对Twofish算法改进的饱和攻击  

An Improvement of the Saturation Attack of Twofish Algorithm

作　　者：吴泽[1] 孙兵[1] 李超[1] 沈璇[1] 

机构地区：[1]国防科技大学理学院,长沙410073

出　　处：《密码学报》2015年第4期363-371,共9页Journal of Cryptologic Research

基　　金：国家自然科学基金项目(61402515)

摘　　要：Twofish算法是Bruce Schneier等人设计的一个分组密码算法,它是AES计划最后一轮的5个候选算法之一.Twofish算法主要是基于ARX结构设计的,该结构包括模加操作,循环移位操作和异或操作.目前对Twofish算法的密码分析结果还比较少,主要是不可能差分分析和饱和攻击.Stefan Lucks等人首次对Twofish算法进行了饱和攻击.饱和攻击的关键是寻找好的饱和区分器.本文通过研究ARX结构的饱和性质,证明了当平衡集通过非线性变换时,ARX中关于平衡性质的三个定理,并利用常数集不改变平衡集的性质改进了Twofish算法4轮饱和区分器,与已知最好结果相比,可以多找到1比特的平衡位置.随后利用这个4轮饱和区分器将对简化轮数的Twofish算法进行了饱和攻击,时间复杂度降低一半.Twofish is a block cipher algorithm designed by Bruce Schneier, it was one of the finalists for the AES. Twofish is based on ARX systems.ARX systems include modular additions, rotations and bitwise XORs. Now the cryptanalysis on Twofish is still very limited, only impossible differential cryptanalysis and Saturation attack have been proposed. Stefan Lucks firstly used the Saturation attack to analyse the Twofish algorithm. The key point is to find a good Saturation distinguisher. This paper mainly studies the Saturation properties of ARX systems. Three theorems related to the balance property are proved. Based on these properties the Saturation distinguisher of Twofish can be improved. Compared with the known results, we can determine the balancedness of one more bit, so we can reduce half of the time complexity of the attacks.

关 键 词：饱和区分器 ARX TWOFISH 

分 类 号：TP309.7[自动化与计算机技术—计算机系统结构]