Applications Of Optimisation Heuristics In The Design And Analysis Of Sequence Cipher

The aim of the research presented in this thesis is to investigate the use of various optimization heuristics in the fields of automated cryptanalysis and automated cryptographic function generation. In this paper, we improve the design of the cryptographic components using optimization algorithm, and a new approach for attacking on some filter generators is introduced.Boolean functions are important parts of sequence ciphers. We must consider various factors in order to design good Boolean functions. For example nonlinearity, algebraic degree, correlation immunity, etc. Basing on the method of Hill Climbing and the properties of Boolean function, an algorithm, which is better than HC algorithm, named IHC is gave out to improve the design of nonlinearity of Boolean functions. By analyzing the output and input sequences of a nonlinear combined generator, we also give out an algorithm (LHC) to evolve the correlation from a random function to a very lower one.For multi-output Boolean functions, we give out the definition of correlation permissibility. By investigating some properties of the balanced multi-output Boolean functions, a feasible strategy of evolvement of function is presented. Based on the method of simulated annealing and using the strategy presented in the 3rd section of capter four, a method is described to tell us how we can get some multi-output Boolean functions with low correlation permissibility and high nonlinearity.The filter generator is an important type of stream cipher. It has an extensive application in the field of information security. In this paper, we discuss a kind of filter generator whose filter functions have less input bits than the degree of the linear feedback shift register (LFSR). By analyzing the structure of the filter generator and its equivalent system, we give out a conditional search algorithm (CSA) to attack this kind of filter generators.