Impossible Differential Cryptanalysis of Reduced-Round SKINNY

SKINNY is a new lightweight tweakable block cipher family proposed by Beierle et al. at CRYPTO 2016. SKINNY has 6 main variants where SKINNY-n-t is a block cipher that operates on n-bit blocks using t-bit tweakey (key and tweak) where n = 64 or 128 and t = n, 2n, or 3n. In this paper, we present impossible differential attacks against reduced-round versions of all the 6 members of the SKINNY family in the single-tweakey model. More precisely, using an 11-round impossible differential distinguisher, we present impossible differential attacks against 18-round SKINNY-n-n, 20-round SKINNY-n-2n and 22-round SKINNY-n-3n (n = 64 or 128). To the best of our knowledge, these are the best attacks against these 6 variants in the single-tweakey model.