We present a new cryptanalytic attack on PN-generators filtered by a Boolean function. The key-idea is to jointly combine the knowledge of the Boolean function and the LFSR structure so as to introduce a new iterative decoding technique. The results we obtained prove to be very good: indeed, even in the case of systems for which classical iterative algorithms appear powerless, our scheme enables us to recover the LFSR initial state with a very low error probability. The latter is such that ending the algorithm by an exhaustive search among sequences of limited length is hardly needed. Therefore, the overall complexity of our scheme remains very small and the algorithm appears as an excellent candidate for cryptanalysis of filtered PN-generators.

No abstract

We present a new algorithm devoted to the cryptanalysis of non-linear filter generators. B y by appropriate permutations. jointly using the knowledge of the Boolean function weight vectors and all the other probabilities can be deduced Moreover by using the Fourier transform, we gain in complexand parity-check equations, the SOJA (soft O u t p u t ity : the probability can be evaluated by : pi(v) = ' -' :(' ), Joint Algorithm) provides a c c u r a t e information on with &(U) = A. riThen, we can show that: the input sequence. rE (0,)