Tweaks to Equihash section; add rationale for mixed endianness.

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

protocol/protocol.tex

@@ -150,6 +150,7 @@
 \newcommand{\block}{\term{block}}
 \newcommand{\blocks}{\term{blocks}}
 \newcommand{\blockHeader}{\term{block header}}
+\newcommand{\blockHeaders}{\term{block headers}}
 \newcommand{\BlockHeaders}{\titleterm{Block Headers}}
 \newcommand{\blockVersionNumber}{\term{block version number}}
 \newcommand{\blockTime}{\term{block time}}
@@ -2271,7 +2272,6 @@ field of a \blockHeader as follows:
 
 \newsavebox{\solutionbox}
 \begin{lrbox}{\solutionbox}
-\setchanged
 \begin{bytefield}[bitwidth=0.45em]{105}
     \bitbox{21}{$\ItoBSP{21}(i_1-1)$}
     \bitbox{21}{$\ItoBSP{21}(i_2-1)$}
@@ -2284,7 +2284,6 @@ field of a \blockHeader as follows:
 \newcommand{\ob}{\bitbox{1}{$1$}}
 \newsavebox{\eqexamplebox}
 \begin{lrbox}{\eqexamplebox}
-\setchanged
 \begin{bytefield}[bitwidth=0.75em]{63}
     \bitbox{21}{$\ItoBSP{21}(68)$}
     \bitbox{21}{$\ItoBSP{21}(41)$}
@@ -2316,7 +2315,14 @@ then the corresponding bit array is:
 and so the first 7 bytes of $\nSolution$ would be
 $[0, 2, 32, 0, 10, 127, 255]$.
 
-\todo{Apologize for the mixing of ends.}
+Note that $\ItoBSP{}$ and $\BStoIP{}$ are big-endian, while the encoding of
+integer fields in $\powtag$ and $\powinput$ is little-endian. The rationale
+for this is that little-endian serialization of \blockHeaders is consistent
+with \Bitcoin, but using little-endian ordering of bits in the solution
+encoding would require bit-reversal (as opposed to only shifting). The
+comparison of $\Xi_r$ values obtained by a big-endian conversion is equivalent
+to lexicographic comparison as specified in \mbox{section IV A.} of
+\cite{Equihash}.
 
 \nsubsubsection{Difficulty filter} \label{difficulty}
 
@@ -2330,7 +2336,7 @@ with simplifications and altered parameters, to adjust difficulty to target
 the desired 2.5-minute block time.
 Unlike \Bitcoin, the difficulty adjustment occurs after every block.
 
-\todo{}
+\todo{Describe the algorithm.}
 
 
 \nsection{Differences from the Zerocash paper}