New approaches to the design of self-synchronizing stream ciphers

Abstract

Self-synchronizing stream ciphers (SSSC) are a commonly used encryption technique for channels with low bit error rate but for which bit synchronization can present a problem. Most presently used such ciphers are based on a block cipher (e.g. DES) in 1-bit cipher feedback mode. In this paper, several alternative design approaches for SSSCs are proposed that are superior to the design based on a block cipher with respect to encryption speed and potentially also with respect to security. A method for combining several SSSCs is presented that allows to prove that the combined SSSC is at least as secure as any of the component ciphers. The problem of designing SSSCs is contrasted with the problem of designing conventional synchronous additive stream ciphers and it is shown that different security criteria must be applied. Furthermore, an efficient algorithm is presented for finding a function of low degree that approximates a given Boolean function, if such an approximation exists. Its significance for the cryptographic security of SSSCs and its applications in coding theory are discussed.