A5/1

A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard, and it is one of several implementations of the A5 security protocol. It was developed in 1987, when GSM was not yet considered for use outside Europe, and it was initially kept secret, but its general design leaked in 1994 and it was entirely reverse engineered in 1999 from a GSM telephone by Marc Briceno. A deliberately weakened variant, A5/2, was developed in 1989 for certain export regions. For each burst A5/1 produces a 114-bit keystream that is combined with the data by exclusive or, and it is initialised with a 64-bit key together with a publicly known 22-bit frame number. A number of serious weaknesses in the cipher have been identified.