  In this paper, we study a block cipher based on cellular automata, proposed
by Seredynski and Bouvry in \cite{semabo04} against \emph{plain-text avalanche
criteria} and \emph{randomness} tests. Our experiments shows that
Seredynski-Bouvry encryption scheme does not pass some NIST statistical tests
by neighborhood radius less than three. It also showed that if the CA rule is
selected carelessly, it weaken the security of scheme. Therefor, the selection
of CA-rule as part of key can not be left to the user. Hence, cryptographic
properties such as balancedness and non-linearity should be considered in the
selection of CA-rules. This approach is more compliant with Kerckhoffs
principle. So security should depend just on security of final data. We also
improve Seredynski-Bouvry encryption scheme to satisfy strict avalanche
criteria and NIST statistical test suite in about half number of iterations
comparing to original scheme. This improvement is achieved by change in the
definition of neighborhood.
