A Real-World Attack Breaking A5/1 within Hours

Abstract. In this paper we present a real-world hardware-assisted attack on the wellknown 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 [BB06,BD00,BSW01,EJ03,Gol97,MJB05,PS00]. 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 [KPP+ 06]. 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 an...

• Type: Conference
• Year: 2008
• Where: CHES
• Authors: Timo Gendrullis, Martin Novotný, Andy Rupp
• Keywords: CHES 2008, Cryptology, Real-world Hardware-assisted Attack, Special-purpose Hardware, Wellknown A5/1 Stream