A Real-World Attack Breaking A5/1 within Hours
- Cite this paper as:
- Gendrullis T., Novotný M., Rupp A. (2008) A Real-World Attack Breaking A5/1 within Hours. In: Oswald E., Rohatgi P. (eds) Cryptographic Hardware and Embedded Systems – CHES 2008. CHES 2008. Lecture Notes in Computer Science, vol 5154. Springer, Berlin, Heidelberg
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  is running on an existing special-purpose hardware device, called COPACOBANA . 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.