In this paper we propose a new key recovery attack on irregular clocked keystream generators where the stream is filtered by a nonlinear Boolean function. We show that the attack is much more efficient than expected from previous analytic methods, and we believe it improves all previous attacks on the cipher model.
In this paper we present a new and improved correlation attack based on maximum likelihood (ML) decoding. Previously the code rate used for decoding has typically been around r = 1/2 14. Our algorithm has low computational complexity and is able to use code rates around r = 1/2 33. This way we get much more information about the key bits. Furthermore, the… (More)
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 rst 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… (More)