Question: In a scenario where a UE (User Equipment) connects to a different MME (Mobility Management Entity) that selects different NAS (Non-Access Stratum) algorithms, what action must the MME take regarding NAS keys and notification to the UE?
Option 1: The MME can continue using the existing NAS keys without notifying the UE.
Option 2: The MME re-derives the NAS keys with new algorithm IDs but does not send a NAS Security Mode Command (NAS SMC) to the UE.
Option 3: The MME sends a NAS SMC to the UE without re-deriving the NAS keys, indicating the change in NAS algorithms.
Option 4: The MME must both re-derive the NAS keys using the new algorithm IDs and send a NAS SMC to the UE to indicate the change and activate the new keys.
Explanation: When a UE connects to a different MME that selects different NAS algorithms, security integrity requires updating the NAS keys to ensure compatibility with the new algorithms. Simply notifying the UE (Option 3) or just re-deriving keys without notification (Option 2) would not suffice for secure communication. Continuing with existing keys (Option 1) would be insecure and incompatible. Thus, the MME must both re-derive the NAS keys with the new algorithm IDs and inform the UE via a NAS SMC to ensure seamless and secure communication, making Option 4 the correct choice.