Improvement of the edit distance attack to clock-controlled LFSR-based stream ciphers

The main idea behind this paper is to improve a known plain-text divide-and-conquer attack that consists in guessing the initial state of a Linear Feedback Shift Register component of a keystream generator, and then trying to determine the other variables of the cipher based on the intercepted keystream. While the original attack requires the exhaustive search over the set of all possible initial states of the involved register, this work presents a new and simple heuristic optimization of such an approach that avoids the evaluation of an important number of initial states when launching a constrained edit distance attack on irregularly clocked shift registers.