= 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