Section: 6.7.3.6 Algorithm negotiation for unauthenticated UEs in LSM
Content: UEs that are in limited service mode (LSM) and that cannot be authenticated by the AMF/SEAF (for whatever reason) may still be allowed to establish emergency session by sending the emergency registration request message. It shall be possible to configure whether the AMF allows unauthenticated UEs in LSM to establish bearers for emergency session or not. If an AMF allows unauthenticated UEs in LSM to establish bearers for an emergency session, then for the NAS protocol, the AMF shall use NIA0 and NEA0 as the integrity and ciphering algorithm respectively. If the AMF allows an unauthenticated UE in LSM to establish bearers for emergency session after it has received the emergency registration request message from the UE, the AMF shall: - Select NIA0 and NEA0, regardless of the supported algorithms announced previously by the UE as the NAS algorithms and signal this to the UE via the NAS security mode command procedure when activating the 5G NAS security context. - Set the UE 5G security capabilities to only contain EIA0, EEA0, NIA0 and NEA0 when sending these to the gNB/ng-eNB in the following messages: - NGAP UE INITIAL CONTEXT SETUP - NGAP UE CONTEXT MODIFICATION REQUEST - NGAP HANDOVER REQUEST NOTE: As a result of that the AMF only sending a UE 5G security capability containing EIA0, EEA0, NIA0 and NEA0 to the gNB/ng-eNB , the gNB/ng-eNB is only able of selecting a null integrity protection for AS integrity protection and a null ciphering algorithm for AS confidentiality protection. That is, if NIA0 is used for NAS integrity protection, then NIA0 or EIA0 will always be used for AS integrity protection. If NIA0 is disabled at the gNB for regulatory requirements and the gNB receives the UE 5G security capabilities to only contain NIA0 for integrity protection algorithms from the AMF in one of the above messages, the gNB shall reject the session. The rules for when the AMF shall select NIA0 for NAS integrity protection, and when the UE shall accept a NAS security mode command selecting NIA0 for NAS integrity protection depends on whether the UE and AMF can be certain that no 5G NAS security context can be established. The rules for determining this is defined in clause 10 of this specification. If the AMF has selected NIA0 as the NAS integrity protection algorithm, the UE shall accept selection of NIA0 or EIA0 as the AS integrity protection algorithm. Selection of AS integrity protection algorithm happens via the AS security mode command procedure or via a handover command. The UE shall under no other circumstances accept selection of null integrity algorithm as the AS integrity protection algorithm.