GSM

Authentication architecture

Based on a secret key shared between the HLR and the station.

• 128 Ki, stored in the station’s SIM card.

• Can only be used after entering a PIN.

Algorithms (initially not public):

• A3 for authentication.

• A8 for generating a session key.

• A5 for encrypting the communication.

A3 and A8 are implemented by SIM cards.

• Can be freely selected by the operator.

Mobile station authentication

MSC fetches a trio from HLR.

• RAND, SRES, Kc.

• More than one is requested.

HLR generates RAND and corresponding trio using the subscriber’s Ki.

• RAND, random value (128 bits).

• SRES = A3 (Ki, RAND) (32 bits)

• Kc = A8 (Ki, RAND) (64 bits)

Usually, operators use COMP128 for A3/A8.

• Recommended by the GSM Consortium.

• [SRES, Kc] = COMP128 (Ki, RAND).