Cryptanalysis of Clock Controlled Shift Registers

Clock control is one of the mechanisms employed to introduce non-linearity into key stream generators built from linear feedback shift registers. The earliest devices were built from stop-and-go registers while the most recent example, the Shrinking Generator proposed in [2], has an irregular clocking scheme where the number of steps between successive outputs is linked to the length of zero-runs in the clocking sequence. Clock controlled shift registers can also be regarded as generalized rotor machines. Thus cascades of clock controlled shift registers can be viewed as the successors of mechanical rotor machines. There is a sufficient body of knowledge to derive analytical results on the period, linear complexity, and statistical properties of such devices [4]. In comparison, little has been published on algorithms for 'practical' cryptanalysis. This contribution attempts to give a survey of the techniques published so far and to put forward open questions and challenges for further work in this area. The presentation will try to convey the main features of the different techniques without going into too much technical detail. The reader will find more information in the references quoted but will notice a general lack of empirical data on the practical efficiency of these methods.