A new family of stream ciphers based on cascaded small s-boxes

Abstract

Many stream cipher designs based on linear feedback shift registers (LFSRs) with non-linear combining functions are susceptible to various versions of the correlation attack. In this paper we propose a new model for stream ciphers which does not make use of LFSRs. Instead, our stream ciphers are based on a cascade of small substitution boxes (s-boxes). Like the RC4 stream cipher designed by Ron Rivest, the cascade stream cipher makes use of evolving s-boxes and pointers. However, instead of using one large s-box we employ a cascade of several small s-boxes. Two parameters of this family of stream ciphers are the size of the individual s-boxes and the length of the cascade. If we use n-bit s-boxes, then each output of the stream cipher is an n-bit block. By way of example, a cascade consisting of 16 2-bit s-boxes would have an effective key length which is adequate for most practical applications. The number of s-boxes in the cascade can be increased if we desire more security. Our studies to date indicate that the cascade cipher has good statistical properties. The new cascade stream cipher requires relatively little storage and executes efficiently in both hardware and software.