Cellular Network Infrastructure » IMSI Pseudonymization

send the IMSI less often, by using the Temporary Mobile Subscriber Identity

(TMSI) where possible.

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

== Location Updating

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"];

}

----

=== With IMSI Pseudonymization

==== SIM Provisioning

==== Successful Location Update With Pseudonymous IMSI

==== Next Pseudonymous IMSI Arrives Via SMS

==== Error Handling

===== SMS is Lost

===== SMS Arrives Late

=== ATT = 0 required