Question: When does the UE start ciphering and deciphering of NAS messages for a NAS signalling connection?
Option 1: Immediately after sending the initial NAS message.
Option 2: After receiving the SECURITY MODE COMMAND message from the MME.
Option 3: After establishing a new EPS bearer context.
Option 4: When the secure exchange of NAS messages has been established for a NAS signalling connection.
Explanation: According to the specification, except for the CONTROL PLANE SERVICE REQUEST message including an ESM message container information element or a NAS message container information element, the UE shall start the ciphering and deciphering of NAS messages when the secure exchange of NAS messages has been established for a NAS signalling connection.