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International audienceCamellia 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

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TEA, XTEA and HIGHT are lightweight block ciphers with 64-bit block sizes and 128-bit keys. The round functions of the three ciphers are based on the simple operations XOR, modular addition and shift/rotation. TEA and XTEA are Feistel ciphers with 64 rounds designed by Needham and Wheeler, where XTEA is a successor of TEA, which was proposed by the same authors as an enhanced version of TEA. HIGHT, which is designed by Hong et al., is a generalized Feistel cipher with 32 rounds. These block ciphers are simple and easy to implement but their diffusion is slow, which allows us to find some impossible properties. This paper proposes a method to identify the impossible differentials for TEA and XTEA by using the weak diffusion, where the impossible differential comes from a bit contradiction. Our method finds a 14-round impossible differential of XTEA and a 13-round impossible differential of TEA, which result in impossible differential attacks on 23-round XTEA and 17-round TEA, respectively. These attacks significantly improve the previous impossible differential attacks on 14-round XTEA and 11-round TEA given by Moon et al. from FSE 2002. For HIGHT, we improve the 26-round impossible differential attack proposed byÖzen et al.; an impossible differential attack on 27-round HIGHT that is slightly faster than the exhaustive search is also given.