newcommand{\GFstar}[1]{\mathbb{F}^\ast_{#1}}
|
|
\newcommand{\ECtoOSP}{\mathsf{EC2OSP}}
|
|
\newcommand{\ECtoOSPXL}{\mathsf{EC2OSP\mhyphen{}XL}}
|
|
\newcommand{\ECtoOSPXS}{\mathsf{EC2OSP\mhyphen{}XS}}
|
|
\newcommand{\ItoOSP}[1]{\mathsf{I2OSP}_{#1}}
|
|
\newcommand{\ItoBSP}[1]{\mathsf{I2BSP}_{#1}}
|
|
\newcommand{\BStoIP}[1]{\mathsf{BS2IP}_{#1}}
|
|
\newcommand{\FEtoIP}{\mathsf{FE2IP}}
|
|
\newcommand{\BNImpl}{\mathtt{ALT\_BN128}}
|
|
\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{\EnforceCommit}[1]{\mathsf{enforce}_{#1}}
|
|
|
|
|
|
\newcommand{\consensusrule}[1]{\subparagraph{Consensus rule:}{#1}}
|
|
\newenvironment{consensusrules}{\subparagraph{Consensus rules:}\begin{itemize}}{\end{itemize}}
|
|
\newcommand{\securityrequirement}[1]{\subparagraph{Security requirement:}{#1}}
|
|
\newenvironment{securityrequirements}{\subparagraph{Security requirements:}\begin{itemize}}{\end{itemize}}
|
|
\newcommand{\pnote}[1]{\subparagraph{Note:}{#1}}
|
|
\newenvironment{pnotes}{\subparagraph{Notes:}\begin{itemize}}{\end{itemize}}
|
|
|
|
|
|
\begin{document}
|
|
|
|
\title{\doctitle \\
|
|
\Large \docversion \\
|
|
\vspace{1ex} \large as intended for the \Zcash release of autumn 2016}
|
|
\author{\Large \leadauthor \\ \Large \coauthors}
|
|
\date{\today}
|
|
\maketitle
|
|
|
|
\tableofcontents
|
|
\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{protected} payment scheme protected 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.
|
|
|
|
Technical terms for concepts that play an important role in \Zcash are
|
|
written in \term{slanted text}. \emph{Italics} are used for emphasis and
|
|
for references between sections of the document.
|
|
|
|
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;
|
|
\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}.
|
|
\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>}. While the production \Zcash network
|
|
has yet to be launched, please feel free to do so in public even if you believe
|
|
the mistake may indicate a security weakness.
|
|
|
|
\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.
|
|
|
|
Value in \Zcash is either \transparent or \xprotected. Transfers of \transparent
|
|
value work essentially as in \Bitcoin and have the same privacy properties.
|
|
\xProtected value is carried by \notes\hairspace\footnote{\label{notesandnullifiers}
|
|
In \Zerocash \cite{BCG+2014}, \notes were called ``coins'', and \nullifiers
|
|
were called ``serial numbers''.},
|
|
which specify an amount and a \payingKey. The \payingKey is part of
|
|
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). However, it is infeasible
|
|
to correlate a commitment with its \nullifier without knowledge of the \note.
|
|
Computing the \nullifier requires the associated private \spendingKey. 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.
|
|
|
|
A \transaction can contain \transparent inputs, outputs, and scripts,
|
|
which all work as in \Bitcoin. They also contain a sequence of zero or
|
|
more \joinSplitDescriptions. Each of these describes a \joinSplitTransfer\hairspace\footnote{
|
|
\joinSplitTransfers in \Zcash generalize ``Mint'' and ``Pour'' \transactions
|
|
in \Zerocash; see \crossref{trstructure} for the differences.}
|
|
which takes in a \transparent value and up to two input \notes, and produces a
|
|
\transparent value and up to two output \notes. 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). Each \joinSplitDescription also includes a computationally sound
|
|
\zkSNARK proof, which proves all of the following:
|
|
|
|
\begin{itemize}
|
|
\item The inputs and outputs 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 its \nullifier will
|
|
not collide with the \nullifier of any other \note.
|
|
\end{itemize}
|
|
|
|
Outside the \zkSNARK, it is 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 two public keys: a \payingKey matching that of \notes
|
|
sent to the address, and a \transmissionKey for a key-private asymmetric encryption
|
|
scheme. ``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 \viewingKey. This facility is
|
|
used to communicate encrypted output \notes on the \blockchain to their
|
|
intended recipient, who can use the \viewingKey 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{Bitcoin-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}
|
|
|
|
The notation $\hexint{}$ followed by a string of \textbf{boldface} hexadecimal
|
|
digits means the corresponding integer converted from hexadecimal.
|
|
|
|
The notation $\bit$ means the set of bit values, i.e. $\setof{0, 1}$.
|
|
$\bitseq{\ell}$ means the set of sequences of $\ell$ bits.
|
|
$\byteseqs$ means the set of bit sequences constrained to be of length
|
|
a multiple of 8 bits.
|
|
|
|
The notation $\ascii{...}$ means the given string represented as a
|
|
sequence of bytes in US-ASCII. For example, $\ascii{abc}$ represents the
|
|
byte sequence $[\hexint{61}, \hexint{62}, \hexint{63}]$.
|
|
|
|
The notation $a..b$, 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}.)
|
|
|
|
The notation $\range{a}{b}$ means the set of integers from $a$ through
|
|
$b$ inclusive.
|
|
|
|
The notation $[f(x)$ for $x$ from $a$ up to $b\,]$ means the sequence
|
|
formed by evaluating $f$ on each integer from $a$ to $b$ inclusive, in
|
|
ascending order. Similarly, $[f(x)$ for $x$ from $a$ down to $b\,]$ means
|
|
the sequence formed by evaluating $f$ on each integer from $a$ to $b$
|
|
inclusive, in descending order.
|
|
|
|
The notation $a\,||\,b$ means the concatenation of sequences $a$ then $b$.
|
|
|
|
The notation $\concatbits(S)$ 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.
|
|
|
|
The notation $\Nat$ means the set of nonnegative integers. $\PosInt$
|
|
means the set of positive integers.
|
|
|
|
The notation $\GF{n}$ 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 $\GF{n}$.
|
|
|
|
The notation $a \bmod q$, for integers $a \geq 0$ and $q > 0$, means the
|
|
remainder on dividing $a$ by $q$.
|
|
|
|
The notation $a \xor b$ means the bitwise exclusive-or of $a$ and $b$,
|
|
defined either on integers or bit sequences depending on context.
|
|
|
|
The notation $\vsum{i=1}{\mathrm{N}} a_i$ means the sum of $a_{\allN{}}$.\;
|
|
$\vxor{i=1}{\mathrm{N}} a_i$ means the bitwise exclusive-or of $a_{\allN{}}$.
|
|
|
|
The notation $\floor{x}$ means the largest integer $\leq x$.
|
|
$\ceiling{x}$ means the smallest integer $\geq x$.
|
|
|
|
The symbol $\bot$ is used to indicate unavailable information or a failed decryption.
|
|
|
|
The notation $T \subseteq U$ indicates that $T$ is an inclusive subset or subtype of $U$.
|
|
|
|
The notation $x \typecolon T$ is used to specify that $x$ has type $T$.
|
|
A cartesian product type is denoted by $S \times T$, and a function type
|
|
by $S \rightarrow T$. 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. An argument to a function can determine other argument or result
|
|
types.
|
|
|
|
Initial arguments to a function or randomized algorithm may be
|
|
written as subscripts, e.g.\ if $x \typecolon X$, $y \typecolon Y$, and
|
|
$\PRF{}{} \typecolon X \times Y \rightarrow Z$, then an invocation of
|
|
$\PRF{}{}(x, y)$ can also be written $\PRF{x}{}(y)$.
|
|
|
|
The following integer constants will be instantiated in \crossref{constants}:
|
|
$\MerkleDepth$, $\NOld$, $\NNew$, $\MerkleHashLength$, $\hSigLength$,
|
|
$\PRFOutputLength$, $\NoteCommitRandLength$, $\RandomSeedLength$, $\AuthPrivateLength$,
|
|
$\NoteAddressPreRandLength$, $\MAXMONEY$. The bit sequence constant
|
|
$\Uncommitted \typecolon \bitseq{\MerkleHashLength}$ will also be defined in
|
|
that section.
|
|
|
|
|
|
\nsection{Concepts}
|
|
|
|
\nsubsection{Payment Addresses and Keys}
|
|
|
|
A \keyTuple $(\AuthPrivate, \TransmitPrivate, \PaymentAddress)$ is
|
|
generated by users who wish to receive payments under this scheme.
|
|
The \viewingKey $\TransmitPrivate$ and the \paymentAddress
|
|
$\PaymentAddress = (\AuthPublic, \TransmitPublic)$ are derived from the
|
|
\spendingKey $\AuthPrivate$.
|
|
|
|
The following diagram depicts the relations between key components.
|
|
Arrows point from a component to any other component(s) that can be derived
|
|
from it.
|
|
|
|
\begin{center}
|
|
\includegraphics[scale=.7]{key_components}
|
|
\end{center}
|
|
|
|
The composition of \paymentAddresses\changed{, \viewingKeys,} 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 or \viewingKey from a \spendingKey.
|
|
|
|
Users can accept payment from multiple parties with a single \paymentAddress
|
|
$\PaymentAddress$ 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.
|
|
|
|
\pnote{
|
|
It is conventional in cryptography to refer to the key used to encrypt
|
|
a message in an asymmetric encryption scheme as the ``public key".
|
|
However, the public key used as the \transmissionKey component
|
|
of an address ($\TransmitPublic$) 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 $\TransmitPublic$ in order to exploit a
|
|
hypothetical weakness in that cryptosystem.
|
|
}
|
|
|
|
\nsubsection{\Notes}
|
|
|
|
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.
