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

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 skipjacksp and skipjacksps 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 skipjacksp .moreover, if p layer satisfies some satiable conditions, we may further obtain m²+ 2 rounds impossible differential for skipjacksps. our results show that we should choose p layer carefully when employing these two structures.