Specify precise encoding of ECDSA public key. Also improve description

of how the public key is bound to the Pour statement.

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

protocol/protocol.tex

@@ -805,10 +805,11 @@ If $\ECDSAs$ is not in the given range, the signature is treated as invalid.
 
 \newsavebox{\pubkeybox}
 \begin{lrbox}{\pubkeybox}
-\begin{bytefield}[bitwidth=0.075em]{520}
-    \bitbox{80}{8 bit $\hexint{04}$}
-    \bitbox{256}{256 bit xxx}
-    \bitbox{256}{256 bit xxx}
+\begin{bytefield}[bitwidth=0.075em]{264}
+    \bitbox{64}{$\zeros{6}$}
+    \bitbox{18}{1}
+    \bitbox{56}{1 bit $\tilde{y}_P$}
+    \bitbox{256}{256 bit $x_P$}
 \end{bytefield}
 \end{lrbox}
 
@@ -820,13 +821,19 @@ The encoding of a signature is:
 
 where $\ECDSAr$ and $\ECDSAs$ are as defined in \cite{sec2-ecdsa}.
 
-The encoding of a public key is:
+The encoding of a public key is as defined in section E.2.3.2 of \cite{std1363}
+for a compressed elliptic curve point with $x$-coordinate $x_P$ and compressed
+$y$-coordinate $\tilde{y}_P$:
 
 \begin{itemize}
   \item[] $\Justthebox{\pubkeybox}{-1.3ex}$
 \end{itemize}
 
-The $\pourPubKey$ is cryptographically bound to $\PourProof$.
+Note that only compressed public keys are valid.
+
+The condition \crossref{nonmalleablepour} in the \zkSNARK statement ensures
+that a holder of all of $\AuthPrivateOld{\allOld}$ has authorized the use of
+the private key corresponding to $\pourPubKey$ to sign this transaction.
 
 
 \subsection{Balance}
@@ -903,7 +910,7 @@ $\snOld{i} = \PRFsn{\AuthPrivateOld{i}}(\CoinAddressRandOld{i})$.
 for each $i \in \setofOld$:
 $\AuthPublicOld{i} = \changed{\PRFaddr{\AuthPrivateOld{i}}(0)}$.
 
-\subparagraph{Non-malleability}
+\subparagraph{Non-malleability} \label{nonmalleablepour}
 
 for each $i \in \setofOld$:
 $\h{i} = \PRFpk{\AuthPrivateOld{i}}(i, \hSig)$.

protocol/zcash.bib

@@ -71,6 +71,15 @@
    note={\mbox{Accessed: 2016-03-14}}
 }
 
+@book{std1363,
+   author={IEEE Computer Society},
+   publisher={Institute of {E}lectrical and {E}lectronic {E}ngineers},
+   year={2000},
+   title={IEEE {S}td 1363-2000: {IEEE} {S}tandard {S}pecifications for {P}ublic-{K}ey {C}ryptography},
+   note={\url{http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=891000&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F7168%2F19282%2F00891000}.
+Accessed 2016-03-15.}
+}
+
 @misc{libsnark,
    key={Libsnark},
    title={libsnark: a C++ library for zkSNARK proofs},