What is a TIMSI?

The TIMSI (Temporary IMSI) is a pseudo-random number generated from the IMSI (International Mobile Subscriber Identity) number.

The TIMSI is utilized in order to remove the need to transmit the IMSI over-the-air. This helps to keep the IMSI more secure.

To track a GSM user via the IMSI/TIMSI, an eavesdropper must intercept the GSM network communication where the TIMSI is initially negotiated.

In addition, because the TIMSI is periodically renegotiated, the eavesdropper must intercept each additional TIMSI re-negotiation session.

GSM Security FAQ

• How is encryption utilized in GSM?
• What algorithm is utilized for authentication in GSM networks?
• What algorithm is utilized for encryption in GSM networks?
• What algorithm is used for key generation in GSM networks?
• What are the components of a GSM network
• What are Ki, Kc, RAND, and SRES?
• How do Authentication and Key generation work in a GSM network?
• Have the A3 and A8 algorithms been broken?
• Have the A5 algorithms been broken?
• What are locking and unlocking?
• What is an IMEI?
• How can I find my IMEI?
• What is an IMSI?
• What is a TIMSI?
• What are the PIN and PUK?
• How do I change my PIN and PUK?
• How do I use CallerID?
• What is a Subscriber Identity Module (SIM)?