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The algorithm used in GSM mobile phones. Alex Biryukov and Adi Shamir have shown this algorithm to be insecure (9/12/99).

“A5/1 is the strong version of the encryption algorithm used by about 100 million GSM customers in Europe to protect the over-the-air privacy of their cellular voice and data communication. The best published attacks against it require between 240 and 245 steps. This level of security makes it vulnerable to hardware-based attacks by large organizations, but not to software-based attacks on multiple targets by hackers.

“In this paper we describe a new attack on A5/1, which is based on subtle flaws in the tap structure of the registers, their noninvertible clocking mechanism, and their frequent resets. The attack can find the key in less than a second on a single PC with 128 MB RAM and two 73 GB hard disks, by analysing the output of the A5/1 algorithm in the first two minutes of the conversation...”

Real Time Cryptanalysis of the Alleged A5/1 on a PC (preliminary draft) Alex Biryukov Adi Shamir
December 9, 1999