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= Specification for IMSI Pseudonymization on the Radio Interface for 2G/3G/4G
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== Introduction
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=== Protecting the IMSI on the Radio Interface is Desirable
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A long-standing issue in the 3GPP specifications is, that mobile phones and
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other mobile equipment (ME) have to send the International Mobile Subscriber
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Identity (IMSI) unencrypted over the air. Each IMSI is a unique identifier for
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the subscriber. Therefore most people can be uniquely identified by recording
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the IMSI that their ME is sending.  The 3GPP specifications provide means for
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implementations to send the IMSI less often by using the Temporary Mobile
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Subscriber Identity (TMSI) where possible.
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But this is not enough. So-called IMSI catchers were invented and are used to
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not only record IMSIs when they have to be sent. But also to force ME to send
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their IMSI by imitating a Base Transceiver Station (BTS). IMSI catchers have
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become small and affordable, even criminals actors without much budget can use
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them to track anybody with a mobile phone.
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5G addresses this problem with the Subscriber Concealed Identifier (SUCI),
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which uses public-key cryptography to ensure that the permanent subscriber
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identity can only be read by the home network (3GPP TS 33.501, Section 6.12.2).
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A comparable, but different approach to conceal the IMSI for 2G, 3G and 4G is
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provided in this specification.
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=== Summary of Proposed Solution
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The solution presented in this document is to periodically change the IMSI of
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the ME to a new pseudonymous IMSI allocated by the Home Location Register (HLR)
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or Home Subscriber Service (HSS). The next pseudonymous IMSI is sent to the SIM
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via Short Message Service (SMS), then a SIM applet overwrites the IMSI of the
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SIM with the new value. The only component that needs to be changed in the
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network besides the SIM/USIM is the HLR/HSS, therefore it should be possible
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even for a Mobile Virtual Network Operator (MVNO) to deploy this privacy
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enhancement.
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=== Summary of Existing Location Updating Procedures in RAN and CN
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The subscriber's SIM is provisioned with the IMSI and cryptographic keys of a
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subscriber, after the subscriber was added with the same data to the HLR/HSS.
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In the Remote Access Network (RAN), the IMSI is sent over the air interface and
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then transmitted to the Core Network (CN), where it is validated by the
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HLR/HSS. The involved components vary by the generation of the network and
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whether the SIM is attempting a Circuit Switched (CS) or Packet Switched (PS)
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connection, but the principle is the same. This document uses 2G CS Location
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Updating for reference, as in <<figure-imsi-regular>>.
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The IMSI is transmitted in the Location Updating Request from ME. The VLR
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needs an authentication challenge specific to the secret keys on the SIM to
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authenticate the SIM, and looks the authentication challenges up by the IMSI.
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If the VLR does not have any more authentication challenges for the IMSI (as it
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happens when the VLR sees the IMSI for the first time), the VLR requests new
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authentication challenges from the HLR. Then the HLR verifies that the IMSI is
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known and, if it is unknown, sends back an error that will terminate the
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Location Updating procedure.
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After the VLR found the authentication challenge, it authenticates the SIM, and
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performs a Classmark Enquiry and Physical Channel Reconfiguration. Then the VLR
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has the required information to finish the Location Updating, and continues
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with Process Update_Location_HLR (3GPP TS 29.002). Afterwards, the VLR assigns
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a new TMSI with the Location Updating Accept, which is acknowledged by the TMSI
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Reallocation Complete. In following Location Updates with the same MSC, the ME
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sends the TMSI instead of the IMSI in the Location Updating Request.
