Synchronization Fault Cryptanalysis for Breaking A5/1

A5/1 pseudo-random bit generator, known from GSM networks, potentially might be used for different purposes, such as secret hiding during cryptographic hardware testing, stream encryption in piconets and others. The main advantages of A5/1 are low cost and a fixed output ratio. We show that a hardware implementation of A5/1 and similar constructions must be quite careful. It faces a danger of a new kind of attack, which significantly reduces possible keyspace, allowing full recovery of A5/1 internal registers' content. We use “fault analysis” strategy: we disturb the A5/1 encrypting device (namely, clocking of the LFSR registers) so it produces an incorrect keystream, and through error analysis we deduce the state of the internal registers. If a secret material is used to initialize the generator, like in GSM, this may enable recovering the secret. The attack is based on unique properties of the clocking scheme used by A5/1, which is the basic security component of this construction. The computations that have to be performed in our attack are about 100 times faster than in the cases of the previous fault-less cryptanalysis methods.