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

摘要： 为评估一类不平衡Feistel密码的安全性能，通过列举的方法，对该密码抵抗差分密码分析和线性密码分析的能力进行了深入的研究。在轮函数是双射的假设条件下，证明了3，4，6，8，10，2r（r≥3）轮密码分别至少有1，1，3，4，5，r个轮函数的输入差分非零，从而若设轮函数的最大差分和线性逼近的概率分别为p和q，则2r（r≥3）轮密码的差分特征和线性特征的概率分别以pr和qr为上界。

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