One method of making a DES--like cryptosystem more secure against
different kind of attacks is to use key--dependent variable
S--boxes. A method of particular interest was introduced by Schneier
\cite{Schn0} who showed how to  derive such S-boxes from a key. A
disadvantage of this approach is that such S--boxes are not
bijective and thus they are not collision free. In this  paper we
analyze a collision attack, similar to Vaudenay \cite{Vaude},
applied to the modified GOST algorithm. We show that if non -
bijective S--boxes are  used in GOST algorithm then it is feasible
to break it when a weak key is in use.
