[ Changed 25th January 1997 ]
There have been a number of recent developments in the design of clock-controlled shift registers, where feedback shift registers are stepped irregularly in an attempt to break up their linearity while maintaining good statistical properties. Among recent developments are the shrinking generator, and the "alleged A5" cipher. At the same time there have been a number of cryptanalytic attacks by Menicocci, by Zivkovic and by Golic, amongst others. I shall talk about basic generators such as the step-1/2 and shrinking generators and the attacks proposed by Zivkovic ("embedding") and Golic ("linearisation"). Then I shall consider the stop-go Gollmann cascades and the attacks proposed by Menicocci and by Park et al. (Here the clocking sequences are XOR'd with the outputs from the clocked registers.) The attacks proposed by Zivkovic have been extended to step-1/2 Gollmann cascades, and have been found equivalent to the "lock-in" attacks discovered earlier.
One of my big points is that most of these attacks are easily parried.
If time permits I shall mention some systems which have not yet been seriously attacked, in the hope of encouraging someone to have a go. Among these are systems with mutual clock-control, for which no very rigorous theory is known.