Content for  TS 33.501  Word version:  18.4.0

TS 23.501 has a scenario when the UE is registered to a VPLMN's serving network via 3GPP access and to another VPLMN's or HPLMN's serving network via non-3GPP access at the same time. When the UE is registered in one PLMN's serving network over a certain type of access (e.g. 3GPP) and is registered to another PLMN's serving network over another type of access (e.g. non-3GPP), then the UE has two active NAS connections with different AMF's in different PLMNs. As described in clause 6.3.2.1, the UE shall independently maintain and use two different 5G security contexts, one per PLMN serving network. The 5G security context maintained by the UE shall contain the full set of 5G parameters, including NAS context parameters for 3GPP and non-3GPP access types per PLMN. In case of connection to two different PLMNs, it is necessary to maintain a complete 5G NAS security context for each PLMN independently, each with all associated parameters (such as two pairs of NAS COUNTs, i.e. one pair for 3GPP access and one pair for non-3GPP access). Each security context shall be established separately via a successful primary authentication procedure with the Home PLMN. All the NAS and AS security mechanisms defined for single registration mode are applicable independently on each access using the corresponding 5G security context.

When the UE is registered in a serving network over two types of access (e.g. 3GPP and non-3GPP), then the UE has two active NAS connections with the same AMF. A common 5G NAS security context is created during the registration procedure over the first access type. In order to realize cryptographic separation and replay protection, the common NAS security-context shall have parameters specific to each NAS connection. The connection specific parameters include a pair of NAS COUNTs for uplink and downlink and unique NAS connection identifier. The value of the unique NAS connection identifier shall be set to "0x01" for 3GPP access and set to "0x02" for non-3GPP access. All other parameters as e.g. algorithm identifiers in the common NAS security context are common to multiple NAS connections. In non-mobility cases, when the UE is simultaneously registered over both types of accesses, and if NAS key re-keying as described in clause 6.9.4.2 or if NAS key refresh as described in clause 6.9.4.3 takes place over one of the accesses (say access A):

1. If the other access (access B) is in CM-CONNECTED state, then the new NAS security context shall only be activated over that access (access A). The UE and the AMF shall not change the NAS security context in use on the other access (say access B). In order to activate the new NAS security context over the other access (access B), the AMF shall trigger a NAS SMC run over that access either in the current running procedure or a subsequent NAS procedure. During the second NAS SMC run (on access B), the AMF shall include the same ngKSI associated with the new NAS security context and the same algorithm choices as for the first access. After a successful second NAS SMC procedure over the other access (access B), both the UE and the AMF shall delete the old NAS security context.
2. Whenever the AMF sends a NAS SMC over access (access A) and AMF considers the UE to not be in CM-CONNECTED state on the other access (access B), the AMF shall additionally activate (if not already in use on the other access) the security context that is active on the other accesses. Similarly, whenever the UE receives a NAS SMC over the access (access A) and UE is not in CM-CONNECTED state on the other access (access B), the UE additionally activates (if not already in use on the other access) the security context on the other access.

In case of 3GPP access mobility or interworking with EPS, the following procedures apply:

1. If the UE is in CM-CONNECTED state on the non-3GPP access, then:
   1. if the AMF does not have the security context the UE is using on the non-3GPP access (e.g. KAMF change on 3GPP access when the AMF changes), then in order to activate the same NAS security context that is in use over the 3GPP access the AMF shall run a NAS SMC procedure on the non-3GPP access; or
   2. in the case of handover from EPS, then a mapped context will be in use on the 3GPP access and a different security context will be active on the non-3GPP access. To align the security contexts in use over both accesses, the AMF shall run a NAS SMC procedure over one access to take into use on that access the security context that is in use on the other access. In the case that a native security context is in use on the non-3GPP access, then the NAS SMC procedure shall be on the 3GPP access to take the native security context into use.
2. Whenever the AMF sends a Registration Accept over the 3GPP access and AMF considers the UE to not be in CM-CONNECTED state on the non-3GPP access, the AMF shall activate (if not already in use on the non-3GPP access) the security context that is in use on the 3GPP access on the non-3GPP access. The AMF shall keep a native security context that was in use on non-3GPP access if the security context in use on the 3GPP access is a mapped security context. In order to take this native security context into use, the AMF shall run a NAS SMC procedure.
   Similarly, whenever the UE receives a Registration Accept over the 3GPP access and UE is not in CM-CONNECTED state on the non-3GPP access, the UE activates (if not already in use on the non-3GPP access) the security context that is in use on the 3GPP access on the non-3GPP access. The UE shall keep a native security context that was in use on non-3GPP access if the security context in use on the 3GPP access is a mapped security context.

To recover from a failure to align the NAS security contexts due to a state mis-match between AMF and UE, the AMF can align the security contexts in use on the 3GPP and non-3GPP access using the a NAS SMC procedure during a subsequent registration procedure (that was either initiated by the UE or sent in response to a Service Reject if the UE sends a Service Request).

Integrity protection for NAS signalling messages shall be provided as part of the NAS protocol.

The input parameters to the NAS 128-bit integrity algorithms as described in Annex D shall be set as follows. The use and mode of operation of the 128-bit integrity algorithms are specified in Annex D.

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.

The supervision of failed NAS integrity checks shall be performed both in the ME and the AMF. In case of failed integrity check (i.e. faulty or missing NAS-MAC) is detected after the start of NAS integrity protection, the concerned message shall be discarded except for some NAS messages specified in TS 24.501. For those exceptions the AMF shall take the actions specified in TS 24.501 when receiving a NAS message with faulty or missing NAS-MAC. Discarding NAS messages can happen on the AMF side or on the ME side.

Confidentiality protection for NAS signalling messages shall be provided as part of the NAS protocol.

The input parameters for the NAS 128-bit ciphering algorithms shall be the same as the ones used for NAS integrity protection as described in clause 6.4.3, with the exception that a different key, KNASenc, is used as KEY, and there is an additional input parameter, namely the length of the key stream to be generated by the encryption algorithms. The use and mode of operation of the 128-bit ciphering algorithms are specified in Annex D.

NAS confidentiality shall be activated using the NAS SMC procedure or after an inter-system handover from EPC. Once NAS confidentiality has been activated, NAS messages without confidentiality protection shall not be accepted by the UE or the AMF. Before NAS confidentiality has been activated, NAS messages without confidentiality protection shall only be accepted by the UE or the AMF in certain cases where it is not possible to apply confidentiality protection. NAS confidentiality shall stay activated until the 5G security context is deleted in either the UE or the AMF.