GSM Security And Encryption
Abstract
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.
Description of GSM Security And Encryption
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.
Block Ciphers
As the name suggests, block ciphers encrypt or decrypt data in blocks or groups of bits. DES uses a 56-bit key and processes data in 64- bit blocks, producing 64-bits of encrypted data for 64-bits of input, and vice-versa. Block algorithms are further characterized by their mode of operation, such as electronic code book (ECB), cipher block chaining (CBC) and cipher feedback (CFB). CBC and CFB are examples of modes of operation where the encryption of successive blocks is dependent on the output of one or more previous encryptions. These modes are desirable because they break up the one-to-one correspondence between ciphertext blocks and plaintext blocks (as in ECB mode). Block ciphers may even be implemented as a component of a stream cipher.
Public Key Algorithms
Public key algorithms are characterized by two keys, a public and private key, which perform complementary functions. Public and private keys exist in pairs and ideally have the property that the private key may not be deduced from the public key, which allows the public key to be openly distributed. Data encrypted with a given public key may only be decrypted with the corresponding private key, and vice versa.
