Cryptanalysis of a Particular Case of Klimov-Shamir Pseudo-Random Generator

Abstract

T-functions have been introduced by Shamir and Klimov in [1]. Those functions can be used in order to design a new class of stream ciphers. We present in this paper an algorithm which can retrieve the internal state of a particular class of pseudo-random generators based on T-functions. This algorithm has time complexity of $O(2^{\frac{n}{4}})$ and has memory complexity of O(n log₂ n ) for pseudo random generators which put out the n/2 most significants bits of their internal state at each time clock, n being the length of the internal state of the pseudo-random generator.