Security Analysis On Several Clock-controlled Generators

The randomicity of keystream sequences and capability of resisting attacks are two symbols of evaluating the intensity of stream cipher. The design of a good keystream generator is the core of research on stream cipher, and security analysis on generators'is one of keystone contents of cryptanalysis. Clock-controlled logic and memorial logic are widely applied to the design of keystream generators.Clock-controlled logic can increase the period and linearity complexity of keystream sequences. Use of combining functions with memories could overcome the tradeoff between the correlation immunity and the algebraic degree, and make them achieve maximum at one time. Both techniques can be both used in the design of the stream ciphers to statisfy more security. And cryptanalysis provides the basic for evaluating properties of the generators using the techniques mentioned above. The security of several clock-controlled generators and combiners with multi-memories are researched in this paper.On the research on the security of clock-controlled generators, the security of two kinds of hand-control generators, STEP[1..D] generators and balanced shrinkinig generators, are firstly studied in this paper, Bsed on the establishment of probability model, the properties of their output sequences are analyzed and the rate of coincidence between the output sequences and the input sequences are analyzed. And we take a correlation attack on balanced shrinkinig generators based on posterior probability. Secondly, we propose a kind of guess-and-determine attack on generalized self-shrinking generators, which has some improvement on time, memory and data complexities. A5/1 algorithm, a representational algorithm of mutual-control model, is taken into account. Then its probability model is established. Moreover it is proved that its output sequences are independent and uniform. And it can resist correlation attack to some extent.Correlation analysis of two kinds of clock-controlled combiners with memory are discussed.The clock-controlled step combiners with multi-memories are firstly considered, the properties of their output sequences and memory sequences are analyzed, the sufficiency and necessary condition of guaranting their output sequences independence, uniform and correlation immunity are studied, and the problem of conversation of energy is reviewed. Secondly, the clock-controlled stop-go combiners with memory are examined, the properties of their output sequences and memory sequences are analyzed.Recursion formula of calculating posteriori probability is obtained, and it's capable to take correlation attack on clock-controlled stop-go combiners based on posterior probability.