On the (Im)Possibility of Practical and Secure Nonlinear Filters and Combiners

A vast amount of literature on stream ciphers is directed to the cryptanalysis of LFSR-based filters and combiners, resulting in various attack models such as distinguishing attacks, (fast) correlation attacks and (fast) algebraic attacks. However, very little is known on the combined effects of these attacks and the resulting cryptographic requirements. In this paper, we present a unified framework for the security of a design against these attacks based on the properties of the LFSR(s) and the Boolean function used. It is explained why building nonlinear filters seems more practical than building nonlinear combiners. We also investigate concrete building blocks that offer a good trade-off in their resistance against these various attacks, and can at the same time be used to build a low-cost synchronous stream cipher for hardware applications.