TMSIs, IMSIs, and BTS Oh My!

It’s been a while since we had anything major to update you with, so here’s something to wet your whistle.

Captured IMSIWhat you are seeing is the Temporary Mobile Subscriber Identity, or TMSI, and the International Mobile Subscriber Identity, or IMSI, for RedQueen’s GSM cell phone attached to the GSM base station on-board the UAV. I’ve blacked out the IMSI information, as it’s essentially the Social Security Number for cell phones. The number 310410 represents USA (310) and the AT&T network(410).

Here are some excerpts from Wikipedia that hopefully help explain:

The TMSI is the identity that is most commonly sent between the mobile and the network. TMSI is randomly assigned by the Visitor Location Register(VLR) to every mobile in the area, the moment it is switched on. The number is local to a location area, and so it has to be updated each time the mobile moves to a new geographical area.

The IMSI is a unique identification associated with all GSM and UMTS network mobile phone users. It is stored as a 64 bit field in the SIM inside the phone and is sent by the phone to the network. It is also used for acquiring other details of the mobile in the Home Location Register (HLR) or as locally copied in the Visitor Location Register(VLR). To prevent eavesdroppers identifying and tracking the subscriber on the radio interface, the IMSI is sent as rarely as possible and a randomly-generated TMSI is sent instead.

 

WhiteQueen

About WhiteQueen

An avid radio control enthusiast and a senior security engineer supporting the U.S. Government. He has had a 20 year Information Technology career including programming, Enterprise Administration, and Information Security. Hobbies include hiking, SCUBA diving, R/C, computers and electronics, as well as a penchant for voiding warranties.
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