Ciphering is one of the security procedures designed to protect the subscriber identity and data. It is an optional procedure in GSM. When ciphering is active, all information exchanged between the mobile and the network on the dedicated radio channels is encrypted. The key previously set between the network and the MS is used to encipher and to decipher the encrypted information.
During the ciphering procedure the ciphering key Kc is set between the network and the MS. Ciphering is initiated after the ciphering key is set on the dedicated signalling channel (SDCCH or FACCH).
The GSM uses stream ciphering, which means that the data flow transmitted on the SDCCH/TCH is the binary bit by bit addition of the user data flow to a ciphering bit stream.
Why it is used
In the days of AMPS phones, it was aquite easy to acquire IMEI numbers right off the air, clone them, and start making calls which are charged to the original owner. This lead to authentication, and in GSM, the ID details from the SIM, not being transmitted over the air.
Algorithms for Ciphering
3. A5/0 (no ciphering)