Feature #4400
Updated by osmith about 4 years ago
h3. Overview In contrary to previous approaches, the HLR decides the pseudo IMSI independently of the SIM applet. The first pseudo IMSI gets allocated, as the SIM card is provisioned. After that pseudo IMSI is used for the first time, the HLR decides the next pseudo IMSI and sends it as end-to-end encrypted message to the SIM. The SIM applet overwrites its current IMSI with the new one, and uses it in the next location update. Afterwards, the HLR will generate the next IMSI and so on. h3. Full session <pre> 2. HLR -> SIM NEW PSEUDO IMSI REQ, session_id=1, imsi_pseudo=123 imsi_pseudo=123, imsi_pseudo_key=(random blob 2) 3. HLR <- SIM NEW PSEUDO IMSI RESP ACK, session_id=1, imsi_pseudo=123 </pre> h3. In Detail 1. The HLR has a table of allocated pseudo IMSIs. When provisioning a new SIM, it randomly decides a new pseudo IMSI. IMSI and pseudo IMSI key. There must be no existing entry in the table with the same pseudo IMSI in the imsi_pseudo column, but the pseudo IMSI may be the real IMSI of an existing entry. The pseudo IMSI key is randomly chosen and used for end2end encrypted SMS between the HLR and the SIM applet. The pseudo IMSI key changes whenever the pseudo IMSI changes. |_. id |_. imsi |_. imsi_pseudo |_. imsi_pseudo_key |_. session_id | | 1 | 100 | 200 | (random blob) | 0 | (Other interesting fields to store in the table may be a boolean for "provisioned", the allocation date and usage count. The usage count would increase whenever the SIM does a successful Location Update with that pseudo IMSI.) 2. The SIM does a successful Location Update with its current pseudo IMSI. (Clean up: if the ACK from the SIM card in step 4 did not arrive in a previous provisioning of a new pseudo IMSI, and the SIM has connected with the newer pseudo IMSI entry, the old pseudo IMSI entry gets deleted now.) Then the HLR creates a new entry with a new pseudo IMSI (generated and imsi_pseudo_key (both generated as described in 1.), and with the session_id increased by one. |_. id |_. imsi |_. imsi_pseudo |_. imsi_pseudo_key |_. session_id | | 1 | 100 | 200 | (random blob) | 0 | | 2 | 100 | 123 | (random blob 2) | 1 | The new information is encoded in an SMS and sent to the SIM. SIM, encrypted with the previous imsi_pseudo_key. <pre> HLR -> SIM NEW PSEUDO IMSI REQ, session_id=1, imsi_pseudo=123 imsi_pseudo=123, imsi_pseudo_key=(random blob 2) </pre> 3. The SIM applet verifies, that the session_id is higher than the last session_id it has seen (initially: 0). If that is not the case, it discards the message. The SIM applet writes the new pseudo IMSI, new pseudo IMSI key and session_id to the SIM card, overwriting the old data. It acknowledges the new data with a SMS back to the HLR: HLR, which is encrypted with the *new* imsi_pseudo_key: <pre> HLR <- SIM NEW PSEUDO IMSI RESP ACK, session_id=1, imsi_pseudo=123 </pre> 4. The HLR verifies, that an entry with the session_id and imsi_pseudo from the NEW PSEUDO IMSI RESP ACK message exists in the table. If not, it discards the message. HLR it deletes the old entry with the same IMSI (in the example: the one with imsi_pseudo=200). |_. id |_. imsi |_. imsi_pseudo |_. imsi_pseudo_key |_. session_id | | 2 | 100 | 123 | (random blob 2) | 1 | h3. Messages getting lost h5. What if "NEW PSEUDO IMSI REQ" gets lost? Both the old and the new pseudo IMSI entry exist in the HLR. The SIM will use the old pseudo IMSI in the next location update. The HLR will try to send _the same_ new pseudo IMSI with the same new session_id, as soon as the next location update is complete. h5. What if "NEW PSEUDO IMSI RESP" gets lost? Both the old and the new pseudo IMSI entry exist in the HLR. The SIM will use the new pseudo IMSI in the next location update. The HLR will then clean up the old pseudo IMSI entry, and proceed with generating a new pseudo IMSI entry and sending it to the SIM, as usually. h3. Messages arriving late h5. What if "NEW PSEUDO IMSI REQ" arrives late? The session_id will not be higher than the session_id, which the SIM card already knows. Therefore, the applet will discard the message. h5. What if "NEW PSEUDO IMSI RESP" arrives late? Session_id and imsi_pseudo from the message will not match what's in the HLR database, so HLR will discard the message. h3. Warning the user if SMS don't arrive An attacker could possibly block the SMS with NEW PSEUDO IMSI REQ from arriving at the SIM applet. In that case, the SIM would continue using the old pseudo IMSI indefinitely. We could possibly count the location updates done with the same pseudo IMSI in the SIM applet, and warn the user if the same pseudo IMSI has been used more than N (e.g. 5) times. (Could be possible by listening to EVENT_DOWNLOAD_LOCATION_STATUS?) h3. End2end encryption When deploying the IMSI pseudonymization, the operator should make sure that the pseudo IMSI related SMS between the HLR and the SIM cannot I need to research more about what can be read or modified by third parties. Otherwise, the next pseudonymous IMSI is leaked, and done in case of modifying the IMSI in the SMS, the SIM may be locked out of the network. OTA SMS are usually encrypted and authenticated (TS 03.48), with algorithms and key lengths that the operator chooses (depending on the SIM and how it is configured). It was considered to add an additional layer of end2end encryption for the pseudonymized IMSIs on top, smartcards, but this is out-of-scope for this project. For if we could reference, once could pre-provision a random "imsi_pseudo_key" do something like AES with the SIM card, store it in java smartcards, the pseudo IMSI table in the HLR, and deploy a new encryption key together with each new pseudo IMSI, attached to the NEW PSEUDO IMSI REQ. approach above should work. h3. Advantages over previous approaches * The range of available IMSIs, and the algorithm for determining IMSIs, can be changed without provisioning new SIM cards * Guaranteed that there are no collisions, since the SIM doesn't need to reproduce the IMSI with a counter value. The HLR determines the IMSI independently and sends it as a whole to the SIM. * SMS arriving late is not an issue CC: @laforge, @neels
