On software simulation of linearisation attacks on FCSR-based stream ciphers

Feedback with Carry Shift Registers (FCSRs) are being explored for their usage as building blocks in stream ciphers. Linearisation attacks are most effective attacks on a class of FCSR-based stream ciphers, which use filtered Galois FCSRs as building blocks. This paper presents techniques for software simulation of these attacks on such ciphers. In order to describe these techniques, the paper uses a small scale variant of the F-FCSR-H v2 type keystream generators, which is known as T-cipher. The paper uses the pseudorandom keystream generator of the T-cipher to develop a statistical analysis. The paper uses this analysis to demonstrate various aspects of the implementation of linearisation attacks on such ciphers. Moreover the paper presents a pseudocode algorithm along with its implementation details for computing the success characteristics of linearisation attacks. The paper also presents enumeration and pseudocode algorithms for solving systems of polynomial equations in the finite field F₂.