New Observations on Impossible Differential Cryptanalysis of Reduced-Round Camellia

Camellia is one of the widely used block ciphers, which has been selected as an international standard by ISO/IEC. In this paper, by exploiting some interesting properties of the key-dependent layer, we improve previous results on impossible differential cryptanalysis of reduced-round Camellia and gain some new observations. First, we introduce some new 7-round impossible differentials of Camellia for weak keys. These weak keys that work for the impossible differential take 3/4 of the whole key space, therefore, we further get rid of the weak-key assumption and leverage the attacks on reduced-round Camellia to all keys by utilizing the multiplied method. Second, we build a set of differentials which contains at least one 8-round impossible differential of Camellia with two FL/FL−1 layers. Following this new result, we show that the key-dependent transformations inserted in Camellia cannot resist impossible differential cryptanalysis effectively. Based on this set of differentials, we present a new cryptanalytic strategy to mount impossible differential attacks on reduced-round Camellia.