Unified Formal Model for Synchronous and Self-Synchronizing Stream Ciphers

Abstract

Stream ciphers encrypt individual characters of a plaintext message one at a time, using an encryption scheme which varies with time. Stream ciphers are commonly classified as synchronous or self-synchronizing. As a general rule, a stream cipher consists of two parts: a key schedule algorithm and a keystream generator. Most research and cryptanalyses of stream ciphers are carried out separately for the two parts, but it is still important, mainly for security reasons, to consider integration aspects and implications between the key schedule algorithm and the keystream generator. Therefore, the paper proposes an unified formal model for synchronous and self-synchronizing stream cipher algorithms from which are derived formal models for both types of stream ciphers.