Differential Factors: Improved Attacks on SERPENT

Abstract

Abstract. A differential attack tries to capture the round keys corresponding to the S-boxes activated by a differential. In this work, we show that for a fixed output difference of an S-box, it may not be possible to distinguish the guessed keys that have a specific difference. We introduce these differences as differential factors. Existence of differential factors can reduce the time complexity of differential attacks and as an example we show that the 10, 11, and 12-round differential-linear attacks of Dunkelman et al. on Serpent can actually be performed with time complexities reduced by a factor of 4, 4, and 8, respectively.