The Improbable Differential Attack: Cryptanalysis of Reduced Round CLEFIA

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In this paper we present a new statistical cryptanalytic technique that we call improbable differential cryptanalysis which uses a differential that is less probable when the correct key is used. We provide data complexity estimates for this kind of attacks and we also show a method to expand impossible differentials to improbable differentials. By using this expansion method, we cryptanalyze 13, 14, and 15-round CLEFIA for the key sizes of length 128, 192, and 256 bits, respectively. These are the best cryptanalytic results on CLEFIA up to this date.

This work was done when the author was a research assistant at Institute of Applied Mathematics, Middle East Technical University, Ankara, Turkey.