Cellular Network Infrastructure » IMSI Pseudonymization

= Specification for IMSI Pseudonymization on the Radio Interface for 2G and Above

== Introduction

A long-standing issue in the 3GPP specifications is, that mobile phones and

other mobile equipment (ME) have to send the International Mobile Subscriber

Identity (IMSI) unencrypted over the air. Each IMSI is uniquely identifying the

person who bought the associated Subscriber Identity Module (SIM) used in the

ME. Therefore most people can be uniquely identified by recording the IMSI that

their ME is sending. Efforts are made in the 2G and above specifications to

send the IMSI less often, and where possible use the Temporary Mobile

Subscriber Identity (TMSI) instead.

But this is not enough. So-called IMSI catchers were invented and are used to

not only record IMSIs when they have to be sent. But also to force ME to send

their IMSI by immitating a Base Transceiver Station (BTS). IMSI catchers have

become small and affordable, even criminals actors without much budget can use

them to track anybody with a mobile phone.

The solution presented in this document is to periodically change the IMSI of

the ME to a new pseudonymous IMSI allocated by the Home Location Register (HLR)

or Home Subscriber Service (HSS). The only component that needs to be changed

in the network besides the SIM is the HLR/HSS, therefore it should be possible

for a Mobile Virtual Network Operator (MVNO) to deploy this privacy

enhancement.

== Location Update

=== Regular

=== With Pseudonymous IMSI

== Implementation Notes

=== Source Code for Reference Implementation

=== Warning the User if the IMSI Does Not Change

=== End to End Encryption of SMS

=== User-configurable Minimum Duration Between IMSI Changes