Question: In which scenario does the User Equipment (UE) include the non-current native NAS key set identifier Information Element (IE)?
Option 1: When the UE is in 5GMM-IDLE mode and performs an inter-system change from N1 mode to S1 mode.
Option 2: When the UE has a valid current native 5G NAS security context and uses it to protect the REGISTRATION REQUEST message.
Option 3: When the UE is in 5GMM-CONNECTED mode and performs an intra-system change within N1 mode.
Option 4: When the UE has a valid non-current native 5G NAS security context, performs an inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, and uses a mapped 5G NAS security context to protect the REGISTRATION REQUEST message.
Explanation: The UE includes the non-current native NAS key set identifier IE when it has a valid non-current native 5G NAS security context and performs an inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, using a mapped 5G NAS security context to protect the REGISTRATION REQUEST message. This ensures secure communication during the inter-system change.