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README.md
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README.md
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@ -5,146 +5,4 @@ mobile subscriber on the radio interface in a 2G, 3G, 4G network.
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Homepage: https://osmocom.org/projects/imsi-pseudo/wiki
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## How it works
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The first pseudo IMSI gets allocated in the HLR, as the SIM card is
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provisioned. After that pseudo IMSI is used for the first time in location
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update, the HLR waits for some time, then decides the next pseudo IMSI and
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sends it together with a delay value as SMS to the SIM. The SIM applet receives
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the SMS and waits the specified delay. Then it overwrites its current IMSI with
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the new one, marks the TMSI as invalid, and initiates the next location update.
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Afterwards, the process repeats.
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```
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HLR <-> SIM LOCATION UPDATE, imsi_pseudo=200
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(time passes)
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HLR -> SIM NEW PSEUDO IMSI, imsi_pseudo=123, delay=60
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(time passes until the SMS arrives)
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(SIM applet waits 60 seconds)
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HLR <-> SIM LOCATION UPDATE, imsi_pseudo=123
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...
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```
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## In detail
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### 1. Provisioning the SIM
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The HLR allocates a new pseudo IMSI as random choice from the pool of available
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IMSIs. The pseudo IMSI must not be used by any other subscriber as pseudo IMSI,
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but may be the real IMSI of another subscriber. The subscriber-specific counter
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imsi_pseudo_i is 0 for the first allocated IMSI for that subscriber.
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| id | imsi | imsi_pseudo | imsi_pseudo_i |
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|------|--------|---------------|---------------|
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| 1 | 100 | 200 | 0 |
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The pseudo IMSI is saved to the SIM as IMSI, instead of the real IMSI. The SIM
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is also provisioned with the IMSI pseudonymization applet.
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### 2. Successful Location Update with pseudo IMSI
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a) If this was the first Location Update after provisioning the SIM, the
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subscriber has only one pseudo IMSI allocated. The HLR waits for some time.
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Then it allocates the next pseudo IMSI from the pool of available IMSIs (as in
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1., but with imsi_pseudo_i increased by one). The HLR sends the new
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pseudo IMSI, the imsi_pseudo_i and a random delay value in one SMS to the SIM.
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The random delay is how long the SIM applet should wait before changing the
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IMSI. This delay prevents easy correlation of the arrival of the SMS with the
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Location Update that will follow in 3. by the SIM. Due to other latencies in
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the network, this is a minimum delay. At this point, the subscriber has two
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allocated pseudo IMSIs:
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| id | imsi | imsi_pseudo | imsi_pseudo_i |
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|------|--------|---------------|---------------|
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| 1 | 100 | 200 | 0 |
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| 2 | 100 | 123 | 1 |
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b) If this was not the first Location Update after provisioning a new SIM, the
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subscriber already has two pseudo IMSIs allocated when doing the Location
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Update. The HLR compares imsi_pseudo_i to find out if the Location Update was
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done with the newer or older pseudo IMSI.
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If the older pseudo IMSI was used, then the SIM applet was not able to set the
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new IMSI. This may be caused by an SMS arriving late, possibly even months
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after it was sent in case the UE was without power for a long period of time.
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Therefore the HLR cannot deallocate the newer pseudo IMSI without risking that
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the SIM would configure that IMSI and then be locked out (unable to do any
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further location updates). Instead, the HLR proceeds like in a), but sends the
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same unused new pseudo IMSI again instead of allocating a new one.
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If the newer pseudo IMSI was used, the SIM applet has successfully set the new
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IMSI. The HLR deallocates the old pseudo IMSI and sends a Purge MS request to
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the VLR with the old pseudo IMSI. Then the HLR proceeds like in a).
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### 3. Arrival of the SMS
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The SIM applet verifies, that imsi_pseudo_i is higher than the last
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imsi_pseudo_i it has seen (initially: 0). If that is not the case, it discards
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the message.
