Cryptanalysis of Achterbahn-Version 2

In English Achterbahn is one of the stream cipher proposals in the eSTREAM project. After the first version had been successfully cryptanalyzed, the second version, denoted Achterbahn-Version 2, was proposed. This paper demonstrates an attack on this second version. In the attack, a quadratic approximation of the output function is considered. The attack uses less keystream bits than the upper limit given by the designers and the computational complexity is significantly less than exhaustive key search.