Modified Alternating Step Generators

Irregular clocking of Feedback Shift Registers is a popular technique to improve parameters of keystream generators in stream ciphers. Another technique is to implement nonlinear functions. We join these techniques and propose Modified Alternating Step Generators built with Linear and Nonlinear Feedback Shift Registers. Adequate nonlinear Boolean functions are used as feedbacks of shift registers and as output functions of the generator in order to increase its resistance to known attacks. We investigate basic parameters of proposed keystream generators, such as period, linear complexity and randomness.