Feistel Schemes and Bi-Linear Cryptanalysis

In this paper we introduce the method of bi-linear cryptanalysis (BLC), designed specifically to attack Feistel ciphers. It allows to construct periodic biased characteristics that combine for an arbitrary number of rounds. In particular, we present a practical attack on DES based on a 1-round invariant, the fastest known based on such invariant, and about as fast as the best Matsui's attack. For ciphers similar to DES, based on small S-boxes, we claim that BLC is very closely related to LC, and we do not expect to find a bi-linear attack much faster than by LC