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[[figure-imsi-regular]]
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.Location Updating in 2G CS with IMSI
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["mscgen"]
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----
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msc {
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  hscale="1.75";
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  ME [label="ME"], BTS [label="BTS"], BSC [label="BSC"], MSC [label="MSC/VLR"],
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  HLR [label="HLR"];
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  // BTS <=> BSC: RSL
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  // BSC <=> MSC: BSSAP, RNSAP
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  // MSC <=> HLR: MAP (process Update_Location_HLR, 3GPP TS 29.002)
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  ME   => BTS [label="Location Updating Request"];
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  BTS  => BSC [label="Location Updating Request"];
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  BSC  => MSC [label="Location Updating Request"];
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  --- [label="If necessary: VLR requests new authentication challenges for this IMSI"];
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  MSC  => HLR [label="Send Auth Info Request"];
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  MSC <=  HLR [label="Send Auth Info Result"];
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  ---;
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  BSC <=  MSC [label="Authentication Request"];
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  BTS <=  BSC [label="Authentication Request"];
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  ME  <=  BTS [label="Authentication Request"];
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  ME   => BTS [label="Authentication Response"];
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  BTS  => BSC [label="Authentication Response"];
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  BSC  => MSC [label="Authentication Response"];
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  BSC <=  MSC [label="Classmark Enquiry"];
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  BTS <=  BSC [label="Classmark Enquiry"];
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  ME  <=  BTS [label="Classmark Enquiry"];
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  ME   => BTS [label="Classmark Change"];
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  BTS  => BSC [label="Classmark Change"];
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  BSC  => MSC [label="Classmark Update"];
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  BSC <=  MSC [label="Physical Channel Reconfiguration"];
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  BTS <=  BSC [label="Ciphering Mode Command"];
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  ME  <=  BTS [label="Ciphering Mode Command"];
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  ME   => BTS [label="Ciphering Mode Complete"];
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  BTS  => BSC [label="Ciphering Mode Complete"];
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  BSC  => MSC [label="Ciphering Mode Complete"];
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  --- [label="Process Update_Location_HLR (3GPP TS 29.002)"];
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  MSC  => HLR [label="Update Location Request"];
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  MSC <=  HLR [label="Insert Subscriber Data Request"];
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  MSC  => HLR [label="Insert Subscriber Data Result"];
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  MSC <=  HLR [label="Update Location Result"];
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  ---;
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  BSC <=  MSC [label="Location Updating Accept"];
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  BTS <=  BSC [label="Location Updating Accept"];
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  ME  <=  BTS [label="Location Updating Accept"];
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  ME   => BTS [label="TMSI Reallocation Complete"];
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  BTS  => BSC [label="TMSI Reallocation Complete"];
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  BSC  => MSC [label="TMSI Reallocation Complete"];
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}
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----
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<<<
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== Required Changes
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[[hlr-imsi-pseudo-storage]]
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=== Pseudonymous IMSI Storage in the HLR
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The HLR must store up to two pseudonymous IMSIs (imsi_pseudo) and their related
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counters (imsi_pseudo_i) per subscriber. Each subscriber initially has one
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pseudonymous IMSI allocated. A subscriber has two valid pseudonymous IMSIs
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only during the transition phase from the old pseudonymous IMSI to the new one.
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The amount of available IMSIs must be higher than the amount of subscribers
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registered with the HLR. If the amount of available IMSIs is too short, the HLR
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can delay assigning new pseudonymous IMSIs until new IMSIs are available again.
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.Examples for additional subscriber data in HLR
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[options="header"]
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|===
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| Subscriber ID | imsi_pseudo | imsi_pseudo_i
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// example IMSIs taken from Wikipedia
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| 123
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| 310150123456789
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| 1
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| 234
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| 502130123456789
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| 1
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| 234
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| 460001357924680
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| 2
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|===
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==== imsi_pseudo
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The value for imsi_pseudo is a random choice from the pool of available IMSIs
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that the HLR controls. The pseudonymous IMSI must not be used by any subscriber
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as pseudonymous IMSI yet, but may be the real IMSI of a subscriber.
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[[hlr-imsi-pseudo-i]]
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==== imsi_pseudo_i
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The counter imsi_pseudo_i indicates how often a subscribers pseudonymous IMSI
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was changed. The value is 1 for the first allocated pseudonymous IMSI of a
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subscriber. When allocating a new pseudonymous IMSI for the same subscriber,
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the new imsi_pseudo_i value is increased by 1. The counter is used by the SIM
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applet to detect and ignore outdated requests related to changing the
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pseudonymous IMSI.
