Generic Attacks on Unbalanced Feistel Schemes with Expanding Functions

Unbalanced Feistel schemes with expanding functions are used to construct pseudo-random permutations from kn bits to kn bits by using random functions from n bits to (k − 1)n bits. At each round, all the bits except n bits are changed by using a function that depends only on these n bits. C.S.Jutla [6] investigated such schemes, which he denotes by Fd k , where d is the number of rounds. In this paper, we describe novel Known Plaintext Attacks (KPA) and Non Adaptive Chosen Plaintext Attacks (CPA-1) against these schemes. With these attacks we will often be able to improve the result of C.S.Jutla. We also give precise formulas for the complexity of our attacks in d, k and n. Key words: Unbalanced Feistel permutations, pseudo-random permutations, generic attacks on encryption schemes, Block ciphers.

• Type: Conference
• Year: 2007
• Where: ASIACRYPT
• Authors: Jacques Patarin, Valérie Nachef, Côme Berbain
• Keywords: ASIACRYPT 2007, Cryptology, Kn Bits, Pseudo-random Permutations, Unbalanced Feistel