The security evaluation of block cipher Twofish

摘要

We propose a new method for evaluating the security of block ciphers like Twofish against truncated differential cryptanalysis. To this end, we propose the new methods for evaluating the truncated differential probability of key-addition and Pseudo Hadamard Transforms (PHT). Using these methods and previously proposed evaluation method for SPN-structures, we can evaluate the truncated differential probability of Twofish. This evaluation indicates that 5-rounds of Twofish (including 1 additional round) can be cryptanalyzed by truncated differential cryptanalysis, and 6-rounds (including 1 additional round) do not.