How to speed-up your NLFSR-based stream cipher

Abstract

Non-linear feedback shift registers (NLFSRs) have been proposed as an alternative to linear feedback shift registers (LFSRs) for generating pseudo-random sequences for stream ciphers. Conventional NLFSRs use the Fibonacci configuration in which the feedback is applied to the last bit only. In this paper, we show how to transform a Fibonacci NLFSR into an equivalent NLFSR in the Galois configuration, in which the feedback can be applied to every bit. Such a transformation can potentially reduce the depth of the circuits implementing feedback functions, thus decreasing the propagation time and increasing the throughput.