4.4.2.5 Establishment of secure exchange of NAS messages

Secure exchange of NAS messages via a NAS signalling connection is usually established by the AMF during the registration procedure by initiating a security mode control procedure. After successful completion of the security mode control procedure, all NAS messages exchanged between the UE and the AMF are sent integrity protected using the current 5G security algorithms, and except for the messages specified in clause 4.4.5, all NAS messages exchanged between the UE and the AMF are sent ciphered using the current 5G security algorithms.

During inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, secure exchange of NAS messages is established between the AMF and the UE by:

• the transmission of NAS security related parameters encapsulated in the AS signalling from the AMF to the UE triggering the inter-system change in 5GMM-CONNECTED mode (see 3GPP TS 33.501 [24]). The UE uses these parameters to generate the mapped 5G NAS security context (see clause 8.6.2 of 3GPP TS 33.501 [24]); and
• after the inter-system change in 5GMM-CONNECTED mode, the transmission of a REGISTRATION REQUEST message from the UE to the AMF. The UE shall send this message integrity protected using the mapped 5G NAS security context and further protect this message as specified in clause 4.4.6 and clause 5.5.1.3.2. After the AMF receives the REGISTRATION REQUEST message:
• if the AMF decides to take the native 5G NAS security context into use, the security mode control procedure is performed. From this time onward, all NAS messages exchanged between the UE and the AMF are sent integrity protected using the native 5G NAS security context, and except for the messages specified in clause 4.4.5, all NAS messages exchanged between the UE and the AMF are sent ciphered using the native 5G NAS security context; or
• if the AMF decides to take the mapped 5G NAS security context into use, from this time onward, all NAS messages exchanged between the UE and the AMF are sent integrity protected using the mapped 5G NAS security context, and except for the messages specified in clause 4.4.5, all NAS messages exchanged between the UE and the AMF are sent ciphered using the mapped 5G NAS security context.

During inter-system change from S1 mode to N1 mode in 5GMM-IDLE mode, if the UE is operating in single-registration mode and:

• if the UE has a valid native 5G NAS security context, the UE shall transmit a REGISTRATION REQUEST message integrity protected with the native 5G NAS security context. The UE shall include the ngKSI indicating the native 5G NAS security context value in the REGISTRATION REQUEST message.

After receiving the REGISTRATION REQUEST message including the ngKSI indicating a native 5G NAS security context value, the AMF shall check whether the ngKSI included in the REGISTRATION REQUEST message belongs to a 5G NAS security context available in the AMF, and shall verify the MAC of the REGISTRATION REQUEST message. If the verification is successful, the AMF deletes the EPS security context received from the source MME if any, and the AMF re-establishes the secure exchange of NAS messages by either:

• replying with a REGISTRATION ACCEPT message that is integrity protected and ciphered using the native 5G NAS security context. From this time onward, all NAS messages exchanged between the UE and the AMF are sent integrity protected and except for the messages specified in clause 4.4.5, all NAS messages exchanged between the UE and the AMF are sent ciphered; or
• initiating a security mode control procedure. This can be used by the AMF to take a non-current 5G NAS security context into use or to modify the current 5G NAS security context by selecting new NAS security algorithms.
• if the UE has no valid native 5G NAS security context, the UE shall send the REGISTRATION REQUEST message without integrity protection and encryption.

After receiving the REGISTRATION REQUEST message without integrity protection and encryption:

• if N26 interface is supported:
• if an EPS security context received from the source MME does not include the NAS security algorithms set to EIA0 and EEA0, the AMF shall either create a fresh mapped 5G NAS security context (see clause 8.6.2 of 3GPP TS 33.501 [24]) or trigger a primary authentication and key agreement procedure to create a fresh native 5G NAS security context; or
• if an EPS security context received from the source MME includes the NAS security algorithms set to EIA0 and EEA0, the AMF shall trigger a primary authentication and key agreement procedure to create a fresh native 5G NAS security context; or
• if N26 interface is not supported, the AMF shall trigger a primary authentication and key agreement procedure.

The newly created 5G NAS security context is taken into use by initiating a security mode control procedure and this context becomes the current 5G NAS security context in both the UE and the AMF. This re-establishes the secure exchange of NAS messages.

During an N1 mode to N1 mode handover, secure exchange of NAS messages is established between the AMF and the UE by:

• the transmission of NAS security related parameters encapsulated in the AS signalling from the target AMF to the UE triggering the N1 mode to N1 mode handover (see 3GPP TS 33.501 [24]). The UE uses these parameters to create a new 5G NAS security context.

The secure exchange of NAS messages shall be continued after N1 mode to N1 mode handover. It is terminated after inter-system change from N1 mode to S1 mode in 5GMM-CONNECTED mode or when the NAS signalling connection is released.

When a UE in 5GMM-IDLE mode establishes a new NAS signalling connection and has a valid current 5G NAS security context, the UE shall transmit the initial NAS message integrity protected with the current 5G NAS security context and further protect this message as specified in clause 4.4.6. The UE shall include the ngKSI indicating the current 5G NAS security context value in the initial NAS message. The AMF shall check whether the ngKSI included in the initial NAS message belongs to a 5G NAS security context available in the AMF, and shall verify the MAC of the NAS message. If the verification is successful, the AMF may re-establish the secure exchange of NAS messages:

• by replying with a NAS message that is integrity protected and ciphered using the current 5G NAS security context. From this time onward, all NAS messages exchanged between the UE and the AMF are sent integrity protected and except for the messages specified in clause 4.4.5, all NAS messages exchanged between the UE and the AMF are sent ciphered; or
• by initiating a security mode control procedure. This can be used by the AMF to take a non-current 5G NAS security context into use or to modify the current 5G NAS security context by selecting new NAS security algorithms.

When a UE attempts multiple registrations in the same or different serving network, both the AMF and the UE shall follow the behavior specified in clause 6.3.2 of 3GPP TS 33.501 [24]. The UE may support multiple records of NAS security context storage for multiple registration (see 3GPP TS 31.102 [22]). If the UE supports multiple records of NAS security context storage for multiple registration, the UE can select the appropriate one among the stored 5G security contexts to protect the initial NAS message (see 3GPP TS 33.501 [24]).

NOTE: For the case when the UE has two records of NAS security context stored and is attempting registration to the PLMN associated with the 5G-GUTI (or an equivalent PLMN) for that access, the UE uses the first NAS security context of that access to protect the initial NAS message. For the case when the UE has two records of NAS security context stored and is attempting registration to the PLMN associated with the second record (or an equivalent PLMN) of that access, the UE uses the second NAS security context of that access to protect the initial NAS message. For other cases when the UE has two records of NAS security context stored and is attempting registration to a PLMN which is not associated with any NAS security context record, the UE uses either record of the NAS security context of that access to protect the initial NAS message.