Switch from blake2b to SHA-256 for nonce computations.

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

protocol/protocol.tex

@@ -108,7 +108,7 @@
 \newcommand{\TransmitDecrypt}[1]{\mathsf{Decrypt}_{#1}}
 \newcommand{\CRH}{\mathsf{CRH}}
 \newcommand{\CRHbox}[1]{\CRH\left(\;\raisebox{-1.3ex}{\usebox{#1}}\;\right)}
-\newcommand{\CryptoBoxSealHash}{\mathsf{blake2b}}
+\newcommand{\CryptoBoxSealHash}{\mathtt{SHA256}}
 \newcommand{\CryptoBoxSealHashbox}[1]{\CryptoBoxSealHash\left(\;\raisebox{-1.3ex}{\usebox{#1}}\;\right)}
 \newcommand{\PRF}[2]{\mathsf{{PRF}^{#2}_\mathnormal{#1}}}
 \newcommand{\PRFaddr}[1]{\PRF{#1}{addr}}
@@ -216,6 +216,7 @@ the leading (initial) $k$ bits of its input.
 $\CRH$ is a collision-resistant hash function. In \Zcash, the $\SHAName$ function
 is used which takes a 512-bit block and produces a 256-bit hash. This is
 different from the $\SHAOrig$ function, which hashes arbitrary-length strings.
+\cite{sha256}
 
 $\PRF{x}{}$ is a pseudo-random function seeded by $x$. Three \emph{independent}
 $\PRF{x}{}$ are needed in our scheme: $\PRFaddr{x}$, $\PRFsn{x}$, and $\PRFpk{x}$.
@@ -369,10 +370,11 @@ ciphertext component as follows:
 Any ciphertext components that fail to decrypt with a given recipient's private key
 will be ignored.
 
-(This is a variation on the $\CryptoBoxSeal$ algorithm defined in libsodium
+This is a variation on the $\CryptoBoxSeal$ algorithm defined in libsodium
 \cite{cryptoboxseal}, but with a single ephemeral key used for all encryptions in a
 given \PourDescription, and with the nonce for each ciphertext component depending
-on the index $i$.)
+on the index $i$. Also, $\CryptoBoxSealHash$ (the full hash, not the compression
+function) is used instead of $\mathsf{blake2b}$.
 
 \subparagraph{Coin Commitments}
 

protocol/zcash.bib

@@ -39,3 +39,12 @@
    howpublished={\url{https://download.libsodium.org/doc/public-key_cryptography/sealed_boxes.html}},
    note={Accessed: 2016-02-01}
 }
+
+@misc{sha256,
+   author={NIST},
+   title={{FIPS} 180-4: Secure {H}ash {S}tandard ({SHS})},
+   month={August},
+   year={2015},
+   note={DOI: 10.6028/NIST.FIPS.180-4},
+   howpublished={\url{http://csrc.nist.gov/publications/PubsFIPS.html#180-4}}
+}