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A unified method for finding impossible differentials of block cipher structures. (English) Zbl 1345.94078

Summary: In this paper, we propose a systematic method for finding impossible differentials for block cipher structures, which we call the unified impossible differential finding method or UID-method. It is more effective than the \(U\)-method introduced by J. Kim et al. [Indocrypt 2003, Lect. Notes Comput. Sci. 2904, 82–96 (2003; Zbl 1123.94352)]. We apply the UID-method to some well-known block cipher structures. Using it, we find a 16-round impossible differential for Gen-Skipjack and a 19-round impossible differential for Gen-CAST256. By this result we can disprove Sung’s long standing conjecture [J. Sung et al., Asiacrypt 2000, Lect. Notes Comput. Sci. 1976, 274–288 (2000; Zbl 0965.94016)] that no such differential is possible for 16 or more rounds. On Gen-MARS and SMS4, the impossible differentials found by the UID-method are much longer than those found by the \(U\)-method. On the Four-Cell and Gen-RC6 block ciphers, our results are the same as the best results previously obtained.

MSC:

94A60 Cryptography

Software:

LBlock
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References:

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