Add reserved field in commitment.

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

protocol/protocol.tex

@@ -174,7 +174,6 @@
 \newcommand{\SpendingKey}{\mathsf{addr_{sk}}}
 \newcommand{\PaymentAddressLeadByte}{\hexint{92}}
 \newcommand{\SpendingKeyLeadByte}{\hexint{??}}
-\newcommand{\NotePlaintextLeadByte}{\hexint{00}}
 \newcommand{\AuthPublic}{\mathsf{a_{pk}}}
 \newcommand{\AuthPrivate}{\mathsf{a_{sk}}}
 \newcommand{\AuthPublicOld}[1]{\mathsf{a^{old}_{pk,\mathnormal{#1}}}}
@@ -629,7 +628,9 @@ \subsection{\Notes}
 
 $\NoteCommitRand$ is randomly generated by the sender. \changed{$\NoteAddressRand$
 is generated from a random seed $\NoteAddressPreRand$ using
-$\PRFrho{\NoteAddressPreRand}$.} Only a commitment to these values is disclosed
+$\PRFrho{\NoteAddressPreRand}$.}
+
+Only a commitment to the contents of the \note is disclosed
 publicly, which allows the tokens $\NoteCommitRand$ and $\NoteAddressRand$ to blind
 the value and recipient \emph{except} to those who possess these tokens.
 
@@ -642,27 +643,29 @@ \subsubsection{\NoteCommitments} \label{comm}
 \newsavebox{\cmbox}
 \begin{lrbox}{\cmbox}
 \setchanged
-\begin{bytefield}[bitwidth=0.036em]{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} &
+\begin{bytefield}[bitwidth=0.03em]{952}
+    \bitbox{28}{1} &
+    \bitbox{28}{0} &
+    \bitbox{28}{1} &
+    \bitbox{28}{1} &
+    \bitbox{28}{0} &
+    \bitbox{28}{0} &
+    \bitbox{28}{0} &
+    \bitbox{28}{0} &
+    \bitbox{112}{$\zeros{112}$} &
     \bitbox{256}{256 bit $\AuthPublic$} &
     \bitbox{128}{64 bit $\Value$} &
-    \bitbox{256}{256 bit $\NoteAddressRand$}
-    \bitbox{256}{256 bit $\NoteCommitRand$} &
+    \bitbox{256}{256 bit $\NoteAddressRand$} &
+    \bitbox{256}{256 bit $\NoteCommitRand$}
 \end{bytefield}
 \end{lrbox}
 
 \changed{
 \hskip 1em $\cm := \FullHashbox{\cmbox}$
 
 \subparagraph{Note:}
-The leading byte of the $\FullHash$ input is $\hexint{B0}$.
+The leading bytes of the $\FullHash$ input are $\hexint{B0}$, followed by 14 zero bytes.
+The zero bytes are reserved for use in a future version of the \Zcash protocol.
 }
 
 \subsubsection{\Nullifiers}
@@ -709,20 +712,20 @@ \subsubsection{\NotePlaintexts{} and \Memos} \label{notept}
 
 The encoding of a \notePlaintext consists of, in order:
 \begin{equation*}
-\begin{bytefield}[bitwidth=0.029em]{1608}
+\begin{bytefield}[bitwidth=0.03em]{1608}
 \changed{
-    \bitbox{192}{8 bit $\NotePlaintextLeadByte$}
+    \bitbox{256}{$\zeros{112}$ (14 bytes)}
   &}\bitbox{192}{$\Value$ (8 bytes)} &
-    \bitbox{256}{$\NoteAddressRand$ (32 bytes)} &
+    \bitbox{256}{$\NoteAddressRand$ (32 bytes)}
     \bitbox{256}{$\NoteCommitRand$ (\changed{32} bytes)} &
-    \changed{\bitbox{800}{$\Memo$ (128 bytes)}}
+    \changed{\bitbox{632}{$\Memo$ (128 bytes)}}
 \end{bytefield}
 \end{equation*}
 
 \begin{itemize}
 \changed{
-    \item A byte, $\NotePlaintextLeadByte$, indicating this version of the
-        encoding of a \notePlaintext.
+    \item 14 zero bytes, which are reserved for use in a future version of the
+\Zcash protocol.
 }
     \item 8 bytes specifying $\Value$.
     \item 32 bytes specifying $\NoteAddressRand$.
@@ -879,7 +882,7 @@ \section{\JoinSplitTransfers{} and Descriptions} \label{pourdesc}
 \Varies & $\nJoinSplit$ & \type{compactSize uint} & The number of \joinSplitDescriptions
 in $\vJoinSplit$. \\ \hline
 
-$1026 \times \nJoinSplit$ & $\vJoinSplit$ &
+$1052 \times \nJoinSplit$ & $\vJoinSplit$ &
 \type{JoinSplitDescription} \type{[$\nJoinSplit$]} &
 The \sequenceOfJoinSplitDescriptions in this \transaction. \\ \hline
 
@@ -926,7 +929,7 @@ \section{\JoinSplitTransfers{} and Descriptions} \label{pourdesc}
 \setchanged 32 &\setchanged $\ephemeralKey$ &\setchanged \type{char[32]} &\mbox{}\setchanged
 A Curve25519 public key $\EphemeralPublic$. \\ \hline
 
-434 & $\encCiphertexts$ & \type{char[217][$\NNew$]} & A sequence of ciphertext
+460 & $\encCiphertexts$ & \type{char[230][$\NNew$]} & A sequence of ciphertext
 components for the encrypted output \notes, $\TransmitCiphertext{\allNew}$. \\ \hline
 
 \setchanged 32 &\setchanged $\randomSeed$ &\setchanged \type{char[32]} &\mbox{}\setchanged