GSM Encryption Algorithmn

GSM:

A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard.

A5/1 is used in Europe and the United States. A5/2 was a deliberate weakening of the algorithm for certain export regions.[1] A5/1 was developed in 1987, when GSM was not yet considered for use outside Europe, and A5/2 was developed in 1989. Though both were initially kept secret, the general design was leaked in 1994

A GSM transmission is organised as sequences of bursts. In a typical channel and in one direction, one burst is sent every 4.615 milliseconds and contains 114 bits available for information. A5/1 is used to produce for each burst a 114 bit sequence of keystream which is XORed with the 114 bits prior to modulation. A5/1 is initialised using a 64-bit key together with a publicly-known 22-bit frame number.When operating in GPRS / EDGE mode, higher bandwidth radio modulation allows for larger 348 bits frames, and A5/3 is then used in a stream cipher mode to maintain confidentiality.