A Related Key Attack on the Feistel Type Block Ciphers

In this paper we show that Biham’s chosen key attack can be generalized to include any block cipher and we give a low complexity chosen key attack on any Feistel type cipher. Then we show that the irregularities in the shift pattern of DES key schedule algorithm is not sufficient for the cryptosystem to resist against related key attacks. We have realized our proposition by a counter example in which the E-box of DES is slightly modified whiles other components and among those, the shift pattern in key schedule algorithm is kept unchanged. We have applied a new related key attack on the resulting DES-like cryptosystem and demonstrated that the security of the system decreases drastically.