Cryptology ePrint Archive: Report 2008/147

A Real-World Attack Breaking A5/1 within Hours

Timo Gendrullis and Martin Novotny and Andy Rupp

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. 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 — and have never been fully implemented.

In contrast to these attacks, our attack which is based on the work by Keller and Seitz [KS01] 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 7 hours on average. Besides providing a detailed description of our attack architecture as well as implementation results, we propose and analyze an optimization that leads again to an improvement of about 16% in computation time.

Category / Keywords: A5/1, GSM, special-purpose hardware, COPACOBANA

Date: received 31 Mar 2008

Contact author: arupp at crypto rub de

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Version: 20080401:022941 (All versions of this report)

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