|
|
|
|
\begin{itemize}
|
|
\item $\AuthPublic \typecolon \PRFOutput$ is the
|
|
\payingKey of the recipient;
|
|
\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 $\NoteCommitRand \typecolon \bitseq{\NoteCommitRandLength}$
|
|
is a random bit sequence used as a \commitmentTrapdoor as
|
|
defined in \crossref{abstractcomm}.
|
|
\end{itemize}
|
|
|
|
Let $\NoteType$ be the type of a \note, i.e.
|
|
$\PRFOutput \times \range{0}{\MAXMONEY} \times \PRFOutput \times \bitseq{\NoteCommitRandLength}$.
|
|
|
|
Creation of new \notes is described in \crossref{send}. When \notes are sent,
|
|
only a commitment (see \crossref{abstractcomm}) to the above values is disclosed
|
|
publically. 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.
|
|
|
|
The \noteCommitment is computed as
|
|
$\NoteCommit(\NoteTuple{}) = \Commit{\NoteCommitRand}(\AuthPublic, \Value, \NoteAddressRand)$,
|
|
where $\Commit{}$ is instantiated in \crossref{concretecomm}.
|
|
|
|
A \nullifier (denoted $\nf$) is derived from the $\NoteAddressRand$ component
|
|
of a \note and the recipient's \spendingKey, as $\PRFnf{\AuthPrivate}(\NoteAddressRand)$.
|
|
A \note is spent by proving knowledge of $\NoteAddressRand$ and $\AuthPrivate$ in
|
|
zero knowledge while publically disclosing its \nullifier $\nf$, allowing $\nf$ to
|
|
be used to prevent double-spending.
|
|
|
|
\nsubsubsection{\NotePlaintexts{} and \Memos}
|
|
|
|
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 $\TransmitPublicNew{\allNew}$,
|
|
and the result forms part of a \notesCiphertext (see \crossref{inband}
|
|
for further details).
|
|
|
|
Each \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
|
$(\Value, \NoteAddressRand, \NoteCommitRand\changed{, \Memo})$.
|
|
|
|
The first three of these fields are as defined earlier.
|
|
|
|
\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.
|
|
}
|
|
|
|
|
|
\nsubsection{Transactions, Blocks, and the Block Chain} \label{blockchain}
|
|
|
|
At a given point in time, the \blockchainview of each \fullnode consists of a
|
|
sequence of one or more valid \blocks. Each \block consists of a sequence of one or
|
|
more \transactions. To each \transaction there is associated an initial \treestate,
|
|
which consists of a \noteCommitmentTree (\crossref{merkletree}), \nullifierSet
|
|
(\crossref{nullifierset}), and data structures associated with \Bitcoin such as
|
|
the UTXO (Unspent Transaction Output) set.
|
|
|
|
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.
|
|
|
|
An \anchor is a Merkle tree root of a \noteCommitmentTree. It uniquely identifies
|
|
a \noteCommitmentTree state given the assumed security properties of the Merkle tree's
|
|
hash function. Since the \nullifierSet is always updated together with the
|
|
\noteCommitmentTree, this also identifies a particular state of the \nullifierSet.
|
|
|
|
In a given node's \blockchainview, \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}
|
|
|
|
\todo{\joinSplitDescriptions also have input and output \treestates.}
|
|
|
|
We rely on Bitcoin-style consensus for \fullnodes to eventually converge on their
|
|
views of valid \blocks, and therefore of the sequence of \treestates in those
|
|
\blocks.
|
|
|
|
|
|
\nsubsection{\JoinSplitTransfers{} and Descriptions} \label{joinsplit}
|
|
|
|
A \joinSplitDescription is data included in a \transaction that describes a \joinSplitTransfer,
|
|
i.e.\ a confidential value transfer. This kind of value transfer is the primary
|
|
\Zcash-specific operation performed by \transactions; it uses, but should not be
|
|
confused with, the \joinSplitStatement used for the \zkSNARK proof and verification.
|
|
|
|
A \joinSplitTransfer spends $\NOld$ \notes $\nOld{\allOld}$ and \transparent input
|
|
$\vpubOld$, and creates $\NNew$ \notes $\nNew{\allNew}$ and \transparent output
|
|
$\vpubNew$.
|
|
|
|
Each \transaction is associated with a \sequenceOfJoinSplitDescriptions.
|
|
|
|
The inputs and outputs of each \joinSplitTransfer{} \MUST balance exactly.
|
|
The \changed{total $\vpubNew$ value adds to, and the total} $\vpubOld$
|
|
value subtracts from the \transparentValuePool of the containing \transaction.
|
|
|
|
\todo{Describe the interaction of \transparent value flows with the \joinSplitDescription's
|
|
\changed{$\vpubOld$ and} $\vpubNew$.}
|
|
|
|
\changed{
|
|
The \anchor of each \joinSplitDescription in a \transaction must refer to either
|
|
some earlier \block's final \treestate, or to the output \treestate of any prior
|
|
\joinSplitDescription in the same \transaction.
|
|
}
|
|
|
|
These conditions act as constraints on the blocks that a \fullnode will
|
|
accept into its \blockchainview.
|
|
|
|
\nsubsection{\NoteCommitmentTree} \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 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.
|
|
|
|
Blocks in the \blockchain are associated (by all nodes) with the \merkleRoot of this tree
|
|
after all of its constituent \joinSplitDescriptions' \noteCommitments have been
|
|
entered into the \noteCommitmentTree associated with the previous \block.
|
|
\todo{Make this more precise.}
|
|
|
|
Each \merkleNode in the \incrementalMerkleTree is associated with a \merkleHash of
|
|
size $\MerkleHashLength$ bytes.
|
|
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{\NullifierSet} \label{nullifierset}
|
|
|
|
Each \fullnode maintains a \nullifierSet alongside the \noteCommitmentTree and UTXO set.
|
|
As valid \transactions containing \joinSplitTransfers are processed, the \nullifiers
|
|
revealed in \joinSplitDescriptions are inserted into this \nullifierSet.
|
|
|
|
If a \joinSplitDescription reveals a \nullifier that already exists in
|
|
the \fullnode's \blockchainview, the containing transaction will be rejected, since
|
|
it would otherwise result in a double-spend.
|
|
|
|
|
|
\nsubsection{Coinbase Transactions}
|
|
|
|
The first \transaction in a block must be a \coinbaseTransaction, which should
|
|
collect and spend any block reward and transaction fees paid by \transactions
|
|
included in this block.
|
|
|
|
\nsubsubsection{Block Subsidy and Transaction Fees}
|
|
|
|
\todo{Describe money supply curve.}
|
|
\todo{Miner's reward = transaction fees + block subsidy - founder's reward}
|
|
|
|
\nsubsubsection{Coinbase outputs}
|
|
|
|
\todo{Coinbase maturity rule.}
|
|
\todo{Any tx with a coinbase input must have no \transparent outputs (vout).}
|
|
|
|
|
|
\nsection{Abstract Protocol}
|
|
|
|
\nsubsection{Abstract Cryptographic Functions}
|
|
|
|
\nsubsubsection{Hash Functions} \label{abstracthashes}
|
|
|
|
$\MerkleCRH \typecolon \MerkleHash \times \MerkleHash \rightarrow \MerkleHash$
|
|
is a collision-resistant hash function used in \crossref{merklepath}.
|
|
It is instantiated in \crossref{merklecrh}.
|
|
|
|
\changed{
|
|
$\hSigCRH{} \typecolon \bitseq{\RandomSeedLength} \times (\PRFOutput)^{\NOld} \times \JoinSplitSigPublic \rightarrow \hSigType$
|
|
is a collision-resistant hash function 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 hash function, 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}.
|
|
}
|
|
|
|
\nsubsubsection{\PseudoRandomFunctions} \label{abstractprfs}
|
|
|
|
$\PRF{x}{}$ is a \pseudoRandomFunction keyed by $x$. \changed{Four} \emph{independent}
|
|
$\PRF{x}{}$ are needed in our protocol:
|
|
|
|
\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{jsstatement}; $\PRFaddr{}$ is also used to
|
|
derive a \paymentAddress from a \spendingKey in \crossref{keycomponents}.
|
|
They are instantiated in \crossref{concreteprfs}.
|
|
|
|
\securityrequirement{
|
|
In addition to being \pseudoRandomFunctions, it is required that $\PRFnf{x}$
|
|
\changed{, $\PRFaddr{x}$, and $\PRFrho{x}$} be collision-resistant across all $x$ ---
|
|
i.e.\ it should not be feasible to find $(x, y) \neq (x', y')$ such that
|
|
$\PRFnf{x}(y) = \PRFnf{x'}(y')$\changed{, and similarly for $\PRFaddr{}$ and $\PRFrho{}$}.
|
|
}
|
|
|
|
\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.
|
|
|
|
$\SymDecrypt{} \typecolon \Keyspace \times \Ciphertext \rightarrow
|
|
\Plaintext \cup \setof{\bot}$ 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. ``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 $\KADerivePublic \typecolon \KAPrivate \rightarrow \KAPublic$ be a function
|
|
that derives the $\KA$ public key corresponding to a given $\KA$ private key.
|
|
|
|
Let $\KAAgree \typecolon \KAPrivate \times \KAPublic \rightarrow \KASharedSecret$
|
|
be the agreement function.
|
|
|
|
\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:
|
|
|
|
Let $\TransmitPrivateSup{1}$ and $\TransmitPrivateSup{2}$ each be chosen uniformly and
|
|
independently at random from $\KAPrivate$.
|
|
|
|
Let $\TransmitPublicSup{j} := \KADerivePublic(\TransmitPrivateSup{j})$.
|
|
|
|
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 $\EphemeralPublic := \KADerivePublic(\EphemeralPrivate)$.
|
|
\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{keycomponents}). If a
|
|
\transmissionKey not in that range is used, it may be distinguishable.
|
|
This is not considered to be a significant security weakness.
|
|
}
|
|
|
|
|
|
\nsubsubsection{Signatures} \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 $(\sk, \vk) \leftarrow \SigGen()$, and
|
|
any $m \typecolon \SigMessage$ and $s \typecolon \SigSignature \leftarrow \SigSign{\sk}(m)$,
|
|
$\SigVerify{\vk}(m, s) = 1$.
