Cellular Network Infrastructure » IMSI Pseudonymization

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

== Introduction

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

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

via Short Message Service (SMS), then a SIM applet overwrites the IMSI of the

SIM with the new value. 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

subscriber, after the subscriber was added with the same data to the HLR/HSS.

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. This document uses 2G CS Location

Updating for reference, as in <<figure-imsi-regular>>.

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

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"];

  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"];

  BSC  => MSC [label="Ciphering Mode Complete"];

  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"];

----

The HLR must store up to two pseudonymous IMSIs (imsi_pseudo) and their related

counters (imsi_pseudo_i) per subscriber. Each subscriber initially has one

pseudonymous IMSI allocated. A subscriber has two valid pseudonymous IMSIs

only during the transition phase from the old pseudonymous IMSI to the new one.

The amount of available IMSIs must be higher than the amount of subscribers

registered with the HLR. If the amount of available IMSIs is too short, the HLR

can delay assigning new pseudonymous IMSIs until new IMSIs are available again.

.Examples for additional subscriber data in HLR

|===

| Subscriber ID | imsi_pseudo | imsi_pseudo_i

// example IMSIs taken from Wikipedia

| 123

| 310150123456789

| 1

| 234

| 502130123456789

| 1

| 460001357924680

| 2

|===

that the HLR controls. The pseudonymous IMSI must not be used by any subscriber

as pseudonymous IMSI yet, but may be the real IMSI of a subscriber.

The counter imsi_pseudo_i indicates how often a subscriber's pseudonymous IMSI

subscriber. When allocating a new pseudonymous IMSI for the same subscriber,

the new imsi_pseudo_i value is increased by 1. The counter is used by the SIM

pseudonymous IMSI.

=== SIM Provisioning

IMSI is stored as IMSI on the subscriber's SIM instead of the real IMSI.

==== SIM applet

The SIM is provisioned with a SIM applet, which is able to change the IMSI once

the next pseudonymous IMSI arrives from the HLR. A reference implementation is

provided in <<reference-src>>.

The SIM applet registers to a suitable SMS trigger (3GPP TS 03.19, Section

6.2). When an SMS from the HLR in the format of <<sms-format>> arrives, the

applet must verify that the SMS is not outdated by comparing imsi_pseudo_i from

the SMS with the last imsi_pseudo_i that was used when changing the IMSI

(initially 1 as in <<hlr-imsi-pseudo-i>>). The new value must be higher,

otherwise the SMS should not be processed further.

The SIM applet registers a timer with min_sleep_time from the SMS. When the

timer triggers, the IMSI of the SIM is overwritten with the new pseudonymous

IMSI, the TMSI and GSM Ciphering key Kc (3GPP TS 31.102, Section 4.4.3.1) are

invalidated. The current imsi_pseudo_i value is stored to compare it with the

next SMS. Afterwards, the EF~IMSI~ changing procedure in 3GPP TS 11.14, Section

6.4.7.1 is executed to apply the new IMSI.

// FIXME: do we need to enforce the LU now, with an arbitrary CM Service

// Request, or would this only be necessary for Osmocom? (OS#4404)

All IMSI Pseudonymization related changes to Process Update_Location_HLR

are outlined in this section are expected to be enabled or disabled entirely

where IMSI pseudonymization is implemented.

["mscgen"]

----

msc {

  hscale="1.75";

  MSC [label="MSC/VLR"], SMSC [label="SMS-SC"], HLR [label="HLR"];

  MSC   => HLR  [label="Update Location Request"];

  --- [label="If new pseudonymous IMSI was used: deallocate and cancel old pseudonymous IMSI"];

  MSC   => HLR  [label="Cancel Location Result"];

  ---;

  MSC   => HLR  [label="Insert Subscriber Data Result"];

  ...;

  |||;

  SMSC box SMSC [label="Deliver SMS to ME"];

}

----

by the IMSI, but by the pseudonymous IMSI instead. Unless the subscriber has

two pseudonymous IMSI allocated and used the old pseudonymous IMSI in the

Update Location Request, this is followed by the existing logic to continue with

Insert Subscriber Data Request.

===== Update Location Request With New Pseudonymous IMSI

If the subscriber has two pseudonymous IMSIs allocated, and the newer entry was

used (higher imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>), this section applies.

The older pseudonymous IMSI is deallocated in the HLR. This is done as early

as possible, so the timeframe where two pseudonymous IMSI are allocated for one

subscriber is short.

A Cancel Location Request with the old pseudonymous IMSI is sent to the VLR, so

the conflicting subscriber entry with the old pseudonymous IMSI is deleted from

the VLR. Receiving a Cancel Location Result is followed by the existing logic

to continue with Insert Subscriber Data Request.

===== Update Location Request With Old Pseudonymous IMSI

If the subscriber has two pseudonymous IMSIs allocated, and the older entry was

used (lower imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>), the newer entry is _not_

deallocated. This could lock out the subscriber from the network if the SMS

with the new pseudonymous IMSI arrives with a delay.

==== Insert Subscriber Data Result

Next_Pseudo_IMSI_Timer starts.

==== Next_Pseudo_IMSI_Timer Expires

available IMSIs in the HLR is high enough, a second pseudonymous IMSI and

related imsi_pseudo_i gets allocated for the subscriber (as described in

<<hlr-imsi-pseudo-storage>>).

If the subscriber still has only one pseudonymous IMSI, because not enough

IMSIs were available in the HLR, the process is aborted here and no SMS with

a next pseudonymous IMSI is sent to the subscriber. The subscriber will get a

new pseudonymous IMSI during the next Location Updating Procedure, if the HLR

has enough IMSIs available at that point.

An SMS is sent to the SMS - Service Centre (SMS-SC) with the newer pseudonymous

IMSI (higher imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>) and related

imsi_pseudo_i value.

[[sms-format]]

==== SMS Format

* min_sleep_time

* imsi_pseudo

* imsi_pseudo_i

=== Next Pseudonymous IMSI SMS is Lost

=== SMS Arrives Late

// FIXME: OS#4486

[[reference-src]]

=== ATT = 0

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