Observations of Skipjack-like Structure with SP/SPS Round Function

Ting Cui (Information Science and Technology Institute, China)

Chenhui Jin (Information Science and Technology Institute, China)

Guoshuang Zhang (Science and Technology on Information Assurance Laboratory, China)

Abstract: Impossible differential cryptanalysis is an important tool for evaluating the security level of a block cipher, and the key step of this cryptanalysis is to find the longest impossible differential. This paper focuses on retrieving impossible differentials for m-cell Skipjack-like structure with SP/SPS round function (named Skipjack_SP and Skipjack_SPS resp.). Up to now, known longest impossible differentials in m-cell Skipjack-like structures is m² rounds. In this paper, we provide some new m² rounds impossible differentials for these two structures. Further, we prove that if P layer is chosen from binary matrices, we can always retrieve m² + 1 rounds impossible differentials for these two structures, and m² + 2 rounds impossible differentials for Skipjack_SP. Moreover, if P layer satisfies some satiable conditions, we may further obtain m² + 2 rounds impossible differential for Skipjack_SPS. Our results show that we should choose P layer carefully when employing these two structures.

Keywords: block cipher, impossible differential, permutation layer, skipjack-like structure

Categories: C.2.0, D.4.6, E.3