|
|
|
|
\Zcash uses two signature schemes, one used for signatures that can be verified
|
|
by script operations such as \ScriptOP{CHECKSIG} and \ScriptOP{CHECKMULTISIG} as
|
|
in \Bitcoin, and one called $\JoinSplitSig$ which is used to sign \transactions
|
|
that contain at least one \joinSplitDescription. The latter is instantiated in
|
|
\crossref{concretesig}. The following defines only the security properties needed
|
|
for $\JoinSplitSig$.
|
|
|
|
\securityrequirement{
|
|
$\JoinSplitSig$ 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 Since a fresh key pair is generated for every \transaction containing
|
|
a \joinSplitDescription and is only used for one signature (see
|
|
\crossref{nonmalleability}), a one-time signature scheme would
|
|
suffice for $\JoinSplitSig$. This is also the reason why only
|
|
security against \emph{non-adaptive} chosen message attack is needed.
|
|
In fact the instantiation of $\JoinSplitSig$ uses a scheme 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 to
|
|
forge a distinct signature on a previously seen message. That is,
|
|
\joinSplitSignatures are intended to be nonmalleable in the sense of
|
|
\cite{BIP-62}.
|
|
\end{pnotes}
|
|
|
|
|
|
\nsubsubsection{Commitment} \label{abstractcomm}
|
|
|
|
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 (``hiding''),
|
|
\item given the \trapdoor and input, the commitment can be verified to ``open''
|
|
to that input and no other (``binding'').
|
|
\end{itemize}
|
|
|
|
\vspace{-3ex}
|
|
A \commitmentScheme $\Commit{}$ defines a type of inputs $\CommitInput$,
|
|
a type of commitments $\CommitOutput$, and a type of \commitmentTrapdoors
|
|
$\CommitTrapdoor$.
|
|
|
|
Let $\Commit{} \typecolon \CommitTrapdoor \times \CommitInput \rightarrow \CommitOutput$
|
|
be a function satisfying the security requirements of computational hiding
|
|
and computational binding, as defined in \todo{need reference}.
|
|
|
|
|
|
\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.
|
|
|
|
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 $(\pk, \vk) \leftarrow \ZKGen()$.
|
|
|
|
\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{Proof of Knowledge:} 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. \todo{Full definition.}
|
|
\end{securityrequirements}
|
|
|
|
These definitions are derived from those in \cite[Appendix C]{BCTV2014}, adapted to
|
|
state concrete rather than asymptotic security. ($\ZKProve{}$ corresponds to $P$,
|
|
$\ZKVerify{}$ corresponds to $V$, and $\ZKSatisfying$ corresponds to $\mathcal{R}_C$
|
|
in the notation of that appendix.)
|
|
|
|
The Proof of Knowledge 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 $(x, w) \in \ZKSatisfying$.
|
|
(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 $(x, w)$.)
|
|
|
|
The \provingSystem is instantiated in \crossref{proofs}.
|
|
$\ZKJoinSplit$ refers to this \provingSystem specialized to the \joinSplitStatement
|
|
given in \crossref{jsstatement}. In this case we omit the key subscripts on
|
|
$\ZKJoinSplitVerify$ and $\ZKJoinSplitProve$, taking them to be the particular
|
|
\provingKey and \verifyingKey defined by the \joinSplitParameters in \crossref{jsparameters}.
|
|
|
|
|
|
\nsubsection{Key Components} \label{keycomponents}
|
|
|
|
Let $\KA$ be a \keyAgreementScheme, instantiated in \crossref{concretekeyagreement}.
|
|
|
|
A new \spendingKey $\AuthPrivate$ is generated by choosing a bit string
|
|
uniformly at random from $\bitseq{\AuthPrivateLength}$.
|
|
|
|
\changed{
|
|
$\AuthPublic$, $\TransmitPrivate$ and $\TransmitPublic$ are derived from
|
|
$\AuthPrivate$
|
|
as follows:}
|
|
{\hfuzz=50pt
|
|
\begin{equation*}
|
|
\begin{aligned}
|
|
\AuthPublic &:= \changed{\PRFaddr{\AuthPrivate}(0)} \\
|
|
\TransmitPrivate &:= \changed{\KAFormatPrivate(\PRFaddr{\AuthPrivate}(1))} \\
|
|
\TransmitPublic &:= \changed{\KADerivePublic(\TransmitPrivate)}
|
|
\end{aligned}
|
|
\end{equation*}
|
|
}
|
|
|
|
\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.
|
|
|
|
Each \joinSplitDescription consists of $(\vpubOld, \vpubNew, \rt,
|
|
\nfOld{\allOld}, \cmNew{\allNew}, \EphemeralPublic, \RandomSeed,
|
|
\h{\allOld}, \JoinSplitProof, \TransmitCiphertext{\allNew})$
|
|
|
|
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 $\nfOld{\allOld} \typecolon (\PRFOutput)^{\NOld}$ is
|
|
the sequence of \nullifiers for the input \notes;
|
|
\item $\cmNew{\allNew} \typecolon (\CommitOutput)^{\NNew}$ is
|
|
the sequence of \noteCommitments for the output \notes;
|
|
\item \changed{$\EphemeralPublic \typecolon \KAPublic$ 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 $\h{\allOld} \typecolon (\PRFOutput)^{\NOld}$ is
|
|
a sequence of tags that bind $\hSig$ to each
|
|
$\AuthPrivate$ of the input \notes;
|
|
\item $\JoinSplitProof \typecolon \ZKProof$ is
|
|
the \zeroKnowledgeProof for the \joinSplitStatement;
|
|
\item $\TransmitCiphertext{\allNew} \typecolon (\Ciphertext)^{\NNew}$ is
|
|
a sequence of ciphertext components for the encrypted output \notes.
|
|
\end{itemize}
|
|
|
|
The $\ephemeralKey$ and $\encCiphertexts$ fields together form the \notesCiphertext.
|
|
|
|
The value $\hSig$ is also computed from $\RandomSeed$, $\nfOld{\allOld}$, and the
|
|
$\joinSplitPubKey$ of the containing \transaction:
|
|
|
|
\begin{itemize}
|
|
\item[] $\hSig := \hSigCRH(\RandomSeed, \nfOld{\allOld}, \joinSplitPubKey)$.
|
|
\end{itemize}
|
|
|
|
$\hSigCRH$ 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 $\ZKJoinSplitVerify((\rt, \nfOld{\allOld}, \cmNew{\allNew},
|
|
\vpubOld, \vpubNew, \hSig, \h{\allOld}), \Proof_{\JoinSplit}) = 1$.
|
|
\end{consensusrules}
|
|
|
|
\nsubsection{Sending \Notes} \label{send}
|
|
|
|
In order to send \xprotected value, the sender constructs a \transaction
|
|
containing one or more \joinSplitDescriptions. This involves first generating
|
|
a new $\JoinSplitSig$ key pair:
|
|
|
|
\hskip 1.5em $(\joinSplitPrivKey, \joinSplitPubKey) \leftarrow \JoinSplitSigGen()$.
|
|
|
|
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. 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 $\bitseq{\NoteCommitRandLength}$.
|
|
\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.
|
|
|
|
After generating all of the \joinSplitDescriptions, the sender obtains the
|
|
$\dataToBeSigned$ (\crossref{nonmalleability}), and signs it with
|
|
the private \joinSplitSigningKey:
|
|
|
|
\hskip 1.5em $\joinSplitSig \leftarrow \JoinSplitSigSign{\text{\small\joinSplitPrivKey}}(\dataToBeSigned)$
|
|
|
|
Then the encoded \transaction including $\joinSplitSig$ is submitted to the network.
|
|
|
|
|
|
\nsubsubsection{\DummyNotes} \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.
|
|
|
|
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 $\EnforceCommit{i}$ to 0.
|
|
\end{itemize}
|
|
|
|
A \dummy output \note is constructed as normal but with zero value, and
|
|
sent to a random \paymentAddress.
|
|
|
|
|
|
\nsubsection{Merkle path validity} \label{merklepath}
|
|
|
|
The depth of the \noteCommitmentTree is $\MerkleDepth$ (defined in \crossref{constants}).
|
|
|
|
Each \merkleNode in the \incrementalMerkleTree is associated with a \merkleHash,
|
|
which is a byte 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 $\Uncommitted$.
|
|
It is assumed to be infeasible to find a preimage \note $\NoteTuple{}$ such that
|
|
$\NoteCommit(\NoteTuple{}) = \Uncommitted$.
|
|
|
|
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 $0 \leq i < 2^h$,
|
|
|
|
\hskip 2em $\MerkleNode{h}{i} := \MerkleCRH(\MerkleNode{h+1}{2i}, \MerkleNode{h+1}{2i+1})$.
|
|
|
|
A \merklePath from \merkleLeafNode $\MerkleNode{\MerkleDepth}{i}$ in the
|
|
\incrementalMerkleTree is the sequence
|
|
|
|
\hskip 2em $[\hairspace\MerkleNode{h}{\MerkleSibling(h, i)} \text{ for }
|
|
h \text{ from } \MerkleDepth \text{ down to } 1\hairspace]$,
|
|
|
|
where
|
|
|
|
\hskip 2em $\MerkleSibling(h, i) = \floor{\frac{i}{2^{\MerkleDepth-h}}} \xor 1$
|
|
|
|
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{concretesig}.
|
|
|
|
\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.
|
|
}
|
|
|
|
The condition enforced by the \joinSplitStatement specified in \crossref{nonmalleablepour}
|
|
ensures that a holder of all of $\AuthPrivateOld{\allOld}$ for each
|
|
\joinSplitDescription has authorized the use of the private signing key corresponding
|
|
to $\joinSplitPubKey$ to sign this \transaction.
