A TIMSI, or Temporary International Mobile Subscriber Identity, number is a random number that is created from a telecommunications subscriber’s IMSI number in order to provide additional security. While an IMSI is unique to each subscriber and is a permanent number that can be used to track a subscriber or interfere with his/her services, a TIMSI is often re-negotiated by the telecommunications company the subscriber is associated with and can only be stolen if an eavesdropper intercepts the initial GSM transmission when the TIMSI is negotiated.
How TIMSI Works
A TIMSI is created by using an algorithm and a user’s TIMSI, although the exact algorithm is not publicly known for security reasons. While a user’s SIM card knows what its TIMSI is, no eavesdropper can identify which phone a TIMSI is coming from by intercepting the data. By communicating with a TIMSI instead of an IMSI, a cell phone and its respective tower are able to secure data and prevent eavesdroppers from identifying a subscriber or his/her location.
TIMSIs are used whenever a cell phone or cell phone tower needs to transmit a subscriber’s data over-the-air, where data can be intercepted by anyone with the proper equipment. Although a phone knows both its IMSI and TIMSI, it only includes the TIMSI with data broadcasts. When a cell phone tower receives information from a particular phone, it converts the phone’s TIMSI into its valid IMSI in order to identify the phone. When the cell phone tower needs to send information back to the cell phone, it reconverts the phone’s IMSI to its pre-negotiated TIMSI.
TIMSIs are advantageous because they keep a cell phone’s IMSI secure from potential eavesdroppers. TIMSIs are pre-negotiated and do not contain any reference to the user’s IMSI. Additionally, TIMSIs are frequently re-negotiated in order to keep security up-to-date and prevent any possible theft of a user’s IMSI.