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=== SIM Provisioning
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IMSI pseudonymization as specified by this document works with SIM and USIM.
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The HLR is allocating a pseudonymous IMSI for the subscriber. This pseudonymous
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IMSI is stored as IMSI on the subscriber's SIM instead of the real IMSI.
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[[sim-app]]
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==== SIM applet
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The SIM is provisioned with a SIM applet, which is able to change the IMSI once
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the next pseudonymous IMSI arrives from the HLR. A reference implementation is
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provided in <<reference-src>>.
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===== Counter Storage
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The following counter variables are stored in the SIM applet.
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[options="header",cols="20%,12%,68%"]
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|===
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| Name | Initial value | Description
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| imsi_pseudo_i
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| 1
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| See <<hlr-imsi-pseudo-i>>.
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| imsi_pseudo_lu
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| 0
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| Amount of Location Updating procedures done with the same pseudonymous IMSI.
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| imsi_pseudo_lu_max
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| (decided by operator)
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| Maximum amount of Location Updating procedures done with the same
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  pseudonymous IMSI, before the SIM applet shows a warning to the subscriber.
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|===
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===== Switch to Next Pseudonymous IMSI
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The SIM applet registers to a suitable SMS trigger (3GPP TS 43.019, Section
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6.2). When an SMS from the HLR in the structure of <<sms-structure>> arrives,
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the applet must verify that the SMS is not outdated by comparing imsi_pseudo_i
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from the SMS with the last imsi_pseudo_i that was used when changing the IMSI
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(initially 1 as in <<hlr-imsi-pseudo-i>>). The new value must be higher,
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otherwise the SMS should not be processed further.
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The SIM applet registers a timer with min_sleep_time from the SMS. When the
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timer triggers, the IMSI of the SIM is overwritten with the new pseudonymous
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IMSI, the TMSI and GSM Ciphering key Kc (3GPP TS 31.102, Section 4.4.3.1) are
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invalidated. The current imsi_pseudo_i from the SMS is stored in the SIM applet
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to compare it with the next SMS. imsi_pseudo_lu is reset to 0. Afterwards,
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the EF~IMSI~ changing procedure in 3GPP TS 11.14, Section 6.4.7.1 is executed
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to apply the new IMSI.
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// FIXME: do we need to enforce the LU now, with an arbitrary CM Service
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// Request, or would this only be necessary for Osmocom? (OS#4404)
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===== Warning the Subscriber If the Pseudonymous IMSI Does Not Change
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An attacker could potentially block the next pseudonymous IMSI SMS on purpose.
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Because the SIM applet cannot decide the next pseudonymous IMSI, it would have
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the same pseudonymous IMSI for a long time. Then it could become feasible for
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an attacker to track the subscriber by their pseudonymous IMSI. Therefore the
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SIM applet should warn the subscriber if the pseudonymous IMSI does not change.
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The SIM applet registers to EVENT_EVENT_DOWNLOAD_LOCATION_STATUS (3GPP TS
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03.19, Section 6.2) and increases imsi_pseudo_lu by 1 when the event is
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triggered. If imsi_pseudo_lu reaches imsi_pseudo_lu_max, the SIM applet
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displays a warning to the subscriber.
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[[process-update-location-hlr]]
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=== Process Update_Location_HLR
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All IMSI Pseudonymization related changes to Process Update_Location_HLR
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(3GPP TS 29.002) are optional. Deviations from the existing specification that
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are outlined in this section are expected to be enabled or disabled entirely
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where IMSI pseudonymization is implemented.