|
|
|
|
|
|
\nsubsection{Balance}
|
|
|
|
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{
|
|
\pnote{
|
|
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,
|
|
\joinSplitDescriptions are indistinguishable regardless of the number of real input
|
|
\notes.
|
|
}
|
|
|
|
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.
|
|
}
|
|
|
|
\nsubsection{\NoteCommitments{} and \Nullifiers}
|
|
|
|
A \transaction that contains one or more \joinSplitDescriptions, 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 \blockchainview \emph{and} \mempool. A \transaction is not
|
|
valid if it attempts to add a \nullifier to the \nullifierSet that already
|
|
exists in the set.
|
|
|
|
\nsubsection{\JoinSplitStatement} \label{jsstatement}
|
|
|
|
A valid instance of $\JoinSplitProof$ assures that given a \term{primary input}:
|
|
|
|
\begin{itemize}
|
|
\item[] $(\rt \typecolon \MerkleHash,
|
|
\nfOld{\allOld} \typecolon (\PRFOutput)^{\NOld},
|
|
\cmNew{\allNew} \typecolon (\CommitOutput)^{\NNew},
|
|
\changed{\vpubOld \typecolon \range{0}{2^{64}-1},}\,
|
|
\vpubNew \typecolon \range{0}{2^{64}-1},\\
|
|
\hphantom{(}
|
|
\hSig \typecolon \hSigType,
|
|
\h{\allOld} \typecolon (\PRFOutput)^{\NOld})$,
|
|
\end{itemize}
|
|
|
|
the prover knows an \term{auxiliary input}:
|
|
|
|
\begin{itemize}
|
|
\item[] $(\treepath{\allOld} \typecolon ((\MerkleHash)^{\MerkleDepth})^{\NOld},
|
|
\nOld{\allOld} \typecolon \NoteType^{\NOld},
|
|
\AuthPrivateOld{\allOld} \typecolon (\bitseq{\AuthPrivateLength})^{\NOld},
|
|
\nNew{\allNew} \typecolon \NoteType^{\NOld}\changed{,}\\
|
|
\hphantom{(}
|
|
\changed{\NoteAddressPreRand \typecolon \bitseq{\NoteAddressPreRandLength},
|
|
\EnforceCommit{\allOld} \typecolon \bitseq{\NOld}})$,
|
|
\end{itemize}
|
|
|
|
where:
|
|
|
|
\begin{itemize}
|
|
\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{itemize}
|
|
|
|
such that the following conditions hold:
|
|
|
|
\subparagraph{Merkle path validity} \label{merklepathvalidity}
|
|
|
|
for each $i \in \setofOld$ \changed{$\mid$ $\EnforceCommit{i} = 1$}:
|
|
$\treepath{i}$ must be a valid \merklePath of depth $\MerkleDepth$, as defined in
|
|
\crossref{merklepath}, from $\NoteCommit(\nOld{i})$ 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}.
|
|
|
|
\subparagraph{Commitment Enforcement}
|
|
|
|
for each $i \in \setofOld$, if $\vOld{i} \neq 0$ then $\EnforceCommit{i} = 1$.
|
|
|
|
\subparagraph{Balance}
|
|
|
|
$\changed{\vpubOld\; +} \vsum{i=1}{\NOld} \vOld{i} = \vpubNew + \vsum{i=1}{\NNew} \vNew{i} \in \range{0}{2^{64}-1}$.
|
|
|
|
\subparagraph{\Nullifier{} integrity}
|
|
|
|
for each $i \in \setofNew$:
|
|
$\nfOld{i} = \PRFnf{\AuthPrivateOld{i}}(\NoteAddressRandOld{i})$.
|
|
|
|
\subparagraph{Spend authority} \label{spendauthority}
|
|
|
|
for each $i \in \setofOld$:
|
|
$\AuthPublicOld{i} = \changed{\PRFaddr{\AuthPrivateOld{i}}(0)}$.
|
|
|
|
\subparagraph{Non-malleability} \label{nonmalleablepour}
|
|
|
|
for each $i \in \setofOld$:
|
|
$\h{i} = \PRFpk{\AuthPrivateOld{i}}(i, \hSig)$.
|
|
|
|
\changed{
|
|
\subparagraph{Uniqueness of $\NoteAddressRandNew{i}$} \label{uniquerho}
|
|
|
|
for each $i \in \setofNew$:
|
|
$\NoteAddressRandNew{i} = \PRFrho{\NoteAddressPreRand}(i, \hSig)$.
|
|
}
|
|
|
|
\subparagraph{Commitment integrity}
|
|
|
|
for each $i \in \setofNew$: $\cmNew{i}$ = $\NoteCommit(\nNew{i})$.
|
|
|
|
\vspace{2.5ex}
|
|
For details of the form and encoding of proofs, see \crossref{proofs}.
|
|
|
|
|
|
\nsubsection{In-band secret distribution} \label{inband}
|
|
|
|
In order to transmit the secret $\Value$, $\NoteAddressRand$, and $\NoteCommitRand$
|
|
(necessary for the recipient to later spend) \changed{and also a \memo} to the
|
|
recipient \emph{without} requiring an out-of-band communication channel, the
|
|
\transmissionKey $\TransmitPublic$ is used to encrypt these
|
|
secrets. The recipient's possession of the associated \keyTuple
|
|
$(\AuthPrivate, \TransmitPrivate, \PaymentAddress)$ is used to reconstruct
|
|
the original \note \changed{ and \memo}.
|
|
|
|
All of the resulting ciphertexts are combined to form a \notesCiphertext.
|
|
|
|
For both encryption and decryption,
|
|
\begin{itemize}
|
|
\item Let $\Sym$ be the \encryptionScheme instantiated in \crossref{concretesym}.
|
|
\item Let $\KDF$ be the \keyDerivationFunction instantiated in \crossref{concretekdf}.
|
|
\item Let $\KA$ be the \keyAgreementScheme instantiated in \crossref{concretekeyagreement}.
|
|
\item 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}.
|
|
|
|
Then to encrypt:
|
|
\begin{itemize}
|
|
\changed{
|
|
\item Generate a new $\KA$ (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 $\DHSecret{i} := \KAAgree(\EphemeralPrivate,
|
|
\TransmitPublicNew{i})$.
|
|
\item Let $\TransmitKey{i} := \KDF(i, \hSig, \DHSecret{i}, \EphemeralPublic,
|
|
\TransmitPublicNew{i})$.
|
|
\item Let $\TransmitCiphertext{i} :=
|
|
\SymEncrypt{\TransmitKey{i}}(\TransmitPlaintext{i})$.
|
|
\end{itemize}
|
|
}
|
|
\end{itemize}
|
|
|
|
The resulting \notesCiphertext is $\changed{(\EphemeralPublic,
|
|
\TransmitCiphertext{\allNew})}$.
|
|
|
|
\nsubsubsection{Decryption by a Recipient}
|
|
|
|
Let $\PaymentAddress = (\AuthPublic, \TransmitPublic)$ be the recipient's
|
|
\paymentAddress, and let $\TransmitPrivate$ be the recipient's \viewingKey.
|
|
|
|
Let $\cmNew{\allNew}$ be the \noteCommitments of each output coin.
|
|
|
|
Then for each $i \in \setofNew$, the recipient will attempt to decrypt that ciphertext
|
|
component as follows:
|
|
|
|
\changed{
|
|
\begin{itemize}
|
|
\item Let $\DHSecret{i} := \KAAgree(\TransmitPrivate, \EphemeralPublic)$.
|
|
\item Let $\TransmitKey{i} := \KDF(i, \hSig, \DHSecret{i}, \EphemeralPublic,
|
|
\TransmitPublicNew{i})$.
|
|
\item Return $\DecryptNote(\TransmitKey{i}, \TransmitCiphertext{i}, \cmNew{i},
|
|
\AuthPublic).$
|
|
\end{itemize}
|
|
|
|
$\DecryptNote(\TransmitKey{i}, \TransmitCiphertext{i}, \cmNew{i}, \AuthPublic)$
|
|
is defined as follows:
|
|
|
|
\begin{itemize}
|
|
\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 $\NoteCommit((\AuthPublic, \ValueNew{i}, \NoteAddressRandNew{i},
|
|
\NoteCommitRandNew{i})) \neq \cmNew{i}$, return $\bot$, else return $\NotePlaintext{i}$.
|
|
\end{itemize}
|
|
}
|
|
|
|
To test whether a \note is unspent in a particular \blockchainview also requires
|
|
the \spendingKey $\AuthPrivate$; the coin is unspent if and only if
|
|
$\nf = \PRFnf{\AuthPrivate}(\NoteAddressRand)$ is not in the \nullifierSet
|
|
for that \blockchainview.
|
|
|
|
\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 on a given \blockchainview,
|
|
as \transactions are added to that view. Also, blockchain reorganisations
|
|
can cause the \transaction in which a \note was output to no longer be on
|
|
the consensus blockchain.
|
|
\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}
|
|
|
|
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}.
|
|
|
|
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 is given explicitly in each box, except for the case of a single
|
|
bit, or for the notation $\zeros{n}$ which represents the sequence of $n$ 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.
