Improved Results on Impossible Differential Cryptanalysis of Reduced-Round Camellia-192/256

As an international standard adopted by ISO/IEC, the block cipher Camellia has been used in various cryptographic applications. In this paper, we reevaluate the security of Camellia against impossible differential cryptanalysis. Specifically, we propose several 7-round impossible differentials with the FL/FL layers. Based on them, we mount impossible differential attacks on 11-round Camellia192 and 12-round Camellia-256. The data complexities of our attacks on 11-round Camellia-192 and 12-round Camellia-256 are about 2 chosen plaintexts and 2 chosen plaintexts, respectively. The corresponding time complexities are approximately 2 11-round encryptions and 2 12-round encryptions. As far as we know, our attacks are 2 times and 2 times faster than the previously best known ones but have slightly more data.