【作者】 陈杰；

【导师】 肖国镇； 胡予濮；

【作者基本信息】 西安电子科技大学 ， 密码学， 2005， 硕士

【摘要】 对密钥流生成器的安全性分析一直是序列密码研究的中心课题。本文主要分析讨论了对流密码的代数攻击,其中包括代数攻击在几种密钥流生成器下的计算复杂度和所需的存储复杂度。同时介绍了我们得到的关于代数攻击以及流密码攻击方法的一些最新研究成果。本文取得的主要结果如下: 1. 系统地研究了对流密码的代数攻击方法。给出了在滤波生成器的非线性部分部分未知和完全未知两种情况下的代数攻击方法及相应的计算复杂度。2. 基于密码函数分拆的思想提出了一种快速求解降次函数的新方法,分析了其计算复杂度,并对该方法进行了改进。我们利用该新算法以更低的计算复杂度计算出了LILI-128 流密码体制的降次函数。3. 基于组合函数的差分特性,提出了一种新型的再同步攻击方法。该方法可以得到含有初始密钥为变元的线性方程组或低次方程组,并最终通过求解方程组恢复出初始密钥。该方法降低了再同步攻击的计算复杂度。我们利用此攻击方法分析了Toyocrypt 密码体制。4. 利用差分分析和代数攻击提出了另一种新型的再同步攻击方法,并利用此攻击方法分析了LILI-128 密码体制。更多还原

【Abstract】 Security analysis of the key stream generators is always at the central of the research on stream ciphers. This thesis mainly analyzes the algebraic attacks on stream ciphers. The computational complexity and memory demand are investigated when algebraic attacks apply to a few stream generators. We also present the recent results we obtained about algebraic attacks and some other attacks on stream ciphers. 1. Algebraic attacks on stream ciphers are investigated. We present the methods of algebraic attacks and computational complexities in which the nonlinear filter generators are only partially known or totally unknown. 2. We present and investigate the computational complexity of a new fast algorithm for constructing depressed functions based on cryptographic functions splitting idea. An improved version of the algorithm is also provided. When applied to LILI-128 cryptographic system to construct depressed functions, the new algorithm obtains a lower computational complexity. 3. We propose a new resynchronization attack based on the difference performances of combinational functions. This attack can get a system of linear equations or of low-degree equations about initial key,and recovers the initial key by solving the system of equations. When implemented to the Toyocrypt scheme, our method gains a lower computational complexity. 4. Another resynchronization attack is also proposed, which is the combination of the differential cryptanalysis and algebraic attack. We use it to cryptanalyze the LILI-128 scheme.更多还原