Cellular Network Infrastructure » IMSI Pseudonymization

# [WIP] Make IMSI Pseudonymization an optional extension of 3GPP TS

Optional additions we need to make, and where to make them:

* Initial provisioning of the SIM: can optionally have a pseudo IMSI

* During location update, the HLR uses the pseudo IMSI for all communication

  with the VLR / MSC

* After successful location update:

  * HLR deallocates a subscriber's previous pseudo IMSI, if it exists, and the

    subscriber has done the location update with the newer pseudo IMSI entry.

    This is the case, if the SIM applet acknowledged the new pseudo IMSI, but

    its ACK SMS did not arrive at the HLR. There are at most two pseudo IMSIs

    allocated for one subscriber.

  * If there is just one pseudo IMSI for the subscriber (no new pseudo IMSI to

    switch to), the HLR allocates a new pseudo IMSI, and increases the

    session_id by one for that new pseudo IMSI, compared to the last pseudo

    IMSI.

  * The HLR sends the new pseudo IMSI, and the associated session_id, to the

    SIM via SMS. No matter, if the new pseudo IMSI was just created, or if it

    existed already.

  * The SIM applet checks, if the session_id is greater than the one that it

    has stored, and rejects the SMS otherwise. If the session_id is fine, it

    overwrites the SIM's IMSI and session_id with the new data. Then the SIM

    sends an ACK packet back to the HLR, containing both the new session_id and

    the new pseudo IMSI.

  * The HLR verifies the session_id and pseudo IMSI in the ACK packet, discards

    the packet if it doesn't know both. If it was not discarded, the HLR

    deallocates the old pseudo IMSI.

* When allocating and deallocating pseudo IMSIs, the HLR flushes information in

  the VLR related to them, so an old TMSI does not point to the wrong pseudo

  IMSI.

TODO:

* extend the list above with the exact sections of the spec, where the new

  information should be placed

* Is there a spec for SIM applets, or do we put the SIM applet behaviour in the

  regular spec for SIM cards, or mention its behavior in the location update

  related change?