A Unified Approach to Related-Key Attacks

This paper introduces a new framework and a generalization of the various flavors of related-key attacks. The new framework allows for combining all the previous related-key attacks into a complex, but much more powerful attack. The new attack is independent of the number of rounds of the cipher. This property holds even when the round functions of the cipher use different subkeys. The strength of our new method is demonstrated by an attack on 4r-round IDEA, for any r. This attack is the first attack on a widely deployed block cipher which is independent of the number of rounds. The variant of the attack with r = 2 is the first known attack on 8-round IDEA.