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The SIM applet registers a timer to wait the specified delay. When the timer
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expires, the applet updates the last imsi_pseudo_i value that it has seen. Then
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it overwrites the IMSI with the next pseudo IMSI and invalidates the TMSI and
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Kc. The applet triggers a refresh, which causes the SIM to do a new Location
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Update with the new IMSI.
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It is important, that the SIM does not invoke the IMSI detach procedure (TS
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04.08 Section 4.3.4). This is controlled by the network. Otherwise it becomes
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trivial to correlate the detach of the old IMSI with the attach or Location
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Update of the new IMSI.
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## Notes
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### What if the SMS gets lost?
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Both the old and the new pseudo IMSI entry exist in the HLR.
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The SIM will use the old pseudo IMSI in the next Location Update. The HLR will
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try to send _the same_ new pseudo IMSI with the same new imsi_pseudo_i, as soon
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as the next Location Update is complete.
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### What if the SMS arrives late?
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The imsi_pseudo_i counter will not be higher than the value the SIM applet
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already knows. Therefore, the applet will discard the message.
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### Warning the user if SMS don't arrive
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An attacker could possibly block the SMS from arriving at the SIM applet. In
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that case, the SIM would continue using the old pseudo IMSI indefinitely.
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We can count the location updates done with the same pseudo IMSI in the SIM
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applet, and warn the user if the same pseudo IMSI has been used more than N
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(e.g. 5) times.
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### End2end encryption
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When deploying the IMSI pseudonymization, the operator should make sure that
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the pseudo IMSI related SMS between the HLR and the SIM cannot be read or
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modified by third parties. Otherwise, the next pseudonymous IMSI is leaked, and
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in case of modifying the IMSI in the SMS, the SIM may be locked out of the
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network.
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OTA SMS are usually encrypted and authenticated (TS 03.48), with algorithms and
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key lengths that the operator chooses (depending on the SIM and how it is
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configured).
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It was considered to add an additional layer of end2end encryption for the
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pseudonymized IMSIs on top, but this is out-of-scope for this project. For
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reference, one could pre-provision a random "imsi_pseudo_key" with the SIM
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card, store it in the pseudo IMSI table in the HLR, and deploy a new encryption
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key together with each new pseudo IMSI, attached to the SMS.
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### User-configurable minimum duration between IMSI changes
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It may be desirable to let users configure their minimum duration between IMSI
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changes. This allows people with a high privacy requirement to switch their
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pseudonymous IMSI more often, and it allows the IMSI change to happen less
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often if it is distracting to the user. The latter depends on the phone's
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software, for example:
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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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Find the specification in the docs subdir.
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@ -1,28 +0,0 @@
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# [WIP] Make IMSI Pseudonymization an optional extension of 3GPP TS
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NOTE: this file is deprecated, it will be replaced by imsi-pseudo-spec.adoc.
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Relevant specs:
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* 3GPP TS 23.008: Organization of subscriber data
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* Add pseudo IMSI and pseudo_imsi_i optionally to be saved in the HLR
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* "Process Update_Location_HLR" of TS 09.02
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* Cancel location in old VLR/SGSN if IMSI pseudonymization is enabled, and a
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previous pseudonymous IMSI exists for the subscriber
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Optional additions we need to make, and where to make them:
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* Initial provisioning of the SIM: can optionally have a pseudo IMSI
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* During location update, the HLR uses the pseudo IMSI for all communication
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with the VLR / MSC
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* Is there anything to update? We just replace the IMSI, so the SIM and the
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VLR / MSC don't act any different
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* After successful location update:
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* See 2. in README.md
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TODO:
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* extend the list above with the exact sections of the spec, where the new
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information should be placed
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* Is there a spec for SIM applets, or do we put the SIM applet behaviour in the
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regular spec for SIM cards, or mention its behavior in the location update
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related change?
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* describe everything in detail, fill in the full contents for the SMS etc.
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