Edit Probability Correlation Attack on the Alternating Step Generator

Abstract

Given an edit transformation defined by the stop/go clocking in the alternating step generator, an edit probability for two binary strings of appropriate lengths is proposed. corresponds alternating An efficient recursive algorithm for the edit probability computation is derived. It is pointed out how this edit probability can be used to mount a statistically optimal correlation attack on each of the clock-controlled shift registers individually. By estimating the underlying false alarm probability, it is shown that the minimum output sequence length required to be known for a successful attack is linear in the length of the respective shift register. This is illustrated by experimental attacks on relatively short shift registers.