Cellular Network Infrastructure » IMSI Pseudonymization

h1. Baseband behavior on IMSI change

h1. With "Proactive SIM" REFRESH command

According to ETSI TS 101 267 / 3GPP TS 11.14 section 6.4.7.1 "EF IMSI changing procedure", after changing EF-IMSI,

valid REFRESH qualifiers are

* "SIM Initialization and File Change Notification"

* "SIM Initialization and Full File Change Notification"

* "SIM Reset"

h2. ATT == 1 (Channel Description: Attach = yes)

h3. Samsung Galaxy S4m

* phone displays "Refreshing SIM" (notification bubble on display, probably annoying to the user)

* immediate IMSI Detach

* 15 seconds delay of being detached

* Location Updating (IMSI Attach) with previous TMSI (!)

* MSC has forgotten the TMSI due to IMSI detach (OsmoMSC peculiarity, usually an MSC should still have the TMSI)

* Identity Request & Response yields new IMSI

h1. Without "Proactive SIM" REFRESH command

| phone | directly when writing new IMSI to SIM file | on CM Service Request | after periodical LU period | after flight mode / phone restart | on Identity Request | 

| Nokia | no reaction                                | uses previous TMSI    | uses previous TMSI         | uses previous TMSI                | returns new IMSI |

| Samsung Galaxy S4m | no reaction                                | uses previous TMSI    | uses previous TMSI         | uses previous TMSI                | returns new IMSI |

h1. Phones Keep the TMSI

Even though the IMSI has changed, the Baseband uses the last TMSI, and hence looks as if it remained completely unchanged, keeps using the previous identity.

After a power cycle, it also uses the last TMSI.

When the MSC has forgotten the last TMSI (e.g. due to IMSI Detach), the Identity Request is answered with the new IMSI, and the phone has thus switched to the new identity.

Needs to make sure the MSC forgets the TMSI.