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[[figure-imsi-pseudo]]
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.Process Update_Location_HLR with IMSI pseudonymization changes
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["mscgen"]
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----
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msc {
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  hscale="1.75";
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  MSC [label="MSC/VLR"], SMSC [label="SMS-SC"], HLR [label="HLR"];
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  MSC   => HLR  [label="Update Location Request"];
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  --- [label="If new pseudonymous IMSI was used: deallocate and cancel old pseudonymous IMSI"];
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  HLR  box HLR  [label="Deallocate old pseudonymous IMSI"];
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  MSC  <=  HLR  [label="Cancel Location Request"];
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  MSC   => HLR  [label="Cancel Location Result"];
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  ---;
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  MSC  <=  HLR  [label="Insert Subscriber Data Request"];
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  MSC   => HLR  [label="Insert Subscriber Data Result"];
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  HLR  box HLR  [label="Start Next_Pseudo_IMSI_Timer"];
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  MSC  <=  HLR  [label="Update Location Result"];
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  MSC  box MSC  [label="Finish Location Updating with ME"],
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  HLR  box HLR  [label="Wait for Next_Pseudo_IMSI_Timer expiry"];
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  |||;
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  ...;
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  |||;
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  HLR  box HLR  [label="Next_Pseudo_IMSI_Timer expired"];
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  HLR  box HLR  [label="\nAllocate new pseudonymous IMSI\nif subscriber has only one allocated\n"];
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  SMSC <=  HLR  [label="Next Pseudonymous IMSI SMS"];
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  SMSC box SMSC [label="Deliver SMS to ME"];
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}
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----
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==== Update Location Request
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When Update Location Request arrives, the HLR does not look up the subscriber
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by the IMSI, but by the pseudonymous IMSI instead. Unless the subscriber has
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two pseudonymous IMSI allocated and used the new pseudonymous IMSI in the
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Update Location Request, this is followed by the existing logic to continue
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with Insert Subscriber Data Request.
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===== Update Location Request With New Pseudonymous IMSI
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If the subscriber has two pseudonymous IMSIs allocated, and the newer entry was
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used (higher imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>), this section applies.
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The older pseudonymous IMSI is deallocated in the HLR. This is done as early
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as possible, so the timeframe where two pseudonymous IMSI are allocated for one
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subscriber is short.
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A Cancel Location Request with the old pseudonymous IMSI is sent to the VLR, so
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the conflicting subscriber entry with the old pseudonymous IMSI is deleted from
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the VLR. Receiving a Cancel Location Result is followed by the existing logic
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to continue with Insert Subscriber Data Request.
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===== Update Location Request With Old Pseudonymous IMSI
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If the subscriber has two pseudonymous IMSIs allocated, and the older entry was
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used (lower imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>), the newer entry is _not_
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deallocated. This could lock out the subscriber from the network if the SMS
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with the new pseudonymous IMSI arrives with a delay.
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==== Insert Subscriber Data Result
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When Insert Subscriber Data Result arrives, a subscriber specific
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Next_Pseudo_IMSI_Timer starts.
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==== Next_Pseudo_IMSI_Timer Expires
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If the subscriber has only one pseudonymous IMSI allocated, and the amount of
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available IMSIs in the HLR is high enough, a second pseudonymous IMSI and
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related imsi_pseudo_i gets allocated for the subscriber (as described in
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<<hlr-imsi-pseudo-storage>>).
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If the subscriber still has only one pseudonymous IMSI, because not enough
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IMSIs were available in the HLR, the process is aborted here and no SMS with
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a next pseudonymous IMSI is sent to the subscriber. The subscriber will get a
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new pseudonymous IMSI during the next Location Updating Procedure, if the HLR
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has enough IMSIs available at that point.
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An SMS is sent to the SMS - Service Centre (SMS-SC) with the newer pseudonymous
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IMSI (higher imsi_pseudo_i, see <<hlr-imsi-pseudo-i>>) and related
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imsi_pseudo_i value.
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[[sms-structure]]
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==== Next Pseudonymous IMSI SMS Structure
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.Next pseudonymous IMSI SMS structure
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[packetdiag]
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----
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{
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	colwidth = 32
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	0-31:	 IMSI_PSEUDO_I
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	32-63:   MIN_SLEEP_TIME
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	64-119:  IMSI_PSEUDO
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	120-127: PAD
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}
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----
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// FIXME
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IMPORTANT: This is a draft. The structure is likely to change after the
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reference implementation phase.
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IMSI_PSEUDO_I: 32 bits::
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See <<hlr-imsi-pseudo-i>>.
