Edit Probability Correlation Attacks on Stop/ Go Clocked Keystream Generators

Abstract. Divide-and-conquer correlation attacks on the alternating step generator, the bilateral stop/ go generator, and the alleged A5 generator are proposed. They are based on appropriately defined edit probabilities incorporating the stop/ go clocking in these generators. Recursive algorithms for the efficient computation of the edit probabilities are derived. It is shown how the edit probabilities can be used to mount statistically optimal correlation attacks on the corresponding subsets of stop/ go clocked shift registers. By using a statistical hypothesis testing method for estimating the underlying false alarm probability, it is argued that the minimum output sequence length required to be known for a successful attack is linear in the total length of the targeted shift registers. This is illustrated by experimental attacks on the alternating step generator and the bilateral stop/ go generator composed of relatively short shift registers.