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{\Large\bf The Skein Hash Function Family} \\
\vspace*{.2in}
{\Large\bf NIST Round 3 Tweak Description} \\
\vspace*{.2in}
{\small 25 Oct 2010}
\end{center}

\section*{Description of Changes}

The only change to the Skein hash function is in the key schedule
parity constant, found in Section~3.3.2 of the newly submitted 
(``tweak'') version 1.3 of the Skein specification document. The
old constant was the value 
  $$C_5\ \  = \texttt{0x5555555555555555}.$$ 
The new constant is 
  $$C_{240} = \texttt{0x1BD11BDAA9FC1A22}.$$

Further details and discussion of the tweak and its implications are found in 
version 1.2 of the Skein specification document, as follows:
\begin{itemize}
\item Section~8.3 (``Key Schedule Constant,'')
\item Section ~9.3 (``Related-Key Attacks for the Threefish Block Cipher'')
\item Section ~9.5.2 (``Rotational Cryptanalysis'')
\item Section ~9.6 (``Empirical Observations for Threefish with Random Rotation Constants'')
\item Section ~9.7 (``Cryptanalysis Summary'')
\item Appendix~B (``Initial Chaining Values'')
\item Appendix~C (``Test Vectors'')
\item Appendix~E (``Empirical data for tweaking the key schedule constant'')
\end{itemize}

In addition, the following items have been updated in the Skein tweak submission package:
\begin{itemize}
\item Reference C source code
\item Optimized C source code (32-bit and 64-bit)
\item Assembly source code (32-bit and 64-bit)
\item Test vectors (KAT\_MCT directory)
\end{itemize}

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