A Complete divide and conquer attack on the alphal stream cipher

Abstract

Alphal is a stream cipher with a key size of 128 bits. It was proposed as a replacement algorithm for the stream cipher A5 to supply confidentiality over mobile communication systems. Alphal consists of four binary linear feedback shift registers. Previous attacks on Alphal only recover the initial state of the shortest register. In this paper we present a complete divide and conquer attack, starting with an improved attack on the shortest register, and continuing the attack to recover the remaining three registers. Although Alphal is a 128-bit stream cipher, the key can be recovered through this divide and conquer attack with complexity 261, using 35,000 bits of known plaintext. © Springer-Verlag 2004.