Paper 2025/1911

Differential Meet-in-the-Middle Attacks on Feistel Ciphers

Abstract

Differential meet-in-the-middle attacks, introduced by Boura et al. in 2023, propose a new way of dealing with differential distinguishers. It allows, in particular, to combine differential attacks with initial structures, that were usually used exclusively for meet-in-the-middle attacks. Several applications of this new technique have been published, but so far the results on Feistel constructions have not improved much upon previous best known attacks. In this paper, we apply them on Feistel constructions with all the improvements proposed so far, and we propose some additional new ideas to generically improve these kinds of attacks. We also propose an automatized tool for optimizing the attacks on Simon-like constructions. Our tool outputs a graphical representation of the attack that makes it very easy to verify. All this has allowed us to provide improved single-key key-recovery attacks on most of the variants of Simon, Simeck and CLEFIA-256, that increase the highest number of rounds attacked by 1 or 2 in nearly all the cases.