GSM Security and Encryption

The motivations for security in cellular telecommunications systems are to secure conversations and signaling data from interception as well as to prevent cellular telephone fraud. With the older analog-based cellular telephone systems such as the Advanced Mobile Phone System (AMPS) and the Total Access Communication System (TACS), it is a relatively simple matter for the radio hobbyist to intercept cellular telephone conversations with a police scanner.
A well-publicized case involved a potentially embarrassing cellular telephone conversation with a member of the British royal family being recorded and released to the media. Another security consideration with cellular telecommunications systems involves identification credentials such as the Electronic Serial Number (ESN), which are transmitted in the clear in analog systems. With more complicated equipment, it is possible to receive the ESN and use it to commit cellular telephone fraud by cloning another cellular phone and placing calls with it. Estimates for cellular fraud in the U.S. in 1993 are as high as $500 million. The procedure wherein the Mobile Station (MS) registers its location with the system is also vulnerable to interception and permits the subscriber s location to be monitored even when a call is not in progress, as evidenced by the recent highly-publicized police pursuit of a famous U.S. athlete.
The security and authentication mechanisms incorporated in GSM make it the most secure mobile communication standard currently available, particularly in comparison to the analog systems described above. Part of the enhanced security of GSM is due to the fact that it is a digital system utilizing a speech coding algorithm, Gaussian Minimum Shift Keying (GMSK) digital modulation, slow frequency hopping, and Time Division Multiple Access (TDMA) time slot architecture. To intercept and reconstruct this signal would require more highly specialized and expensive equipment than a police scanner to perform the reception, synchronization, and decoding of the signal. In addition, the authentication and encryption capabilities discussed in this paper ensure the security of GSM cellular telephone conversations and subscriber identification credentials against even the determined eavesdropper.