Quantum Generic Attacks on Feistel Schemes

The Feistel scheme is an important structure in the block ciphers. The security of the Feistel scheme is related to distinguishability with a random permutation. In this paper, efficient quantum algorithms for distinguishing classical 3,4-round and unbalanced Feistel scheme with contracting functions from random permutation are proposed. Our algorithms realize an exponential speed-up over classical algorithms for these problems. Furthermore, the method presented in this paper can also be used to consider unbalanced Feistel schemes with expanding functions.