Experimental Analysis of Guess-and-Determine Attacks on Clock-Controlled Stream Ciphers

Abstract

Guess-and-Determine (GD) attacks have recently been proposed for the effective analysis of word-oriented stream ciphers. This paper discusses GD attacks on clock-controlled stream ciphers, which use irregular clocking for a non-linear function. The main focus is the analysis of irregular clocking for GD attacks. We propose GD attacks on a typical clock-controlled stream cipher AA5, and calculate the process complexity of our proposed GD attacks. In the attacks, we assume that the clocking of linear feedback shift registers (LFSRs) is truly random. An important consideration affecting the practicality of these attacks is the question of whether these assumptions are realistic. Because in practice, the clocking is determined by the internal states. We implement miniature ciphers to evaluate the proposed attacks, and show that they are applicable. We also apply the GD attacks to other clock controlled stream ciphers and compare them. Finally, we discuss some properties of GD attacks on clock-controlled stream ciphers and the effectiveness of the clock controllers. Our research results contain information that are useful in the design of clock-controlled stream ciphers.