Improved linear consistency attack on irregular clocked keystream generators

Abstract

In this paper we propose a new attack on a general model for irregular clocked keystream generators. The model consists of two feedback shift registers of lengths l1 and l2, where the first shift register produces a clock control sequence for the second. This model can be used to describe among others the shrinking generator, the step-1/step-2 generator and the stop and go generator. We prove that the maximum complexity for attacking such a model is only O(2l1). © International Association for Cryptologic Research 2004.