Revisiting impossible differentials of MARS-like structures
The MARS-like structure is a generalised Feistel structure. In 2015, Xue and Lai proved that there always exist (3n − 1)-round impossible differentials of MARS-like structures with n subblocks, as long as the round function is bijective. In this study, the length of the impossible differentials is extended by 1 round supposing that the MARS-like structure adopts the bijective round function of SP-type, i.e. the round function is decomposed into a substitution layer followed by a linear diffusion layer. It is surprised that such result is irrelevant to both the specifics of the S-boxes in the substitution layer and the specifics of the linear transformations in the diffusion layer.