Linear cryptanalysis for a class of feistel ciphers
一类Feistel密码的线性分析

In this paper, a new method is proposed for seeking the upper bounds of maximum linear bias for block ciphers, which is especially applicable to a class of Feistel ciphers that key is XORed with data. This technique consists of two steps. Firstly, the mathematical relationship between linear bias of ciphers and linear bias of round function F and S-box respectively is given by carrying out strictly mathematical expression of linear bias for ciphers. Next, the upper bounds of linear bias for ciphers are determined by seeking the solution with minimum weight for linear equation group. Using this method the upper bounds of linear bias within 32 rounds are given.