= 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
[[warn-no-imsi-change]]
=== Warning the User if the IMSI Does Not Change
=== User-configurable Minimum Duration Between IMSI Changes

<<<
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