Reduced complexity correlation attacks on two clock-controlled generators

Abstract

The Shrinking Generator and the Alternating Step Generator are two of the most well known clock-controlled stream ciphers. We con- sider correlation attacks on these two generators, based on an identified relation to the decoding problem for the deletion channel and the ins- ertion channel, respectively. Several ways of reducing the decoding com- plexity are proposed and investigated, resulting in “divide-and-conquer” attacks on the two generators having considerably lower complexity than previously known attacks.