Correlation attacks on clock-controlled shift registers in keystream generators

A concept of constrained many-to-one string edit distances, with or without memory, based on edit operations of deletion and substitution is introduced, and the recursive algorithms for their efficient computation are derived. Their use for correlation attacks on clock-controlled shift register based keystream generators is proposed, the divide and conquer effect is examined, and the generalized reducibility property of functions with memory is pointed out as a design criterion.