Cryptanalysis of Feistel Networks with Secret Round Functions

Generic distinguishers against Feistel Network with up to 5 rounds exist in the regular setting and up to 6 rounds in a multi-key setting. We present new cryptanalyses against Feistel Networks with 5, 6 and 7 rounds which are not simply distinguishers but actually recover completely the unknown Feistel functions. When an exclusive-or is used to combine the output of the round function with the other branch, we use the so-called yoyo game which we improved using a heuristic based on particular cycle structures. The complexity of a complete recovery is equivalent to O(22n ) encryptions where n is the branch size. This attack can be used against 6- and 7-round Feistel Networks in time respectively O(2n2n−1 +2n ) and O(2n2n +2n ). However when modular addition is used, this attack does not work. In this case, we use an optimized guess-and-determine strategy to attack 5 rounds with complexity O(2n23n/4 ). Our results are, to the best of our knowledge, the first recovery attacks against g...

• Type: Journal
• Year: 2015
• Where: SACRYPT
• Authors: Alex Biryukov, Gaëtan Leurent, Léo Perrin
• Keywords: Cryptology, SACRYPT 2015