Meet-in-the-middle technique for integral attacks against feistel ciphers

Abstract

In this paper, an improvement for integral attacks against Feistel ciphers is discussed. The new technique can reduce the complexity of the key recovery phase. This possibly leads to an extension of the number of attacked rounds. In the integral attack, an attacker guesses a part of round keys and performs the partial decryption. The correctness of the guess is judged by examining whether the XOR sum of the results becomes 0 or not. In this paper, it is shown that the computation of the XOR sum of the partial decryptions can be divided into two independent parts if the analysis target adopts the Feistel network or its variant. Then, correct key candidates are efficiently obtained with the meet-in-the-middle approach. The effect of our technique is demonstrated for several Feistel ciphers. Improvements on integral attacks against LBlock, HIGHT, and CLEFIA are presented. Particularly, the number of attacked rounds with integral analysis is extended for LBlock. © 2013 Springer-Verlag Berlin Heidelberg.