The Delicate Issues of Addition with Respect to XOR Differences

In this paper we analyze the previous attacks on the block cipher SHACAL-1 and show that all the differential-based attacks fail due to mistreatment of XOR differences through addition. We show that the previously published differential and rectangle attacks on SHACAL-1 fail as some of the underlying differentials are impossible. The related-key rectangle attacks on the cipher generally fail, but if some conditions are imposed on the key (i.e., for a weak key class) they work. After identifying the flaws in previous attacks, we present possible fixes to these attacks. We then present some modified differentials which lead to a related-key rectangle attack which can be applied to 2504 weak keys. Our observations are then used to improve a related-key rectangle attack on IDEA by a factor of 2.