Espresso: A Stream Cipher for 5G Wireless Communication Systems

The demand for more efficient ciphers is a likely to sharpen with new generation of products and applications. Previous cipher designs typically focused on optimizing only one of the two parameters - hardware size or speed, for a given security level. In this paper, we present a methodology for designing a class of stream ciphers which takes into account both parameters simultaneously. We combine the advantage of the Galois configuration of NLFSRs, short propagation delay, with the advantage of the Fibonacci configuration of NLFSRs, which can be analyzed formally. According to our analysis, the presented stream cipher Espresso is the fastest among the ciphers below 1500 GE, including Grain-128 and Trivium

A Method for Generating Full Cycles by a Composition of NLFSRs

Abstract. Non-Linear Feedback Shift Registers (NLFSR) are a generalization of Linear Feedback Shift Registers (LFSRs) in which a current state is a nonlinear function of the previous state. The interest in NLFSRs is motivated by their ability to generate pseudo-random sequences which are usually hard to break with existing cryptanalytic methods. However, it is still not known how to construct large n-stage NLFSRs which generate full cycles of 2 n possible states. This paper presents a method for generating full cycles by a composition of NLFSRs. First, we show that an n ∗ k-stage register with period O(2 2n) can be constructed from k n-stage NLFSRs by adding to their feedback functions a logic block of size O(n ∗ k). This logic block implements Boolean functions representing the set of pairs of states whose successors have to be exchanged in order to join cycles. Then, we show how to join all cycles into one by using one more logic block of size O(n ∗ k 2) and an extra time step. The presented method is feasible for generating very large full cycles.