Cryptanalysis of the class of maximum period galois NLFSR-based stream ciphers

Abstract

Espresso cipher is designed targeting 5G wireless communication systems. To achieve high efficiency, a maximum period Galois NLFSR is used as the only building block. The Galois NLFSR is constructed by a scalable method which converts a maximum LFSR to a Galois NLFSR. Based on this method, a new class of stream ciphers, namely maximum period Galois NLFSR-based stream ciphers can be designed. However, we identify a conditional equivalence problem in the design method and adopt the Type-II-to-Fibonacci transformation algorithm. We apply the algorithm to the Espresso cipher and successfully transform the Galois NLFSR to a Fibonacci LFSR with a nonlinear output function. The Espresso cipher is t..