Cellular Network Infrastructure » IMSI Pseudonymization

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

== Introduction

=== Protecting the IMSI on the Radio Interface is Desirable

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.

=== Summary of Proposed Solution

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 next pseudonymous IMSI is sent to the SIM

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

enhancement.

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

The subscriber's SIM is provisioned with the IMSI and cryptographic keys of a

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

with Process Update_Location_HLR (3GPP TS 29.002). 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="If necessary: VLR requests new authentication challenges for this IMSI"];

  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"];

  --- [label="Process Update_Location_HLR (3GPP TS 29.002)"];

  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

[[hlr-imsi-pseudo-storage]]

=== Pseudonymous IMSI Storage in the HLR

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

| 234

| 460001357924680

| 2

|===

==== imsi_pseudo

The value for imsi_pseudo is a random choice from the pool of available IMSIs

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.

[[hlr-imsi-pseudo-i]]

==== imsi_pseudo_i

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

was changed. The value is 1 for the first allocated pseudonymous IMSI of a

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

applet to detect and ignore outdated requests related to changing the

pseudonymous IMSI.

=== SIM Provisioning

The HLR is allocating a pseudonymous IMSI for the subscriber. This pseudonymous

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

[[sim-app]]

==== 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 structure of <<sms-structure>> 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)

[[process-update-location-hlr]]

=== Process Update_Location_HLR

All IMSI Pseudonymization related changes to Process Update_Location_HLR

(3GPP TS 29.002) are optional. Deviations from the existing specification that

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

where IMSI pseudonymization is implemented.

[[figure-imsi-pseudo]]

.Process Update_Location_HLR with IMSI pseudonymization changes

["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"];

  HLR  box HLR  [label="Deallocate old pseudonymous IMSI"];

  MSC  <=  HLR  [label="Cancel Location Request"];

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

  ---;

  MSC  <=  HLR  [label="Insert Subscriber Data Request"];

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

  HLR  box HLR  [label="Start Next_Pseudo_IMSI_Timer"];

  MSC  <=  HLR  [label="Update Location Result"];

  MSC  box MSC  [label="Finish Location Updating with ME"],

  HLR  box HLR  [label="Wait for Next_Pseudo_IMSI_Timer expiry"];

  |||;

  ...;

  |||;

  HLR  box HLR  [label="Next_Pseudo_IMSI_Timer expired"];

  HLR  box HLR  [label="\nAllocate new pseudonymous IMSI\nif subscriber has only one allocated\n"];

  SMSC <=  HLR  [label="Next Pseudonymous IMSI SMS"];

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

}

----

==== Update Location Request

When Update Location Request arrives, the HLR does not look up the subscriber

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

When Insert Subscriber Data Result arrives, a subscriber specific

Next_Pseudo_IMSI_Timer starts.

==== Next_Pseudo_IMSI_Timer Expires

If the subscriber has only one pseudonymous IMSI allocated, and the amount of

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-structure]]

==== Next Pseudonymous IMSI SMS Structure

// FIXME

IMPORTANT: This is a draft. The structure is likely to change after the

reference implementation phase.

.Next pseudonymous IMSI SMS structure

[packetdiag]

----

{

	colwidth = 32

	0-31:	 IMSI_PSEUDO_I

	32-63:   MIN_SLEEP_TIME

	64-119:  IMSI_PSEUDO

	120-127: PAD

}

----

IMSI_PSEUDO_I: 32 bits::

See <<hlr-imsi-pseudo-i>>.

MIN_SLEEP_TIME: 32 bits::

Amount of seconds, which the SIM applet should wait before changing to the new

pseudonymous IMSI. Since it is unclear when the SMS will arrive (ME might be

turned off), this is a minimum amount.

IMSI_PSEUDO: 60 bits::

Telephony Binary Coded Decimal (TBCD, 3GPP TS 29.002) version of the next

pseudonymous IMSI.

PAD: 8 bits::

Padding at the end, should be filled with 1111 as in the TBCD specification.

== Error Scenarios

=== Next Pseudonymous IMSI SMS is Lost

If the SMS with the next pseudonymous IMSI does not arrive, the SIM will start

the next Location Updating Procedure with the old pseudonymous IMSI. Because

the HLR has both the old and the new pseudonymous IMSI allocated at this point,

the subscriber is not locked out of the network.

An attacker might block the next pseudonymous IMSI SMS on purpose. Then the

subscriber would have the same pseudonymous IMSI for a long time. A suitable

defense is warning the subscriber if the IMSI does not change

(<<warn-no-imsi-change>>).

=== Next Pseudonymous IMSI SMS arrives out of order

The next pseudonymous IMSI SMS may arrive out of order. Either, because the

network is not able to deliver them in order, or even because an attacker would

perform a replay attack.

If the SMS arrives out of order, the imsi_pseudo_i counter will not be higher

than the value the SIM applet (<<sim-app>>) has stored. Therefore, the applet

will discard the message and the subscriber is not locked out of the network.

// === SMS Arrives Before Timer Expires

// FIXME: OS#4486

[[reference-src]]

== Reference Implementation with Source Code

A reference implementation for the SIM applet (<<sim-app>>) is available in

source code under the Apache-2.0 license at:

https://osmocom.org/projects/imsi-pseudo

The HLR modifications described in <<hlr-imsi-pseudo-storage>> and

<<process-update-location-hlr>> were implemented for reference in OsmoHLR from

the Osmocom project, licensed under AGPL-3.0. Information about the source code

and related branches for IMSI pseudonymization can be found at the above URL as

well.

== Recommendations for Real-World Implementations

=== BCCH SI3: ATT = 0

When changing from one pseudonymous IMSI to the next, it is important that the

ME does not detach from the network. Otherwise it would be trivial for an

attacker to correlate the detach with the attach of the same ME with the next

pseudonymous IMSI.

This is controlled with the ATT flag in the SYSTEM INFORMATION TYPE 3 (SI3)

message on the Broadcast Control Channel (BCCH), see 3GPP TS 44.018 Section

10.5.2.11. It must be set to 0.

// FIXME: verify how it set with operators in germany (OS#4404)

=== End to End Encryption of SMS

When deploying the IMSI pseudonymization, the operator should make sure that

the next pseudonymous IMSI SMS (<<sms-structure>>) cannot be read or modified

by third parties. Otherwise, the next pseudonymous IMSI is leaked, and if the

pseudonymous IMSI in the SMS was changed, the SIM would be locked out of the

network.

The safest way to protect the next pseudonymous IMSI SMS is a layer of end to

end encryption from the HLR to the SIM. It was considered for this

specification, but found to be out of scope.

[[warn-no-imsi-change]]

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

=== User-configurable Minimum Duration Between IMSI Changes

<<<

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