Cryptanalysis of Achterbahn-128/80

A key recovery attack on the stream cipher Achterbahn128/80, a cipher in the second phase of eSTREAM, is given. The key observation is a high dependency between some input bits to the Boolean combining function generating the keystream. It results in the first known attacks on both the 128-bit and the 80-bit variants of the cipher. The amount of keystream bits required in the attacks is less than 2, the maximum frame length.