Flaws in the initialisation process of stream ciphers

Abstract

The initialisation process is a key component in modern stream cipher design. A well-designed initialisation process should not reveal any information about the secret key, or possess properties that may help to facilitate attacks. This paper analyses the initialisation processes of shift register based stream ciphers and identifies four flaws which lead to compression, state convergence, the existence of slid pairs and possible weak Key-IV combinations. These flaws are illustrated using the A5/1 stream cipher as a case study. We also provide some design recommendations for the intialisation process in stream ciphers, to overcome these and other flaws.