LEX算法的输出位置分析

摘要

LEX is a candidate algorithm that may work in phase 3 of eSTREAM stream cipher project.The basic idea of it is to output from the AES cipher parts of the internal state as the keystream of LEX algorithm.We analyse the impact of the output locations on security of the LEX,examine the security of LEX in the circumstance that its output locations are the same in both the odd round and the even round,and apply the guess and determine attack to LEX cipher for analysis,when the 236.3 bytes keysteams sequence is known,the 128-bit initial key of LEX can be entirely recovered with only 281.4 round LEX encipher operations,which is far less than the current analysis result.This shows that the existing output locations of LEX are chosen reasonably.%LEX算法是进入eSTREAM计划第三阶段的一个候选算法，该算法的基本思想是从AES算法中输出一部分中间状态作为LEX算法的密钥流。研究输出位置对LEX算法的安全性影响，考察输出位置在奇数轮和偶数轮相同的情况下LEX算法的安全性，利用猜测决定攻击对该LEX算法进行分析，在已知236．3字节的密钥流序列的条件下，仅需要281．4轮LEX加密就可完全恢复128比特初始密钥，远远小于对LEX算法的现有分析结果。该结果表明，现有LEX算法的输出位置的选择是合理的。