|
|
|
|
\begin{comment}
|
|
\todo{Update example for big-bit-endian order.}
|
|
|
|
\newsavebox{\exampleabox}
|
|
\begin{lrbox}{\exampleabox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=1.3em]{32}
|
|
\bitbox{1}{0} &
|
|
\bitbox{1}{1} &
|
|
\bitbox{1}{0} &
|
|
\bitbox{1}{0} &
|
|
\bitbox{16}{16 bit $\hexint{ABCD}$} &
|
|
\bitbox{12}{12 bit $\hexint{123}$} &
|
|
\end{bytefield}
|
|
\end{lrbox}
|
|
|
|
\newsavebox{\examplebbox}
|
|
\begin{lrbox}{\examplebbox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=1.3em]{32}
|
|
\bitbox{4}{4 bit $\hexint{2}$} &
|
|
\bitbox{4}{4 bit $\hexint{D}$} &
|
|
\bitbox{4}{4 bit $\hexint{C}$} &
|
|
\bitbox{4}{4 bit $\hexint{B}$} &
|
|
\bitbox{4}{4 bit $\hexint{A}$} &
|
|
\bitbox{4}{4 bit $\hexint{3}$} &
|
|
\bitbox{4}{4 bit $\hexint{2}$} &
|
|
\bitbox{4}{4 bit $\hexint{1}$} &
|
|
\end{bytefield}
|
|
\end{lrbox}
|
|
|
|
\newsavebox{\examplecbox}
|
|
\begin{lrbox}{\examplecbox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=1.3em]{32}
|
|
\bitbox{8}{8 bit $\hexint{D2}$} &
|
|
\bitbox{8}{8 bit $\hexint{BC}$} &
|
|
\bitbox{8}{8 bit $\hexint{3A}$} &
|
|
\bitbox{8}{8 bit $\hexint{12}$} &
|
|
\end{bytefield}
|
|
\end{lrbox}
|
|
|
|
For example, the following diagrams are all equivalent:
|
|
\begin{itemize}
|
|
\item[] $\Justthebox{\exampleabox}$
|
|
\item[] $\Justthebox{\examplebbox}$
|
|
\item[] $\Justthebox{\examplecbox}$
|
|
\end{itemize}
|
|
|
|
and represent the byte sequence $[\hexint{D2}, \hexint{BC}, \hexint{3A}, \hexint{12}]$.
|
|
\end{comment}
|
|
|
|
\nsubsection{Constants} \label{constants}
|
|
|
|
Define:
|
|
|
|
\begin{itemize}
|
|
\item[] $\MerkleDepth = \changed{29}$
|
|
\item[] $\NOld = 2$
|
|
\item[] $\NNew = 2$
|
|
\item[] $\MerkleHashLength = 256$
|
|
\item[] $\hSigLength = 256$
|
|
\item[] $\PRFOutputLength = 256$
|
|
\item[] $\NoteCommitRandLength = \changed{256}$
|
|
\item[] $\changed{\RandomSeedLength = 256}$
|
|
\item[] $\AuthPrivateLength = \changed{252}$
|
|
\item[] $\NoteAddressPreRandLength = \changed{252}$
|
|
\item[] $\Uncommitted = \zeros{\MerkleHashLength}$
|
|
\item[] $\MAXMONEY = \changed{2.1 \times 10^{15}}$.
|
|
\end{itemize}
|
|
|
|
|
|
\nsubsection{Concrete Cryptographic Functions}
|
|
|
|
\nsubsubsection{Merkle Tree Hash Function} \label{merklecrh}
|
|
|
|
$\MerkleCRH$ is used to hash \incrementalMerkleTree \merkleHashes.
|
|
It is instantiated by the $\SHAName$ function, which takes a 512-bit block
|
|
and produces a 256-bit hash. \cite{NIST2015}
|
|
|
|
\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}
|
|
|
|
\hskip 2em $\MerkleCRH(\mathsf{left}, \mathsf{right}) := \CRHbox{\merklebox}$.
|
|
|
|
\pnote{
|
|
$\SHA$ is not the same as the $\FullHashName$ function, which hashes arbitrary-length
|
|
sequences.
|
|
}
|
|
|
|
\securityrequirement{
|
|
$\SHA$ must be collision-resistant, and it must be infeasible to find a preimage $x$
|
|
such that $\SHA(x) = \zeros{256}$.
|
|
}
|
|
|
|
\nsubsubsection{\hSigText{} Hash Function} \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{
|
|
\hskip 1.5em $\hSigCRH(\RandomSeed, \nfOld{\allOld}, \joinSplitPubKey) := \GeneralCRH{256}(\ascii{ZcashComputehSig},\; \hSigInput)$
|
|
|
|
where
|
|
|
|
\hskip 1.5em $\hSigInput := \Justthebox{\hsigbox}$.
|
|
}
|
|
|
|
$\GeneralCRH{\ell}(p, x)$ is instantiated by unkeyed $\Blake{\ell}$
|
|
\cite{ANWW2013}\cite{RFC-7693} in sequential mode, with an output
|
|
digest length of $\ell/8$ bytes, 16-byte personalization string $p$,
|
|
and input $x$.
|
|
|
|
\pnote{
|
|
$\Blake{\ell}$ is not the same as $\Blake{512}$ truncated to $\ell$ bits.
|
|
}
|
|
|
|
\securityrequirement{
|
|
$\Blake{256}(\ascii{ZcashComputehSig}, x)$ must be collision-resistant.
|
|
}
|
|
|
|
\nsubsubsection{Equihash Generator} \label{equihashgen}
|
|
|
|
$\EquihashGen{n, k}$ is a specialized hash function 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}
|
|
% Blech. Dijkstra was right \cite{EWD831}.
|
|
Let $\EquihashGen{n, k}(S, i) := T_{h+1\hairspace..\hairspace h+n}$, where
|
|
\begin{itemize}
|
|
\item $m := \floor{\frac{512}{n}}$;
|
|
\item $h := (i-1 \bmod m)\, n$;
|
|
\item $T := \GeneralCRH{n m}(\powtag,\, S \,||\, \powcount(\floor{\frac{i-1}{m}}))$.
|
|
\end{itemize}
|
|
|
|
Indices of bits in $T$ are 1-based. $\GeneralCRH{\ell}(p, x)$ is defined
|
|
as in the previous section.
|
|
|
|
\securityrequirement{
|
|
$\Blake{\ell}(\powtag, x)$ must be collision-resistant, for any $\ell$ and
|
|
$\powtag$ used in the protocol.
|
|
}
|
|
|
|
\nsubsubsection{\PseudoRandomFunctions} \label{concreteprfs}
|
|
|
|
The \changed{four} independent PRFs described in \crossref{abstractprfs} are
|
|
all instantiated using the $\SHAName$ function:
|
|
|
|
\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 := \CRHbox{\addrbox} \\
|
|
\PRFnf{\AuthPrivate}(\NoteAddressRand) &:= \CRHbox{\nfbox} \\
|
|
\PRFpk{\AuthPrivate}(i, \hSig) &:= \CRHbox{\pkbox} \\
|
|
\setchanged \PRFrho{\NoteAddressPreRand}(i, \hSig) &\setchanged := \CRHbox{\rhobox}
|
|
\end{aligned}
|
|
\end{equation*}
|
|
|
|
\begin{securityrequirements}
|
|
\item The $\SHAName$ function must be collision-resistant.
|
|
\item The $\SHAName$ function 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 $\SHA$, 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 $\FullHashName$ hash
|
|
function --- see \crossref{concretecomm}.
|
|
(The specific bit patterns chosen here are motivated by the possibility of future
|
|
extensions that either increase $\NOld$ and/or $\NNew$ to 3, or that add an
|
|
additional bit to $\AuthPrivate$ to encode a new key type, or that require an
|
|
additional PRF.)
|
|
}
|
|
}
|
|
|
|
\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 uses 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{\KeyAgreement} \label{concretekeyagreement}
|
|
|
|
\changed{
|
|
The \keyAgreementScheme specified in \crossref{abstractkeyagreement} is
|
|
instantiated using Curve25519 \cite{Bern2006} as follows.
|
|
|
|
Let $\KAPublic$ and $\KASharedSecret$ be the type of Curve25519 public keys
|
|
(i.e.\ a sequence of 32 bytes), and let $\KAPrivate$ 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 $\CurveBase$ 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 $\KAFormatPrivate(x) := \Clamp(x)$.
|
|
|
|
Define $\KAAgree(n, q) := \CurveMultiply(n, q)$.
|
|
}
|
|
|
|
\nsubsubsection{\KeyDerivation} \label{concretekdf}
|
|
|
|
\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 $\Blake{256}$ as follows:
|
|
|
|
\hskip 1.5em $\KDF(i, \hSig, \DHSecret{i}, \EphemeralPublic, \TransmitPublicNew{i}) :=
|
|
\Blake{256}(\kdftag, \kdfinput)$
|
|
|
|
where:
|
|
|
|
\hskip 1.5em $\kdftag := \Justthebox{\kdftagbox}$
|
|
|
|
\hskip 1.5em $\kdfinput := \Justthebox{\kdfinputbox}$.
|
|
}
|
|
|
|
\nsubsubsection{Signatures} \label{concretesig}
|
|
|
|
$\JoinSplitSig$ is specified in \crossref{abstractsig}.
|
|
|
|
\changed{It is instantiated as $\JoinSplitSigSpecific$ \cite{BDL+2012},
|
|
with the additional requirement that $\EdDSAs$ (the integer represented
|
|
by $\EdDSAS$) must be less than the prime
|
|
$\ell = 2^{252} + 27742317777372353535851937790883648493$,
|
|
otherwise the signature is considered invalid.
|
|
$\JoinSplitSigSpecific$ is defined as using $\JoinSplitSigHashName$ internally.
|
|
}
|
|
|
|
\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}
|
|
|
|
\changed{
|
|
The encoding of a signature is:
|
|
}
|
|
\begin{itemize}
|
|
\item[] $\Justthebox{\sigbox}$
|
|
\end{itemize}
|
|
|
|
\changed{
|
|
where $\EdDSAR$ and $\EdDSAS$ are as defined in \cite{BDL+2012}.
|
|
|
|
The encoding of a public key is as defined in \cite{BDL+2012}.
|
|
}
|
|
|
|
\nsubsubsection{Commitment} \label{concretecomm}
|
|
|
|
\newsavebox{\cmbox}
|
|
\begin{lrbox}{\cmbox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=0.032em]{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 $\Commit{}$ specified in \crossref{abstractcomm} is
|
|
instantiated using $\FullHashName$ as follows:
|
|
|
|
\hskip 1em $\Commit{\NoteCommitRand}(\Value, \AuthPublic, \NoteAddressRand) := \FullHashbox{\cmbox}$.
|
|
|
|
\pnote{
|
|
The leading byte of the $\FullHash$ input is $\hexint{B0}$.
