algorithm 5 (A5)

With GSM, data is encrypted on the user and traffic channels to maintain confidentiality. The data is encrypted on the air interface between the mobile station and the base station using the A5 algorithm, a Europe-wide standardized algorithm that is only accessible to manufacturers.

The A5 algorithm is used in the cell phone, and a command over the network tells it which user data or signaling data is encrypted.

Encryption using the A5 algorithm is symmetric encryption using the cipher key( Kc) in the mobile station( MS) and the mobile switching center ( MSC). In this process, the user and signaling data are encrypted and decrypted using the same session key and algorithm.

According to the procedure, the A5 algorithm generates a random bit stream from a 64-bit key and the 22-bit TDMA frame number, which is linked to the GSM data frame.