In \crossref{internalh}, add a security argument for why the SHA-256-based commitment scheme

NoteCommit^Sprout is binding and hiding, under reasonable assumptions about SHA256Compress.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -237,6 +237,7 @@
   \def\tempstring{#1}%
   \xStrSubstitute{\tempstring}{MAEA2010}{MÁEÁ2010}[\tempstring]%
   \xStrSubstitute{\tempstring}{Hisil2010}{Hı\cedilla{s}ıl2010}[\tempstring]%
+  \xStrSubstitute{\tempstring}{Damgard1989}{Damgård1989}[\tempstring]%
   \tempstring
   \restoreexpandmode
 }
@@ -1543,6 +1544,8 @@ electronic commerce and payment, financial privacy, proof of work, zero knowledg
 \newcommand{\NoteCommitGenTrapdoor}[1]{\NoteCommitAlg{#1}\mathsf{.GenTrapdoor}}
 \newcommand{\NoteCommitInput}[1]{\NoteCommitAlg{#1}\mathsf{.Input}}
 \newcommand{\NoteCommitOutput}[1]{\NoteCommitAlg{#1}\mathsf{.Output}}
+\newcommand{\CommitPrimeAlg}{\mathsf{COMM}'}
+\newcommand{\CommitPrime}[1]{\CommitPrimeAlg_{#1}}
 \newcommand{\ValueCommitAlg}[1]{\mathsf{ValueCommit}^\mathsf{#1\kern-0.1em}}
 \newcommand{\ValueCommit}[2]{\ValueCommitAlg{#1}_{#2}}
 \newcommand{\ValueCommitTrapdoor}[1]{\ValueCommitAlg{#1}\mathsf{.Trapdoor}}
@@ -14096,6 +14099,26 @@ A side benefit is that this reduces the cost of computing the
 evaluations needed to compute each \noteCommitment from three to two,
 saving a total of four \shaCompress evaluations in the \joinSplitStatement.
 
+\sproutspecificpnote{
+The full \shaHash algorithm is used for $\NoteCommitAlg{Sprout}$, with randomness
+appended after the commitment input. The commitment input can be split into two
+blocks, call them $x$ of length $64$ bytes, and $y$ of the remaining length ($9$ bytes).
+Let $\CommitPrime{r}(z \typecolon \byteseq{41})$ be the \commitmentScheme that applies
+$\SHACompress$ with the first $32$ bytes of $z$ in the IV, and the rest of $z$
+($9$ bytes), the randomness $r$ ($32$ bytes), and padding up to $64$ bytes in the
+$\SHACompress$ input block. Then we have
+$\NoteCommit{Sprout}{r}(x \bconcat y) = \CommitPrime{r}(\SHACompress(x) \bconcat y)$.
+Suppose we make the reasonable assumption that $\CommitPrimeAlg$ is a computationally
+\binding and \hiding \commitmentScheme. If $\SHACompress$ is \collisionResistant with
+the standard IV\footnote{If $\SHACompress$ is not \collisionResistant with the
+standard IV, then \shaHash is not \collisionResistant for a $2$-block input.}, then
+$\NoteCommitAlg{Sprout}$ is as secure for \binding as $\CommitPrimeAlg$. Also
+$\NoteCommitAlg{Sprout}$ is as secure for \hiding as $\CommitPrimeAlg$ (without
+any assumption on $\SHACompress$). This effectively rules out potential concerns
+about the Merkle--Damgård structure \cite{Damgard1989} of \shaHash causing any
+security problem for $\NoteCommitAlg{Sprout}$.
+} %sproutspecificpnote
+
 \sproutspecificpnote{
 \Sprout \noteCommitments are not statistically \hiding, so for \Sprout notes,
 \Zcash does not support the ``everlasting anonymity'' property described in
@@ -14524,6 +14547,9 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
 \begin{itemize}
     \item In \crossref{joinsplit}, clarify that balance for \joinSplitTransfers is enforced
           by the \joinSplitStatement, and that there is no consensus rule to check it directly.
+    \item In \crossref{internalh}, add a security argument for why the \shaHash-based
+          \commitmentScheme $\NoteCommitAlg{Sprout}$ is \binding and \hiding, under reasonable
+          assumptions about $\SHACompress$.
 \end{itemize}
 
 

protocol/zcash.bib

@@ -473,6 +473,26 @@ Received March~20, 2012.}
    urldate={2021-03-08}
 }
 
+@inproceedings{Damgard1989,
+   presort={Damgard1989},
+   shorthand={Damgård1989},
+   author={Ivan Damgård},
+   title={A Design Principle for Hash Functions},
+   date={1990}, % publication year
+   booktitle={Advances in Cryptology - CRYPTO~'89.
+Proceedings of the 9th Annual International Cryptology Conference
+(Santa Barbara, California, USA, August~20--24, 1989)},
+   volume={435},
+   series={Lecture Notes in Computer Science},
+   editor={Giles Brassard},
+   pages={416--427},
+   publisher={Springer},
+   isbn={978-0-387-34805-6},
+   doi={10.1007/0-387-34805-0_39},
+   url={https://link.springer.com/chapter/10.1007/0-387-34805-0_39},
+   urldate={2022-01-19}
+}
+
 @misc{NIST2016,
    presort={NIST2016},
    author={NIST},