Security of Reduced Version of the Block Cipher Camellia against Truncated and Impossible Differential Cryptanalysis

摘要

This paper describes truncated and impossible differential cryptanalysis of the 128-bit block cipher Camellia, which was proposed by NTT and Mitsubishi Electric Corporation. Our work improves on the best known truncated and impossible differential cryptanalysis. As a result, we show a nontrivial 9-round byte characteristic, which may lead to a possible attack of reduced-round version of Camellia without input/output whitening, FL or FL~(-1) in a chosen plain text scenario. Previously, only 6-round differentials were known, which may suggest a possible attack of Camellia reduced to 8-rounds. Moreover, we show a nontrivial 7-round impossible differential, whereas only a 5-round impossible differential was previously known. This cryptanalysis is effective against general Feistel structures with round functions composed of S-D (Substitution and Diffusion) transformation.