|
|
}
|
|
|
|
\todo{Security requirements on $\FullHashName$.}
|
|
|
|
\nsubsection{\NotePlaintexts{} and \Memos} \label{notept}
|
|
|
|
Transmitted \notes are stored on the blockchain in encrypted form, together with
|
|
a \noteCommitment $\cm$.
|
|
|
|
The \notePlaintexts associated with a \joinSplitDescription are encrypted to the
|
|
respective \transmissionKeys $\TransmitPublicNew{\allNew}$,
|
|
and the result forms part of a \notesCiphertext (see \crossref{inband}
|
|
for further details).
|
|
|
|
Each \notePlaintext (denoted $\NotePlaintext{}$) consists of
|
|
$(\Value, \NoteAddressRand, \NoteCommitRand\changed{, \Memo})$.
|
|
|
|
The first three of these fields are as defined earlier.
|
|
\changed{$\Memo$ is a 512-byte \memo associated with this \note.
|
|
|
|
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 $\hexint{F6}$ or greater is reserved for use in future \Zcash
|
|
protocol extensions.
|
|
}
|
|
|
|
The encoding of a \notePlaintext consists of, in order:
|
|
\begin{equation*}
|
|
\begin{bytefield}[bitwidth=0.029em]{1608}
|
|
\changed{
|
|
\bitbox{192}{$8$-bit $\NotePlaintextLeadByte$}
|
|
&}\bitbox{192}{$64$-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, $\NotePlaintextLeadByte$, indicating this version of the
|
|
encoding of a \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}
|
|
|
|
|
|
\nsubsection{Encodings of Addresses and Keys}
|
|
|
|
This section describes how \Zcash encodes \paymentAddresses\changed{, \viewingKeys,}
|
|
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{Bitcoin-Base58}.
|
|
|
|
$\SHAName$ outputs are always represented as sequences of 32 bytes.
|
|
|
|
The language consisting of the following encoding possibilities is prefix-free.
|
|
|
|
\nsubsubsection{Transparent Payment Addresses}
|
|
|
|
These are encoded in the same way as in \Bitcoin \cite{Bitcoin-Base58}.
|
|
|
|
\nsubsubsection{Transparent Private Keys}
|
|
|
|
These are encoded in the same way as in \Bitcoin \cite{Bitcoin-Base58}.
|
|
|
|
\nsubsubsection{Protected Payment Addresses}
|
|
|
|
A \paymentAddress consists of $\AuthPublic$ and $\TransmitPublic$.
|
|
$\AuthPublic$ is a $\SHAName$ output.
|
|
$\TransmitPublic$ is a \changed{Bern2006} public key, for use with the
|
|
encryption scheme defined in \crossref{inband}.
|
|
|
|
The raw encoding of a \paymentAddress starts with:
|
|
|
|
\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 \Zcash \paymentAddress
|
|
on the production network. (Addresses on the test network use
|
|
$[\PaymentAddressTestnetLeadByte, \PaymentAddressTestnetSecondByte]$
|
|
instead.)
|
|
}
|
|
\item 256 bits specifying $\AuthPublic$.
|
|
\item \changed{256 bits} specifying $\TransmitPublic$, \changed{using the
|
|
normal encoding of a Curve25519 public key \cite{Bern2006}}.
|
|
\end{itemize}
|
|
|
|
To allow for future extensions, when reading the raw encoding, up to 66
|
|
additional bytes after the fields defined above \MUST be accepted and stored
|
|
with the address, but otherwise ignored. In this version of the protocol there
|
|
is no defined meaning for these additional bytes, and so they \MUST be omitted
|
|
when an address is generated.
|
|
|
|
\nsubsubsection{Spending Keys}
|
|
|
|
A \spendingKey consists of $\AuthPrivate$, which is a sequence of \changed{252} bits.
|
|
|
|
The raw encoding of a \spendingKey consists of, in order:
|
|
|
|
\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 4 zero padding bits.
|
|
}
|
|
\item \changed{252} bits specifying $\AuthPrivate$.
|
|
\end{itemize}
|
|
|
|
\changed{
|
|
The zero padding occupies the most significant 4 bits of the third byte.
|
|
|
|
\pnote{
|
|
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.
|
|
}
|
|
}
|
|
|
|
|
|
\nsubsection{\ZeroKnowledgeProvingSystem} \label{proofs}
|
|
|
|
\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{PGHR2013} and \cite{BCGTV2013}.
|
|
|
|
The pairing implementation is $\BNImpl$.
|
|
|
|
Let $q = 21888242871839275222246405745257275088696311157297823662689037894645226208583$.
|
|
|
|
Let $r = 21888242871839275222246405745257275088548364400416034343698204186575808495617$.
|
|
|
|
Let $b = 3$.
|
|
|
|
($q$ and $r$ are prime.)
|
|
|
|
The pairing is of type $\GroupG{1} \times \GroupG{2} \rightarrow \GroupG{T}$, where:
|
|
\begin{itemize}
|
|
\item $\GroupG{1}$ is a Barreto--Naehrig curve over $\GF{q}$ with equation
|
|
$y^2 = x^3 + b$. This curve has embedding degree 12 with respect to $r$.
|
|
\item $\GroupG{2}$ is the subgroup of order $r$ in the twisted Barreto-Naehrig curve
|
|
over $\GF{q^2}$ with equation $y^2 = x^3 + b/xi$. We represent elements of $\GF{q^2}$ as
|
|
polynomials $a_1 t + a_0 \typecolon \GF{q}[t]$, modulo the irreducible polynomial
|
|
$t^2 + 1$.
|
|
\item $\GroupG{T}$ is $\mu_r$, the subgroup of $r^\mathrm{th}$ roots of unity in
|
|
$\GFstar{q^{12}}$.
|
|
\end{itemize}
|
|
|
|
Let $\PointP{1} \typecolon \GroupG{1} = (1, 2)$.
|
|
|
|
\begin{tabular}{@{}l@{}r@{}l@{}}
|
|
Let $\PointP{2} \typecolon \GroupG{2} =\;$
|
|
% are these the right way round?
|
|
&$(11559732032986387107991004021392285783925812861821192530917403151452391805634$ & $\,t\;+$ \\
|
|
&$ 10857046999023057135944570762232829481370756359578518086990519993285655852781$ & $, $ \\
|
|
&$ 4082367875863433681332203403145435568316851327593401208105741076214120093531$ & $\,t\;+$ \\
|
|
&$ 8495653923123431417604973247489272438418190587263600148770280649306958101930$ & $). $
|
|
\end{tabular}
|
|
|
|
$\PointP{1}$ and $\PointP{2}$ are generators of $\GroupG{1}$ and $\GroupG{2}$ respectively.
|
|
|
|
A proof consists of a tuple
|
|
$(\Proof_A \typecolon \GroupG{1},\;
|
|
\Proof'_A \typecolon \GroupG{1},\;
|
|
\Proof_B \typecolon \GroupG{2},\;
|
|
\Proof'_B \typecolon \GroupG{1},\;
|
|
\Proof_C \typecolon \GroupG{1},\;
|
|
\Proof'_C \typecolon \GroupG{1},\;
|
|
\Proof_K \typecolon \GroupG{1},\;
|
|
\Proof_H \typecolon \GroupG{1})$.
|
|
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 \arithmeticCircuit verifying the \joinSplitStatement,
|
|
or its expression as a \rankOneConstraintSystem, are not specified here.
|
|
In practice it will be necessary to use the specific proving and verification keys
|
|
generated for the \Zcash production \blockchain (see \crossref{jsparameters}),
|
|
and a \provingSystem implementation that is interoperable with the \Zcash fork
|
|
of \libsnark, to ensure compatibility.
|
|
}
|
|
|
|
\nsubsubsection{Encoding of Points} \label{pointencoding}
|
|
|
|
\newsavebox{\gonebox}
|
|
\begin{lrbox}{\gonebox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=0.05em]{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 $\ItoOSP{32}(x)$}
|
|
\end{bytefield}
|
|
\end{lrbox}
|
|
|
|
\newsavebox{\gtwobox}
|
|
\begin{lrbox}{\gtwobox}
|
|
\setchanged
|
|
\begin{bytefield}[bitwidth=0.05em]{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 $\ItoOSP{64}(x)$}
|
|
\end{bytefield}
|
|
\end{lrbox}
|
|
|
|
Define $\ItoOSP{} \typecolon (k \typecolon \Nat) \times \range{0}{256^k\!-\!1} \rightarrow \range{0}{255}^k$
|
|
such that $\ItoOSP{\ell}(n)$ is the sequence of $\ell$ bytes representing $n$ in
|
|
big-endian order.
|
|
|
|
For a point $P \typecolon \GroupG{1} = (x_P, y_P)$:
|
|
\begin{itemize}
|
|
\item The field elements $x_P$ and $y_P \typecolon \GF{q}$ 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}
|
|
|
|
For a point $P \typecolon \GroupG{2} = (x_P, y_P)$:
|
|
\begin{itemize}
|
|
\item A field element $w \typecolon \GF{q^2}$ is represented as
|
|
a polynomial $a_{w,1} t + a_{w,0} \typecolon \GF{q}[t]$ modulo $t^2 + 1$.
|
|
Define $\FEtoIP \typecolon \GF{q^2} \rightarrow \range{0}{q^2\!-\!1}$ such that
|
|
$\FEtoIP(w) = a_{w,1} q + a_{w,0}$.
|
|
\item Let $x = \FEtoIP(x_P)$, $y = \FEtoIP(y_P)$, and $y' = \FEtoIP(-y_P)$.
|
|
\item Let $\tilde{y} = \begin{cases} 1, &\text{if } y > y' \\0, &\text{otherwise.} \end{cases}$
|
|
\item $P$ is encoded as $\Justthebox{\gtwobox}$.
|
|
\end{itemize}
|
|
|
|
\subparagraph{Non-normative notes:}
|
|
\begin{itemize}
|
|
\item The use of big-endian byte order is different from the encoding
|
|
of most other integers in this protocol. The above encodings 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 on $\GroupG{1}$, and the
|
|
SORT compressed form (i.e.\ $\ECtoOSPXS$) for points on $\GroupG{2}$.
