Section: 6.4.3.2 NAS integrity activation
Content: NAS integrity shall be activated using the NAS SMC procedure or after an inter-system handover from EPC. Replay protection shall be activated when integrity protection is activated, except when the NULL integrity protection algorithm is selected. Replay protection shall ensure that the receiver only accepts each incoming NAS COUNT value once using the same NAS security context. Once NAS integrity has been activated, NAS messages without integrity protection shall not be accepted by the UE or the AMF. Before NAS integrity has been activated, NAS messages without integrity protection shall only be accepted by the UE or the AMF in certain cases where it is not possible to apply integrity protection. NAS integrity shall stay activated until the 5G security context is deleted in either the UE or the AMF. It shall not be possible to change from non-NULL integrity protection algorithm to NULL integrity protection.