International Association
for Cryptologic Research

In this paper, security analysis of block ciphers with key length greater than block length is proposed. For a well-designed block cipher with key length k and block length n s.t. k>n and for all P, C, there are 2^{k-n} keys which map P to C. For given block cipher, if there is an efficient algorithm that can classify such keys, we propose an algorithm will be able to recover the secret key with complexity O(max{2^n, 2^{k-n}}). We apply this method on 2-round block cipher KASUMI.