CIPHER STRENGTH EVALUATION DEVICE AND PROGRAM RECORDING MEDIUM THEREOF

摘要

PROBLEM TO BE SOLVED: To make it possible to evaluate the strength of cipher against attach by deriving a relation between an input different value and an output difference value usable for the attach using truncated differential. SOLUTION: Truncated differential of a round function (F-function) of repeated cipher to be evaluated is stored in a storage part 12 beforehand. A block length (m) of the repeated cipher to be evaluated, the number of steps R, and a sub-block length (T) are inputted from an input part 11. At an r-th step (1<=r<=R) of an F-function processing part 13, the truncated differentials of the round function stored in the storage part 12 are subjected to a prescribed processing to all possible input difference values X(r) to the r-th round function. When the (r) is equal to the R, an input difference value X(1) and an output difference value X(R+1) of the truncated differential of this Feistel ciphering device are outputted from an output part 14 under a special condition.