On the security of FCSR-based pseudorandom generators

This article describes new theoretical results concerning the general behavior of an FCSR automaton that allow to better understand the initial parameters that must be chosen to use this automaton as a basic block of a filtering stream cipher. The results demonstrated here especially concern the structure of the subjacent graph of an FCSR automaton, its entropy and the number of iterations of the FCSR transition function required to reach the main part of the graph. A linear weakness and a way to discard the induced potential attack are also given. The parameters chosen for the two candidates F-FCSR-16 and F-FCSR-H make this attack impossible.