Fix crossrefs.

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

protocol/protocol.tex

@@ -1665,7 +1665,7 @@ for a given key. The security notions INT-CTXT and IND-CPA are as defined in
 \cite{BN2007}.
 }
 
-\nsubsubsection{\KeyAgreement} \label{abstractkeyagreement}
+\nsubsubsection{\KeyAgreement} \label{concretekeyagreement}
 
 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.
@@ -2591,7 +2591,7 @@ The motivations for this change were as follows:
         encrypted in each \joinSplitDescription.
 \end{itemize}
 
-\nsubsection{Omission in \Zerocash security proof}
+\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
@@ -2729,7 +2729,7 @@ of $\PRFaddr{}$ was found by Daira Hopwood.
     \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. \cite{ticket836}
+    \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.