Question: When can a UE accept NAS messages without confidentiality protection after NAS confidentiality has been activated?
Option 1: Always, as long as the message is from a trusted source.
Option 2: Only during an inter-system handover from EPC.
Option 3: Never, unless the 5G security context is deleted in either the UE or the AMF.
Option 4: In certain cases where it is not possible to apply confidentiality protection before NAS confidentiality has been activated.
Explanation: According to the text, once NAS confidentiality has been activated, NAS messages without confidentiality protection shall not be accepted by the UE or the AMF. However, there are certain cases where it is not possible to apply confidentiality protection before NAS confidentiality has been activated, and in those cases, NAS messages without confidentiality protection may still be accepted.