Advanced truncated differential cryptanalysis of GOST block cipher

n this paper, we use the ideas presented by Courtois and Mourouzis to study the security of two variants of GOST, which are considered as the simpler and most secure variants [9]; the one with the S-boxes replaced by the Identity Map and the ISO version which is assumed to be the strongest one. The advanced differential attacks we present are of the form of Depth-First Key search, which uses a 20 round distinguisher in the middle (or equivalently 26-round distinguisher for the simpler version of GOST with Identity Map) [11]. The main idea is that we consider a partition of the 32 rounds by placing in the middle the constructed distinguisher. Then, based on the weak diffusion we can extend these very strong statistical distinguishers to efficiently good filters for some external rounds. Then, by guessing some key bits for external rounds and determining some plaintext and ciphertext pairs of specified input-output differences we can extend the construction to an attack against the full block cipher. Thus, the technique we apply is a generic cryptanalytic framework of First-Search key search type which involves several optimization tasks obtained from the specific structure of the given encryption algorithm.