Cryptanalysis of alternating step generators

Alternate clocking of linear feedback shift registers is the popular technique used to increase the linear complexity of binary sequences produced by keystream generators designed for stream ciphers. The analysis of the best known attacks on the alternating step generator led us to add nonlinear filtering functions and the nonlinear scrambler to the construction. In this paper we give complexities of these attacks applied to the modified alternating step generator with nonlinear filters and the scrambler. We also suggest minimum lengths of registers in the original alternating step generator to make it resistant to the attacks.