Cryptanalysis of Achterbahn

We present several attacks against the Achterbahn stream cipher, which was proposed to the eSTREAM competition. We can break the reduced and the full version with complexity of 2 and 2 steps. Extensions of our attacks are also described to break modified versions of the Achterbahn stream cipher, which were proposed following the publication of preliminary cryptanalysis results. These attacks highlight some problems in the design principle of Achterbahn, i.e., combining the outputs of several nonlinear (but small) shift registers using a nonlinear (but rather sparse) output function.