Attacking of Stream Cipher Systems Using a Genetic Algorithm

Stream cipher is one of the hard electronic cipher systems because of high security and difficulty in breaking it. This work is considered a new approach to cryptanalysis based on the application of direct search algorithm called genetic algorithm. It concentrates on showing the applicability of genetic algorithm concepts as a powerful tool in breaking cryptographic systems. The cryptanalysis is based on attacking stream cipher systems by finding the equivalent linear system. The goal of the genetic algorithm is finding the shortest linear feedback shift register that generates the known key stream through finding the initial state, feedback polynomial, and the shift register length. Previously, there were methods to find the linear equivalence by using Massey algorithm and neural networks. Now, the proposed method is considered the first attempt to find it via genetic algorithm with variable chromosomes lengths within genetic population leading to minimized average number of generations and accordingly less computational time. The proposed artificial system has been applied successfully to break a number of linear and nonlinear stream cipher systems, such as Hadmard system and Bruer system.