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A Real-World Attack Breaking A5/1 within Hours
T. Gendrullis, M. Novotny, A. Rupp
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| A5/1 is a stream cipher used for securing GSM communication in most
countries all over the world. Although it has been intensively
analyzed in the past none of the proposed attacks against A5/1 has
been fully implemented. We present an attack based on the work by
Keller and Seitz which reduces the complexity of a plain brute force
attack significantly by early recognizing contradictions while
guessing the initial state of A5/1.
We implemented our attack on a special purpose hardware device called
COPACOBANA. With the knowledge of only 64 bits of keystream we are now
able to reveal the corresponding internal 64-bit state of the cipher
in about 7 hours on average. Furthermore, we present an architectural
optimization of our hardware design that leads again to an improvement
of about 16% in computation time. |
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