International Association for Cryptologic Research (IACR)
Abstract
In this paper, we present new general techniques for practical
security evaluation against differential and linear cryptanalysis
for an extensive class of block ciphers similar to the cipher
GOST. We obtain upper bounds of the average differential and
linear characteristic probabilities for an arbitrary GOST-like
cipher. The obtained bounds have similar form to the upper bounds
of the average differential and linear characteristic
probabilities known for some Markov Feistel ciphers. But, the
expressions of our bounds contain new parameters (different from
the classical differential and linear probabilities) of the
cipher\u27s s-boxes. These parameters are very natural for
GOST-like ciphers, since they inherit the type of operation (key
addition modulo 2m) used in these ciphers. The methods our
proofs are based on are of independent interest and can be used
for investigation both of a wider class of block ciphers and of a
wider class of attacks.
Application of our results to GOST shows that maximum values of
the average differential and linear characteristic probabilities
of this cipher (with 32 rounds and some s-boxes) are bounded by
2−59.57 and 2−42, respectively. The last two estimates
of practical security of GOST against the differential and linear
cryptanalysis are not quite impressive. But, as far as we know,
they are the best of such estimates obtained by an accurate
mathematical proof