Differential Related-Cipher Attacks on Block Ciphers with Flexible Number of Rounds

Related-Cipher attack was introduced by Hongjun Wu in 2002. We can consider related ciphers as block ciphers with the same round function but different round number and their key schedules do not depend on the total round number. This attack can be applied to block ciphers when one uses some semi-equivalent keys in related ciphers. In this paper we introduce differential related-cipher attacks on block ciphers, which combine related-cipher attacks with differential cryptanalysis. We apply this attack to the block cipher ARIA and SC2000. Furthermore, related-cipher attack can be combined with other block cipher attacks such as linear cryptanalysis, higher-order differential cryptanalysis, and so on. In this point of view we also analyze some other block ciphers which use flexible number of rounds, SAFER++ and CAST-128.