Question: What happens to the NAS Transport message when a UE sends or receives an MO/MT SMS over NAS via 3GPP/non-3GPP and has already activated NAS security with the AMF?
Option 1: The NAS Transport message is only integrity protected using the NAS security context.
Option 2: The NAS Transport message is only ciphered using the NAS security context.
Option 3: The NAS Transport message is both ciphered and integrity protected using the NAS security context by the UE/AMF.
Option 4: The NAS Transport message is not protected by any NAS security mechanisms.
Explanation: According to the specified procedures, when a UE has already activated NAS security with the AMF before sending/receiving SMS, the NAS Transport message shall be both ciphered and integrity protected using the NAS security context by the UE/AMF. This ensures that the message is secure and protected from unauthorized access or tampering.