Content for  TS 33.501  Word version:  18.4.0

The 5G authentication and key agreement protocols provide increased home control. Compared to EPS AKA in EPS, this provides better security useful in preventing certain types of fraud as explained in more detail below. This increased home control comes in the following forms in 5GS:

• In the case of EAP-AKA', the AUSF in the home network obtains confirmation that the UE has been successfully authenticated when the EAP-Response/AKA'-Challenge received by the AUSF has been successfully verified, cf. subclause 6.1.3.1 of the present document.
• In the case of 5G AKA, the AUSF in the home network obtains confirmation that the UE has been successfully authenticated when the authentication confirmation received by the AUSF in Nausf_UEAuthentication_Authenticate Request message has been successfully verified, cf. subclause 6.1.3.2 of the present document.

When 3GPP credentials are used in above cases, the result is reported to the UDM. Details are described in clause 6.1.4.1a. The feature of increased home control is useful in preventing certain types of fraud, e.g. fraudulent Nudm_UECM_Registration Request for registering the subscriber's serving AMF in UDM that are not actually present in the visited network. But an authentication protocol by itself cannot provide protection against such fraud. The authentication result needs to be linked to subsequent procedures, e.g. the Nudm_UECM_Registration procedure from the AMF in some way to achieve the desired protection. The actions taken by the home network to link authentication confirmation (or the lack thereof) to subsequent procedures are subject to operator policy and are not standardized. But informative guidance is given in subclause 6.1.4.2 as to what measures an operator could usefully take. Such guidance may help avoiding a proliferation of different solutions. The feature of increased home control is also used to allow the UDM to keep track of the AUSF that stores the KAUSF to be used during e.g. the control plane solution for Steering of Roaming or UE Parameter Update procedures; i.e. the AUSF that stores the latest KAUSF generated after successful completion of the latest primary authentication reported to the UDM. After the UDM is informed that the UE has been successfully (re-)authenticated, the UDM shall store the AUSF instance which reported the successful authentication. If the UDM has been previously informed that the UE was authenticated by a different AUSF instance, the UDM may request the old AUSF to clear the stale security parameters (KAUSF, SOR counter and UE parameter update counter). If the UDM determines to delete the security parameters in the old AUSF, then the UDM shall use the Nausf_UEAuthentication_deregister service operation (see clause 14.1.5).

The information sent from the AUSF to the UDM that a successful or unsuccessful authentication of a subscriber has occurred, shall be used to link authentication confirmation to subsequent procedures. The AUSF shall send the Nudm_UEAuthentication_ResultConfirmation service operation for this purpose as shown in Figure 6.1.4.1a-1.

This subclause gives informative guidance on how a home operator could link authentication confirmation (or the lack thereof) to subsequent Nudm_UECM_Registration procedures from AMF to achieve protection against certain types of fraud, as mentioned in the preceding subclause. Approach 1: The home network records the time of the most recent successfully verified authentication confirmation of the subscriber together with the identity of the 5G visited network that was involved in the authentication. When a new Nudm_UECM_Registration Request arrives from a visited network, the home network checks whether there is a sufficiently recent authentication of the subscriber by this visited network. If not, the Nudm_UECM_Registration Request is rejected. The rejection message may include, according to the home networks policy, an indication that the visited network should send a new Nausf_UEAuthentication_Authenticate Request (cf. subclause 6.1.2 of the present document) for fetching a new authentication vector before repeating the Nudm_UECM_Registration Request. Approach 2: As a variant of the above Approach 1, Approach 2 is based on a more fine-grained policy applied by the home network; the home network could classify roaming partners into different categories, depending on the trust - e.g. derived from previous experience placed in them, for example as follows:

• For a visited network in the first category, the home network would require a successful authentication 'immediately preceding' the Nudm_UECM_Registration Request from an AMF.
• For a visited network in the second category, the home network would only check that an authentication in a network visited by the subscriber was sufficiently recent (taking into account that there may have been a security context transfer between the visited networks).
• For a visited network in the third category, the home network would perform no checks regarding Nudm_UECM_Registration Requests and authentication at all.

Further approaches are possible, depending on the home operator's policy.

The support of Home Network triggered authentication is optional for the HN and the SN. If both the networks (HN and SN) support Home Network triggered primary authentication, the following clauses apply.

The UDM may initiate primary authentication based on procedures initiated by the UE (e.g. UE registration in 5GC) or towards the UE (e.g. SoR/UPU) or events from other NFs, considering the local policy into account as well.

Step 0a and step 0b are the pre-requisite of the whole procedure. The UDM may execute other procedures (e.g. SoR/UPU) depending on the reason that motivated the UDM triggered (re-)authentication procedure in step 1.