Editing My 3GPP 33.501 notes (section)

== 6.3 Security Contexts ==

=== 6.3.2 Multiple registrations in same or different serving networks ===

==== 6.3.2.0 General ====

There are two cases where the UE can be multiple registered in different PLMN's serving networks or in the same PLMN's serving networks. The first case is when the UE is registered in one PLMN serving network over a certain type of access (e.g. 3GPP) and is registered to another PLMN serving network over the other type of access (e.g. non-3GPP). The second case is where the UE is registered in the same AMF in the same PLMN serving network over both 3GPP and non-3GPP accesses. The UE will establish two NAS connections with the network in both cases.
:NOTE: 	The UE uses the same subscription credential(s) for multiple registrations in the same or different serving networks.

==== 6.3.2.1 Multiple registrations in different PLMNs ====
The UE shall independently maintain and use two different 5G security contexts, one per PLMN's serving network. Each security context shall be established separately via a successful primary authentication procedure with the Home PLMN.

The ME shall store the two different 5G security contexts on the USIM if the USIM supports the 5G parameters storage. If the USIM does not support the 5G parameters storage, then the ME shall store the two different 5G security contexts in the ME non-volatile memory. Both of the two different 5G security contexts are current 5G security context.

The latest K<sub>AUSF</sub> result of the successful completion of the latest primary authentication shall be used by the UE and the HN regardless over which access network type (3GPP or non-3GPP) it was generated.
 
The HN shall keep the latest K<sub>AUSF</sub> generated during successful authentication over a given access even if the UE is deregistered from that access, but the UE is registered via another access.

==== 6.3.2.2 Multiple registrations in the same PLMN ====

When the UE is registered in the same AMF in the same PLMN serving network over both 3GPP and non-3GPP accesses, the UE shall establish two NAS connections with the network. Upon receiving the registration request message, the AMF should check whether the UE is authenticated by the network. The AMF may decide to skip a new authentication run in case there is an available 5G security context for this UE by means of 5G-GUTI, e.g. when the UE successfully registered to 3GPP access.

If the UE registers to the same AMF via non-3GPP access, the AMF can decide not to run a new authentication if it has an available security context to use. In this case, the UE shall directly take into use the available common 5G NAS security context and use it to protect the registration over the non-3GPP access. If there are stored NAS counts for the non-3GPP access for the PLMN in the UE, then the stored NAS counts for the non-3GPP access for the PLMN shall be used to protect the registration over the non-3GPP access. Otherwise, the common 5G NAS security context is taken into use for the first time (partial) over non-3GPP access. In this case, the UL NAS COUNT value and DL NAS COUNT value for the non-3GPP access needs to be set to zero by the UE before the UE is taking the 5G NAS security context into use over non 3GPP access.

The AMF and the UE shall establish a common NAS security context consisting of a single set of NAS keys and algorithm at the time of first registration over any access. The AMF and the UE shall also store parameters specific to each NAS connection in the common NAS security context including two pairs of NAS COUNTs for each access (i.e. 3GPP access and non-3GPP access). The connection specific parameters are specified in clause 6.4.2.2 of the present document.