|
|
\item Testing $y > y'$ for the compression of $\GroupG{2}$ 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 $\GroupG{1}$, and
|
|
\cite[Appendix A.12.11]{IEEE2004} for $\GroupG{2}$.
|
|
\end{itemize}
|
|
|
|
When computing square roots in $\GF{q}$ or $\GF{q^2}$ 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.
|
|
|
|
\nsubsubsection{Encoding of \ZeroKnowledgeProofs} \label{proofencoding}
|
|
|
|
\newsavebox{\proofbox}
|
|
\begin{lrbox}{\proofbox}
|
|
\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 proof is encoded by concatenating the encodings of its elements:
|
|
|
|
\vspace{1.5ex}
|
|
\hskip 0.2em $\Justthebox{\proofbox}$
|
|
\vspace{1ex}
|
|
|
|
The resulting proof size is 296 bytes.
|
|
|
|
\vspace{0.8ex}
|
|
|
|
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 $\range{0}{q\!-\!1}$ or (in the case of $\Proof_B$) $\range{0}{q^2\!-\!1}$;
|
|
\item the encoding represents a point on the relevant curve.
|
|
\end{itemize}
|
|
|
|
\nsubsection{\JoinSplitParameters} \label{jsparameters}
|
|
|
|
For the testnet in release v0.11.2.z9 and later, the $\FullHashName$ hashes of the
|
|
\provingKey and \verifyingKey for the \joinSplitStatement, encoded in \libsnark
|
|
format, are:
|
|
|
|
\begin{verbatim}
|
|
226913bbdc48b70834f8e044d194ddb61c8e15329f67cdc6014f4e5ac11a82ab z9-proving.key
|
|
4c151c562fce2cdee55ac0a0f8bd9454eb69e6a0db9a8443b58b770ec29b37f5 z9-verifying.key
|
|
\end{verbatim}
|
|
|
|
The \Zcash production \blockchain will use parameters obtained by a multi-party
|
|
computation, which has yet to be performed.
|
|
|
|
|
|
\nsection{Consensus Changes from \Bitcoin}
|
|
|
|
\nsubsection{Encoding of \Transactions} \label{txnencoding}
|
|
|
|
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{uint32\_t} & Transaction version number; either 1 or 2. \\ \hline
|
|
|
|
\Varies & $\txInCount$ & \type{compactSize uint} & Number of \transparent inputs in this transaction. \\ \hline
|
|
|
|
\Varies & $\txIn$ & $\txIn$ & Transparent inputs, encoded as in \Bitcoin. \\ \hline
|
|
|
|
\Varies & $\txOutCount$ & \type{compactSize uint} & Number of \transparent outputs in this transaction. \\ \hline
|
|
|
|
\Varies & $\txOut$ & $\txOut$ & Transparent outputs, encoded as in \Bitcoin. \\ \hline
|
|
|
|
4 & $\lockTime$ & \type{uint32\_t} & A Unix epoch time or block number, encoded as in \Bitcoin. \\ \hline
|
|
|
|
\Varies\;$\dagger$ & $\nJoinSplit$ & \type{compactSize uint} & The number of \joinSplitDescriptions
|
|
in $\vJoinSplit$. \\ \hline
|
|
|
|
\Longunderstack{1802 $\times$ \\ $\nJoinSplit\,\dagger$} & $\vJoinSplit$ & \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}.
|
|
|
|
The changes relative to \Bitcoin version 1 transactions as described in \cite{Bitcoin-Format} are:
|
|
\begin{itemize}
|
|
\item The \transactionVersionNumber{} can be either 1 or 2. A version 1 \transaction is
|
|
equivalent to a version 2 \transaction with $\nJoinSplit = 0$. Software that parses
|
|
\blocks{} \MUSTNOT assume, when an encoded \block starts with an $\versionField$
|
|
field representing a value other than 1 or 2 (e.g.\ future versions potentially
|
|
introduced by hard forks), that it will be parseable according to this format.
|
|
\item The $\nJoinSplit$, $\vJoinSplit$, $\joinSplitPubKey$, and $\joinSplitSig$ fields
|
|
have been added.
|
|
\end{itemize}
|
|
|
|
Software that creates \transactions{} \SHOULD use version 1 for \transactions with no
|
|
\joinSplitDescriptions.
|
|
|
|
\pnote{
|
|
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 support BIP 68, or the related BIPs
|
|
9, 112 and 113.
|
|
}
|
|
|
|
\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{int64\_t} &\mbox{}\setchanged
|
|
A value $\vpubOld$ that the \joinSplitTransfer removes from the \transparentValuePool. \\ \hline
|
|
|
|
8 & $\vpubNewField$ & \type{int64\_t} & A value $\vpubNew$ that the \joinSplitTransfer inserts
|
|
into the \transparentValuePool. \\ \hline
|
|
|
|
32 & $\anchorField$ & \type{char[32]} & A merkle root $\rt$ of the \noteCommitmentTree at
|
|
some block height 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 $\JoinSplitProof$
|
|
(see \crossref{proofencoding}). \\ \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 $\ephemeralKey$ and $\encCiphertexts$ fields together form the \notesCiphertext.
|
|
|
|
|
|
\nsubsection{\BlockHeaders}
|
|
|
|
The \Zcash \blockHeader format is as follows:
|
|
|
|
\begin{center}
|
|
\hbadness=1000
|
|
\begin{tabularx}{0.92\textwidth}{|c|l|p{10.7em}|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 & $\hashReserved$ & \type{char[32]} & A reserved field which should be ignored. \\ \hline
|
|
|
|
4 & $\nTime$ & \type{uint32\_t} & The \blockTime is a Unix epoch time when the miner
|
|
started hashing the header (according to the miner). This \MUST be greater than or equal
|
|
to the median time of the previous 11 blocks. A \fullnode{} \MUSTNOT accept \blocks with
|
|
headers more than two hours in the future according to its clock. \\ \hline
|
|
|
|
4 & $\nBits$ & \type{uint32\_t} & An encoded version of the target threshold this \block's
|
|
header hash must be less than or equal to, in the same nBits format used by \Bitcoin.
|
|
\cite{Bitcoin-nBits} \\ \hline
|
|
|
|
32 & $\nNonce$ & \type{char[32]} & An arbitrary field miners change to modify the
|
|
header hash in order to produce a hash below the target threshold. \\ \hline
|
|
|
|
1344 & $\nSolution$ & \type{char[1344]} & The Equihash solution, which \MUST be valid
|
|
according to \crossref{equihash}. \\ \hline
|
|
|
|
\end{tabularx}
|
|
\end{center}
|
|
|
|
The changes relative to \Bitcoin version 4 blocks as described in \cite{Bitcoin-Block} are:
|
|
\begin{itemize}
|
|
\item The \blockVersionNumber{} \MUST be 4. Previous versions are not supported. Software
|
|
that parses blocks \MUSTNOT assume, when an encoded \block starts with an $\nVersion$
|
|
field representing a value other than 4 (e.g.\ future versions potentially introduced
|
|
by hard forks), that it will be parseable according to this format.
|
|
\item The $\hashReserved$ and $\nSolution$ fields have been added.
|
|
\item The type of the $\nNonce$ field has changed from \type{uint32\_t} to \type{char[32]}.
|
|
\end{itemize}
|
|
|
|
\pnote{
|
|
There is no relation between the values of the $\versionField$ field of a \transaction, and
|
|
the $\nVersion$ field of a \blockHeader.
|
|
}
|
|
|
|
\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}.
|
|
|
|
A \block satisfies the Proof of Work if and only if:
|
|
\begin{itemize}
|
|
\item The $\nSolution$ field encodes a \validEquihashSolution according to \crossref{equihash}.
|
|
\item The \blockHeader satisfies the difficulty check according to \crossref{difficulty}.
|
|
\end{itemize}
|
|
|
|
|
|
\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
|
|
$X_{1..\mathrm{N}}$ of $n$-bit strings, find $2^k$ distinct $X_{i_j}$ such that
|
|
$\vxor{j=1}{2^k} X_{i_j} = 0$.
|
|
|
|
In Equihash, $\mathrm{N} = 2^{\frac{n}{k+1}+1}$, and the sequence $X_{1..\mathrm{N}}$ 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 $\nTime$}
|
|
\bitbox{128}{32-bit $\nBits$} \\
|
|
\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 $\ItoBSP{} \typecolon (u \typecolon \Nat) \times \range{0}{2^u\!-\!1} \rightarrow \bitseq{u}$
|
|
such that $\ItoBSP{u}(x)$ is the sequence of $u$ bits representing $x$ in
|
|
big-endian order.
|
|
|
|
Define $\BStoIP{} \typecolon (u \typecolon \Nat) \times \bitseq{u} \rightarrow \range{0}{2^u\!-\!1}$
|
|
such that $\BStoIP{u}$ is the inverse of $\ItoBSP{u}$.
|
|
|
|
Define $\Xi_r(a, b) := \BStoIP{2^{r-1} \ell}(\concatbits(X_{i_{a..b}}))$.
|
|
|
|
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}
|
|
|
|
For all $r \in \range{1}{k\!-\!1}$, for all $w \in \range{0}{2^{k-r}\!-\!1}$:
|
|
\begin{itemize}
|
|
\item $\vxor{j=1}{2^r} X_{i_{w 2^r + j}}$ has $\frac{nr}{k+1}$ leading zeroes; and
|
|
\item $\Xi_r(w 2^r + 1, w 2^r + 2^{r-1}) < \Xi_r(w 2^r + 2^{r-1} + 1, w 2^r + 2^r)$.
|
|
\end{itemize}
|
|
|
|
\pnote{
|
|
This does not include a difficulty condition, because here we are defining validity
|
|
of an Equihash solution independent of difficulty.
