Extract MCC and MNC from the SIB1 PLMN identity and store them in the
UE NAS context to propogate them to NAS security procedures.
Previously, the Serving Network Name (SNN) used during authentication
was constructed from the IMSI, implicitly assuming the UE was attached
to its home network. In roaming scenarios, this produced an incorrect
SNN, causing authentication and key derivation failures.
The NAS key derivation functions (transferRES, derive_kausf, derive_kseaf)
now build the SNN using the MCC/MNC broadcast by the serving network in SIB1,
ensuring that authentication uses the correct serving PLMN during roaming.
Aaron van Diepen
abbd670e5d