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MIN_SLEEP_TIME: 32 bits::
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Amount of seconds, which the SIM applet should wait before changing to the new
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pseudonymous IMSI. Since it is unclear when the SMS will arrive (ME might be
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turned off), this is a minimum amount.
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IMSI_PSEUDO: 60 bits::
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Telephony Binary Coded Decimal (TBCD, 3GPP TS 29.002) version of the next
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pseudonymous IMSI.
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PAD: 8 bits::
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Padding at the end, should be filled with 1111 as in the TBCD specification.
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== Error Scenarios
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=== Next Pseudonymous IMSI SMS is Lost
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If the SMS with the next pseudonymous IMSI does not arrive, the SIM will start
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the next Location Updating Procedure with the old pseudonymous IMSI. Because
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the HLR has both the old and the new pseudonymous IMSI allocated at this point,
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the subscriber is not locked out of the network.
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=== Next Pseudonymous IMSI SMS Arrives Out of Order
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The next pseudonymous IMSI SMS may arrive out of order. Either, because the
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network is not able to deliver them in order, or even because an attacker would
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perform a replay attack.
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If the SMS arrives out of order, the imsi_pseudo_i counter will not be higher
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than the value the SIM applet (<<sim-app>>) has stored. Therefore, the applet
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will discard the message and the subscriber is not locked out of the network.
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// === SMS Arrives Before Timer Expires
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// FIXME: OS#4486
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== Recommendations for Real-World Implementations
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=== BCCH SI3: ATT = 0
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When changing from one pseudonymous IMSI to the next, it is important that the
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ME does not detach from the network. Otherwise it would be trivial for an
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attacker to correlate the detach with the attach of the same ME with the next
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pseudonymous IMSI.
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This is controlled with the ATT flag in the SYSTEM INFORMATION TYPE 3 (SI3)
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message on the Broadcast Control Channel (BCCH), see 3GPP TS 44.018 Section
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10.5.2.11. It must be set to 0.
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// FIXME: verify how it set with operators in germany (OS#4404)
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=== End to End Encryption of SMS
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When deploying the IMSI pseudonymization, the operator should make sure that
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the next pseudonymous IMSI SMS (<<sms-structure>>) cannot be read or modified
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by third parties. Otherwise, the next pseudonymous IMSI is leaked, and if the
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pseudonymous IMSI in the SMS was changed, the SIM would be locked out of the
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network.
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The safest way to protect the next pseudonymous IMSI SMS is a layer of end to
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end encryption from the HLR to the SIM.  The existing means for OTA SMS
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security (3GPP TS 23.048) provide mechanisms for integrity protection,
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confidentiality as well as replay protection and must be implemented when using
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IMSI pseudonymization.
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=== User-configurable Minimum Duration Between IMSI Changes
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It may be desirable to let subscribers configure their minimum duration between
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IMSI changes. This allows subscribers with a high privacy requirement to switch
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their pseudonymous IMSI more often, and it allows the pseudonymous IMSI change
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to happen less frequently if it is distracting to the subscriber.
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How distracting the pseudonymous IMSI change is, depends on the ME. The
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following examples were observed:
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// FIXME: might need an update after SYS#4481
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* A Samsung GT-I9100 Galaxy SII smartphone with Android 4.0.3 displays a
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  message at the bottom of the screen for about 5 seconds, but the user
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  interface remains usable.
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* A Samsung GT-E1200 feature phone displays a waiting screen for 16 to 17
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  seconds and is unusable during that time.
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[[reference-src]]
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== Reference Implementation with Source Code
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A reference implementation for the SIM applet (<<sim-app>>) is available in
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source code under the Apache-2.0 license at:
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https://osmocom.org/projects/imsi-pseudo
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The HLR modifications described in <<hlr-imsi-pseudo-storage>> and
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<<process-update-location-hlr>> were implemented for reference in OsmoHLR from
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the Osmocom project, licensed under AGPL-3.0. Information about the source code
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and related branches for IMSI pseudonymization can be found at the above URL as
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well.
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