184 lines
7.4 KiB
Plaintext
184 lines
7.4 KiB
Plaintext
= Specification for IMSI Pseudonymization on the Radio Interface for 2G and Above
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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 uniquely identifying the
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person who bought the associated Subscriber Identity Module (SIM) used in the
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ME. Therefore most people can be uniquely identified by recording the IMSI that
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their ME is sending. Efforts are made in the 2G and above specifications to
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send the IMSI less often, by using the Temporary Mobile Subscriber Identity
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(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 immitating 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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=== 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 is the HLR/HSS, therefore it should be possible even
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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 an Update Location Request procedure with the HLR. Afterwards, the VLR
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assigns a new TMSI with the Location Updating Accept, which is acknowledged by
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the TMSI Reallocation Complete. In following Location Updates with the same
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MSC, the ME sends the TMSI instead of the IMSI in the Location Updating
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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="VLR requests new authentication challenges for this IMSI if necessary"];
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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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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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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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=== 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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|===
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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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==== imsi_pseudo_i
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The counter imsi_pseudo_i indicates how often a subscriber's 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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=== Successful Location Update With Pseudonymous IMSI
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// HLR may choose not to give out next IMSI if it is short on available IMSIS
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=== Next Pseudonymous IMSI Arrives Via SMS
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== Error Scenarios
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=== Next Pseudonymous IMSI SMS is Lost
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=== SMS Arrives Late
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== Reference Implementation with Source Code
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== Recommendations for Real-World Implementations
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=== ATT = 0
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=== End to End Encryption of SMS
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=== Warning the User if the IMSI Does Not Change
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=== User-configurable Minimum Duration Between IMSI Changes
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<<<
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include::./common/chapters/gfdl.adoc[]
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