中文摘要：

对分组密码算法CLEFIA进行不可能差分分析．CLEFIA算法是索尼公司在2007年快速软件加密大会（FSE）上提出来的．结合新发现和新技巧，可有效过滤错误密钥，从而将算法设计者在评估报告中给出的对11圈CLEFIA-192／256的攻击扩展到11圈 CLEFIA-128／192／256，复杂度为2^103.1次加密和2^103.1个明文．通过对明文附加更多限制条件，给出对12圈CLEFIA-128／192，256的攻击，复杂度为2^119.1次加密和2^119.1个明文．而且，引入一种新的生日筛法以降低预计算的时间复杂度．此外，指出并改正了Tsunoo等人对12圈CLEFIA的攻击中复杂度计算方面的错误．

英文摘要：

An improved impossible differential attack on the block cipher CLEFIA is presented. CLEFIA was proposed by Sony Corporation at FSE 2007. Combining some observations with new tricks, the wrong keys are filtered out more efficiently, and the original impossible differential attack on 11-round CLEFIA-192/256 published by the designers, is extended to CLEFIA-128/192/256, with about 2^103.1 encryptions and 2^103.1 chosen plaintexts. By putting more constraint conditions on plaintext pairs, we present an attack on 12-round CLEFIA for all three key lengths with 2^119.1 encryptions and 2^119.1 chosen plaintexts. Moreover, a birthday sieve method is introduced to decrease the complexity of the precomputation. And an error about the time complexity evaluation in Tsunoo et al.＇s attack on 12-round CLEFIA is pointed out and corrected.