Differential Cryptanalysis in Stream Ciphers

Abstract

In this paper we present a general framework for the application of the ideas of differential cryptanalysis to stream ciphers. We demonstrate that some differences in the key (or the initial state or the plaintext) are likely to cause predicted differences in the key stream or in the internal state. These stream differences can then be used to analyze the internal state of the cipher and retrieve it efficiently. We apply our proposed ideas to stream ciphers of various designs, e.g., regularly clocked LFSRs, irregularly clocked LFSRs such as A5/1, and permutation-based stream ciphers such as RC4.