关键词: 不平衡Feistel密码, 轮函数, 差分特征概率, 线性特征概率

Abstract: To evaluate the security of a class of unbalanced Feistel ciphers，the security analysis against differential and linear attacks is investigated deeply using enumeration method.If the round function is bijective，then the number of active round function for 3，4，6，8，10，2r（r≥3）-round ciphers is not less than 1，1，3，4，5，r respectively.So the upper bounds of maxi-
mum differential and linear characteristic probabilities for 2r（r≥3）-round ciphers are pr and qr respectively if maximum differential and liner approximation probabilities for round function are p and q.

Key words: unbalanced Feistel ciphers, round function, differential characteristic probabilities, linear characteristic probabilities