Question: Upon receiving an RRCReject message in response to an RRCResumeRequest for Service-Based Data Transfer (SDT) and after T380 has expired, what action(s) does the UE take regarding its security keys?
Option 1: The UE updates all existing security keys (KgNB, KRRCenc, KRRCint, KUPint, KUPenc).
Option 2: The UE retains the current KgNB key but discards and re-establishes the KRRCenc, KRRCint, KUPint, and KUPenc keys.
Option 3: The UE sets a flag to trigger a future update of security keys but takes no immediate action on them.
Option 4: The UE discards the current KgNB key, KRRCenc key, KRRCint key, KUPint key, and KUPenc key derived in accordance with the specified protocol.
Explanation: When an RRCReject is received for SDT after T380 has expired, the UE discards specific security keys (KgNB, KRRCenc, KRRCint, KUPint, KUPenc) to ensure security integrity. This action resets the connection attempt, aligning with secure communication protocols in telecom networks, ensuring subsequent successful connections operate with freshly generated, secure keys.