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中文摘要：
LBlock-s算法是CAESAR竞赛候选认证加密算法LAC中的主体算法,算法结构与LBlock算法基本一致,只是密钥扩展算法采用了扩散效果更好的增强版设计.利用新密钥扩展算法中仍然存在的子密钥间的迭代关系,通过选择合适的14轮不可能差分特征,我们给出了对21轮LBlock-s算法的不可能差分分析.攻击需要猜测的子密钥比特数为72比特,需要的数据量为2^（63）个选择明文,时间复杂度约为2^（67.61）次21轮加密.利用部分匹配技术,我们也给出了直到23轮LBlock-s算法低于密钥穷举量的不可能差分分析结果.这些研究可以为LAC算法的整体分析提供参考依据.
英文摘要：
LBlock-s is the kernel block cipher of the authentication encryption algorithm LAC submitted to CAESAR competition. The general structure of LBlock-s is almost the same as that of LBlock, but LBlock-s adopts an improved key schedule algorithm with better diffusion property. Using the shifting relation of subkeys derived by the key schedule algo- rithm, an impossible differential cryptanalysis on 21-round LBlock-s was presented based on a 14-round impossible differen- tial. The time and data complexities are 267. 67.61 21-round encryptions and 263 chosen plaintexts respectively, and the number of subkey bits needed to be guessed is 72. Using partial-matching method, an impossible differential cryptanalysis on LBlock-s up to 23-round was also presented with time complexity less than exhaustion of all key bits. This work is useful for the secu- rity analysis of LAC algorithm.
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