IMSI
International Mobile Subscriber Identity
Internationale Mobilfunk-Teilnehmer-Kennung
The IMSI identifies the subscriber (the SIM) in the mobile network and is thus the counterpart to the device-side International Mobile Equipment Identity (IMEI). It is stored on the SIM and consists of a country and network code plus a subscriber-specific number. Because of its sensitivity, the network replaces it with temporary identifiers wherever possible.
History & facts. The IMSI is structured into Mobile Country Code, Mobile Network Code and a subscriber-specific identifier. It is master data of network registration; to avoid transmitting it constantly over the air interface, networks use temporary identities. Notorious are so-called IMSI catchers, which can induce devices to reveal their IMSI by impersonating a base station.
Outlook & recommendation. In data-protection terms the IMSI is a highly sensitive identifier (PII) that makes a person uniquely traceable via their connection. Newer mobile generations encrypt the permanent subscriber identity more strongly to hinder catcher attacks. For forensic and investigation-related work: handling IMSI data is subject to strict legal limits — it is not freely usable incidental data.