GSM Security

Most GSM providers use a version of COMP128 for both the A3 authentication algorithm and the A8 key generation algorithm.

Ian Goldberg and David Wagner of the University of California at Berkeley demonstrated that all A8 implementations they looked at, including the few that did not use COMP128, were deliberately weakened. The A8 algorithm takes a 64-bit key, but ten key bits were set to zero. The attack on the A8 algorithm demonstrated by Goldberg and Wagner takes just 2^19 queries to the GSM SIM *Subscriber Identity Module), which takes roughly 8 hours.

Josyula R. Rao, Pankaj Rohatgi and Helmut Scherzer of IBM and Stephane Tinguely of the Swiss Federal Institute of Technology have published Partitioning Attacks: Or How to Rapidly Clone Some GSM Cards which shows a method by which COMP128 can be broken in less than a minute.

The COMP128-2 and COMP128-3 algorithms have been developed to address the security issues of COMP128-1. COMP128-2 and COMP128-3 are secret algorithms which have not been subject to cryptanalysis. COMP128-3 fixes the issue where 10 bits of the Session Key (Kc) were set to zero.

GSM network operators are slowly migrating from COMP128 (also known as COMP128-1) to COMP28-2 or COMP128-3. Because the A3 and A8 algorithms are stored in the Subscriber Identity Module, this requires changing the GSM subscribers SIM cards.

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