|
|
}
|
|
|
|
An Equihash solution with $n = 200$ and $k = 9$ is encoded in the $\nSolution$
|
|
field of a \blockHeader as follows:
|
|
|
|
\newsavebox{\solutionbox}
|
|
\begin{lrbox}{\solutionbox}
|
|
\begin{bytefield}[bitwidth=0.45em]{105}
|
|
\bitbox{21}{$\ItoBSP{21}(i_1-1)$}
|
|
\bitbox{21}{$\ItoBSP{21}(i_2-1)$}
|
|
\bitbox{42}{$\cdots$}
|
|
\bitbox{21}{$\ItoBSP{21}(i_{512}-1)$}
|
|
\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}{$\ItoBSP{21}(68)$}
|
|
\bitbox{21}{$\ItoBSP{21}(41)$}
|
|
\bitbox{21}{$\ItoBSP{21}(2^{21}-1)$} \\
|
|
\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}
|
|
|
|
\hskip 1.5em $\Justthebox{\solutionbox}$
|
|
|
|
\vspace{1ex}
|
|
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:
|
|
|
|
\hskip 1.5em $\Justthebox{\eqexamplebox}$
|
|
|
|
and so the first 7 bytes of $\nSolution$ would be
|
|
$[0, 2, 32, 0, 10, 127, 255]$.
|
|
|
|
\begin{pnotes}
|
|
\item $\ItoBSP{}$ and $\BStoIP{}$ are big-endian, while the encoding of
|
|
integer fields in $\powheader$ and in the instantiation of $\EquihashGen{}$
|
|
is 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). The comparison of $\Xi_r$
|
|
values obtained by a big-endian conversion is equivalent to lexicographic
|
|
comparison as specified in \cite[section IV A]{BK2016}.
|
|
\item When $\EquihashGen{}$ is used to construct the input list, the index
|
|
$i$ runs sequentially from $1$ to $N$, allowing the number of calls
|
|
to $\BlakeGeneric$ used in the instantiation of $\EquihashGen{}$ to
|
|
be reduced by a factor of $\floor{\frac{512}{n}}$ (which is a factor
|
|
of 2 for $n = 200$).
|
|
\end{pnotes}
|
|
|
|
\nsubsubsection{Difficulty filter} \label{difficulty}
|
|
|
|
The difficulty filter is unchanged from \Bitcoin, and is calculated using
|
|
\SHAd on the whole \blockHeader (including $\nSolution$).
|
|
|
|
\nsubsubsection{Difficulty adjustment} \label{diffadjustment}
|
|
|
|
\Zcash uses a difficulty adjustment algorithm based on DigiShield v3/v4,
|
|
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.
|
|
|
|
\todo{Describe the algorithm.}
|
|
|
|
|
|
\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 \xprotected
|
|
value in a single \Zcash \transaction, e.g.\ to spend a \protectedNote
|
|
that has just been created. (In \Zcash, we refer to value stored in
|
|
UTXOs as \transparent, and value stored in \joinSplitTransfer output
|
|
\notes as \xprotected.)
|
|
This was not possible in the \Zerocash design without using multiple
|
|
transactions. It also allows \transparent and \xprotected 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}.
|
|
|
|
|
|
\nsubsection{Unification of Mints and Pours}
|
|
|
|
In the original \Zerocash protocol, there were two kinds of transaction
|
|
relating to \protectedNotes:
|
|
\begin{itemize}
|
|
\item a ``Mint'' transaction takes value from \transparent UTXOs as
|
|
input and produces a new \protectedNote as output.
|
|
\item a ``Pour'' transaction takes up to $\NOld$ \protectedNotes
|
|
as input, and produces up to $\NNew$ \protectedNotes and a
|
|
\transparent UTXO as output.
|
|
\end{itemize}
|
|
|
|
Only ``Pour'' transactions included a \zkSNARK proof.
|
|
|
|
In \Zcash, the sequence of operations added to a \transaction
|
|
(described in \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.
|
|
|
|
|
|
\nsubsection{Faerie Gold attack and fix} \label{faeriegold}
|
|
|
|
When a \protectedNote 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}.
|
|
|
|
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.
|
|
|
|
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 valid \blockchainview
|
|
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 $\Blake{256}$
|
|
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.)
|
|
|
|
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 the \joinSplitStatement
|
|
(see \crossref{uniquerho}).
|
|
|
|
Now even if the creator of a \joinSplitDescription does not choose
|
|
$\NoteAddressPreRand$ randomly, uniqueness of \nullifiers and
|
|
collision resistance of both $\Blake{256}$ and $\PRFrho{}$ will ensure
|
|
that the derived $\NoteAddressRand$ values are unique, at least for
|
|
any two \joinSplitDescriptions that get into a valid \blockchainview.
|
|
This is sufficient to prevent the Faerie Gold attack.
|
|
|
|
|
|
\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
|
|
values of $\NoteAddressRand$ 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 $\FullHashName$ 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 a generalized \joinSplitTransfer which always uses a
|
|
\zeroKnowledgeProof, it does not require the nesting. A side benefit is
|
|
that this reduces the number of $\SHA$ evaluations needed to compute
|
|
each \noteCommitment from three to two, saving a total of four $\SHA$
|
|
evaluations in the \joinSplitStatement.
|
|
|
|
\pnote{
|
|
\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.
|
|
}
|
|
|
|
\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}.
|
|
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.
|
|
|
|
For resistance to Faerie Gold attacks as described in
|
|
\crossref{faeriegold}, \Zcash depends on collision resistance of both
|
|
$\GeneralCRH{256}$ and $\PRFrho{}$. 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 instantiating $\GeneralCRH{\ell}$
|
|
as $\Blake{\ell}$ in \Zcash, and avoiding truncation along any path from the
|
|
inputs to the computation of $\hSig$ to the uses of $\NoteAddressRand$.
|
|
|
|
Since the PRFs are instantiated using $\SHA$ 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.
|
|
|
|
|
|
\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 scheme based on Curve25519 key agreement, 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}.
|
|
|
|
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}.
|
|
\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 key agreement is defined in a way that avoids these concerns
|
|
due to the curve structure and the ``clamping'' of private keys.
|
|
\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 because
|
|
$\TransmitPublic$ is included in the KDF input, being able to break the
|
|
Elliptic Curve Diffie-Hellman Problem on Curve25519 (without breaking
|
|
$\SymSpecific$ as an authenticated encryption scheme or $\Blake{256}$ as
|
|
a KDF) would not help to decrypt the \notesCiphertext unless
|
|
$\TransmitPublic$ is known or guessed.
|
|
\item 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 $\Blake{256}$ is a
|
|
``weak PRF''.
|
|
Note that $\hSig$ is authenticated, by the ZK proof, 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.
|
|
\item The scheme used by \Zcash includes an optimization that uses 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 hash function $H$, 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. 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.
|
|
|
|
|
|
\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
|
|
$\AuthPrivateX{1}$ and $\AuthPrivateX{2}$ 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.
|
|
|
|
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 $\cmOldX{1} = \cmOldX{2}$, then the fact that
|
|
$\snOldX{1} \neq \snOldX{2}$ implies that the witness $a$ contains
|
|
two distinct openings of $\cmOldX{1}$ (the first opening contains
|
|
$(\AuthPrivateOldX{1}, \NoteAddressRandOldX{1})$, while the second
|
|
opening contains $(\AuthPrivateOldX{2}, \NoteAddressRandOldX{2})$).
|
|
This violates the binding property of the commitment scheme $\Commit{}$."
|
|
\end{itemize}
|
|
|
|
In fact the openings do not contain $\AuthPrivateOld{i}$; they contain
|
|
$\AuthPublicOld{i}$.
|
|
|
|
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 \Zcash, which \emph{are} collision-resistant assuming
|
|
that $\SHA$ 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{spendauthority}),
|
|
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 it 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 $\Commit{}$ in \Zcash. $\TransmitPublic$ is also
|
|
not needed as part of a \note: it is not an input to $\Commit{}$ nor
|
|
is it constrained by the \Zerocash \POUR{} \statement or the
|
|
\Zcash \joinSplitStatement. $\cm$ can be computed from the other fields.
|
|
\item The length of proof encodings given in the paper is 288 bytes.
|
|
This differs from the 296 bytes specified in \crossref{proofencoding},
|
|
because the paper did not take into account the need to encode compressed
|
|
$y$-coordinates. The fork of \libsnark used by \Zcash uses a different
|
|
format to upstream \libsnark, in order to follow \cite{IEEE2004}.
|
|
\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, and no doubt others.
|
|
|
|
The Faerie Gold attack was found by Zooko Wilcox.
|
|
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.
|
|
|
|
|
|
\nsection{Change history}
|
|
|
|
\subparagraph{2016.0-beta-1.2}
|
|
|
|
\begin{itemize}
|
|
\item Allow up to 66 ignored but retained bytes after the raw encoding
|
|
of a payment address, to support extensibility.
|
|
\end{itemize}
|
|
|
|
\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}
|
|
|
|
\subparagraph{2016.0-beta-1}
|
|
|
|
\begin{itemize}
|
|
\item Major reorganisation 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}
|
|
|
|
\subparagraph{2016.0-alpha-3.1}
|
|
|
|
\begin{itemize}
|
|
\item Change main font to Quattrocento.
|
|
\end{itemize}
|
|
|
|
\subparagraph{2016.0-alpha-3}
|
|
|
|
\begin{itemize}
|
|
\item Change version numbering convention (no other changes).
|
|
\end{itemize}
|
|
|
|
\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}
|
|
|
|
\subparagraph{2.0-alpha-2}
|
|
|
|
\begin{itemize}
|
|
\item Change from truncated $\Blake{512}$ to $\Blake{256}$.
|
|
\item Clarify endianness, and that uses of $\BlakeGeneric$ 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{merklepathvalidity} about how this condition
|
|
corresponds to conditions in the \Zerocash paper.
|
|
\item Changes to terminology around keys.
|
|
\end{itemize}
|
|
|
|
\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
|
|
|
|
\end{document}
|