A Real-World Attack Breaking A5/1 within Hours

Abstract

In this paper we present a real-world hardware-assisted attack on the well-known A5/1 stream cipher which is (still) used to secure GSM communication in most countries all over the world. During the last ten years A5/1 has been intensively analyzed [1,2,3,4,5,6,7]. However, most of the proposed attacks are just of theoretical interest since they lack from practicability — due to strong preconditions, high computational demands and/or huge storage requirements — or have never been fully implemented.

In contrast to these attacks, our attack which is based on the work by Keller and Seitz [8] is running on an existing special-purpose hardware device, called COPACOBANA [9]. With the knowledge of only 64 bits of keystream the machine is able to reveal the corresponding internal 64-bit state of the cipher in about 6 hours on average. We provide a detailed description of our attack architecture as well as implementation results.