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%%  Example 07-04-2 
%%  Copyright (C) 2017 Herbert Voss
%% 
%% ==== 
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\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
\usepackage[T1]{fontenc}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands
\begin{document}
\begin{equation}
\begin{array}{rcl}
\underline{RS} & = & \left(\begin{array}{cccccccc}
01 & a4 & 55 & 87 & 5a & 58 & db & 9e\\
a4 & 56 & 82 & f3 & 1e & c6 & 68 & e5\\
02 & a1 & fc & c1 & 47 & ae & 3d & 19\\
a4 & 55 & 87 & 5a & 58 & db & 9e & 03\end{array}\right)\\
\\
\left(\begin{array}{c}
  s_{i,0}\\  s_{i,1}\\  s_{i,2}\\  s_{i,3}
\end{array}\right) & = & \underline{RS}\cdot
\left(\begin{array}{c}
  m_{8i+0}\\ m_{8i+1}\\ \cdots\\ m_{8i+6}\\ m_{8i+7}
\end{array}\right)\\[\bigskipamount]
S_{i} & = & \sum\limits_{j=0}^{3}s_{i,j}\cdot2^{8j}\qquad i=0,1,...,k-1\\[\medskipamount]
S & = & \left(S_{k-1},S_{k-2},...,S_{1},S_{0}\right)
\end{array}
\end{equation}
\end{document}
