= 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, by using the Temporary Mobile Subscriber Identity
(TMSI) where possible.

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

== Summary of Existing Location Updating Procedures in RAN and CN

The SIM is provisioned with the IMSI (3GPP TS 23.008 section 2.1.9) and
cryptographic keys, that it uses to authenticate with the network. In the
Remote Access Network (RAN), the IMSI is sent over the air interface and then
transmitted to the Core Network (CN), where it is validated by the HLR/HSS.
The involved components vary by the generation of the network and whether the
SIM is attempting a Circuit Switched (CS) or Packet Switched (PS) connection.
But the principle is the same and looks like <<figure-imsi-regular>> for 2G CS
Location Updating with IMSI.

The IMSI is transmitted in the Location Updating Request from ME. The VLR
needs an authentication challenge specific to the secret keys on the SIM to
authenticate the SIM, and looks the authentication challenges up by the IMSI.
If the VLR does not have any more authentication challenges for the IMSI (as it
happens when the VLR sees the IMSI for the first time), the VLR requests new
authentication challenges from the HLR. Then the HLR verifies that the IMSI is
known and, if it is unknown, sends back an error that will terminate the
Location Updating procedure.

After the VLR found the authentication challenge, it authenticates the SIM, and
performs a Classmark Enquiry and Physical Channel Reconfiguration. Then the VLR
has the required information to finish the Location Updating, and continues
with an Update Location Request procedure with the HLR. Afterwards, the VLR
assigns a new TMSI with the Location Updating Accept, which is acknowledged by
the TMSI Reallocation Complete. In following Location Updates with the same
MSC, the ME sends the TMSI instead of the IMSI in the Location Updating
Request.

[[figure-imsi-regular]]
.Location Updating in 2G CS with IMSI
["mscgen"]
----
msc {
  hscale="1.75";
  ME [label="ME"], BTS [label="BTS"], BSC [label="BSC"], MSC [label="MSC/VLR"],
  HLR [label="HLR"];

  // BTS <=> BSC: RSL
  // BSC <=> MSC: BSSAP, RNSAP
  // MSC <=> HLR: MAP (process Update_Location_HLR, 3GPP TS 29.002)

  ME   => BTS [label="Location Updating Request"];
  BTS  => BSC [label="Location Updating Request"];
  BSC  => MSC [label="Location Updating Request"];

  --- [label="VLR requests new authentication challenges for this IMSI if necessary"];
  MSC  => HLR [label="Send Auth Info Request"];
  MSC <=  HLR [label="Send Auth Info Result"];
  ---;

  BSC <=  MSC [label="Authentication Request"];
  BTS <=  BSC [label="Authentication Request"];
  ME  <=  BTS [label="Authentication Request"];
  ME   => BTS [label="Authentication Response"];
  BTS  => BSC [label="Authentication Response"];
  BSC  => MSC [label="Authentication Response"];
  BSC <=  MSC [label="Classmark Enquiry"];
  BTS <=  BSC [label="Classmark Enquiry"];
  ME  <=  BTS [label="Classmark Enquiry"];
  ME   => BTS [label="Classmark Change"];
  BTS  => BSC [label="Classmark Change"];
  BSC  => MSC [label="Classmark Update"];
  BSC <=  MSC [label="Physical Channel Reconfiguration"];
  BTS <=  BSC [label="Ciphering Mode Command"];
  ME  <=  BTS [label="Ciphering Mode Command"];
  ME  =>  BTS [label="Ciphering Mode Complete"];
  BTS  => BSC [label="Ciphering Mode Complete"];
  BSC  => MSC [label="Ciphering Mode Complete"];

  MSC  => HLR [label="Update Location Request"];
  MSC <=  HLR [label="Insert Subscriber Data Request"];
  MSC  => HLR [label="Insert Subscriber Data Result"];
  MSC <=  HLR [label="Update Location Result"];

  BSC <=  MSC [label="Location Updating Accept"];
  BTS <=  BSC [label="Location Updating Accept"];
  ME  <=  BTS [label="Location Updating Accept"];
  ME   => BTS [label="TMSI Reallocation Complete"];
  BTS  => BSC [label="TMSI Reallocation Complete"];
  BSC  => MSC [label="TMSI Reallocation Complete"];
}
----


== Required Changes
=== SIM Provisioning
=== Successful Location Update With Pseudonymous IMSI
=== Next Pseudonymous IMSI Arrives Via SMS

== Error Scenarios
=== Next Pseudonymous IMSI SMS is Lost
=== SMS Arrives Late

== Reference Implementation with Source Code

== Recommendations for Real-World Implementations
=== ATT = 0
=== End to End Encryption of SMS
=== Warning the User if the IMSI Does Not Change
=== User-configurable Minimum Duration Between IMSI Changes

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