PROVABLE SECURITY AGAINST A DIFFERENTIAL ATTACK

The purpose of this paper is to show that DES-like iterated ciphers th at are provably resistant against differential attacks exist. The main result on the security of a DES-like cipher with independent round ke ys is Theorem 1, which gives an upper bound to the probability of s-ro und differentials, as defined in [4], and this upper bound depends onl y on the round function of the iterated cipher. Moreover, it is shown that functions exist such that the probabilities of differentials are less than or equal to 2(3-n), where n is the length of the plaintext b lock. We also show a prototype of an iterated block cipher, which is c ompatible with DES and has proven security against differential attack .