## 3 Overview of 3 GPP AKMA
### AKMA Architecture and Operation
#### 3.1.3 AKMA Procedures  
The AKMA procedure consists of two phases: the initial phase and the authentication phase, as illustrated in Figures 4 and 5, respectively. **AKMA initial phase:** Before a UE contacts the AF for AKMA purposes, sufficient AKMA key material must be available to derive application-specific keys, as specified in the AKMA key hierarchy shown in Figure 3. The process begins with the AUSF generating the AKMA anchor key \(K_{AKMA}\) following the 5G primary authentication. The AUSF generates AKMA key material only for UEs that are authorized to use AKMA, as indicated by the AKMA subscription data stored in the UDM. For this, the UDM provides the AUSF with the authentication vector, the AKMA indicator, and RID for authorized UEs in the Nudm_UEAuthentication_Get Response message during the primary authentication. Once the primary authentication is successful, the AUSF derives the AKMA anchor key \(K_{AKMA}\) and its identifier A-KID from the \(K_{AUSF}\), which is then stored. In turn, the derived values are securely transmitted along with the UE's identifier (SUPI) to the AAnF. Upon a receipt, the AAnF stores such key material for future AKMA application key generation, forming the AKMA context for a specific UE as SUPI, A-KID, and \(K_{AKMA}\). On the UE side, the  
Figure 3: AKMA key design. \(K_{AKMA}\) and A-KID are generated before the UE initiates communication with an AKMA AF, ensuring the key material is ready at both the AAnF and the UE for further use. **AKMA authentication phase:** To ensure secure communication between the UE and the AF, the AKMA framework uses a dedicated AKMA application key (\(K_{AF}\)). If the UE wants to access the 5G cAPP, it begins the AKMA authentication phase by sending the AF the Application Session Establishment Request - message including its A-KID. If the AF does not have an active context for this A-KID, it sends the Naant_AKMA_ApplicationKey_Get Request message - with its identity (AF_ID) to the AAnF. The AAnF verifies if the UE is authorized to use AKMA by checking the presence of the \(K_{AKMA}\) identified by the A-KID. If the UE is authorized, the AAnF derives the \(K_{AF}\) from the \(K_{AKMA}\) and stores the \(K_{AF}\)'s expiration time. It then sends the \(K_{AF}\), SUPI, and expiration time to the AF. If the UE is not authorized, the AAnF indicates this failure to the AF. Note that during this phase the UE and the AF execute the \(\text{Ua}^{*}\) protocol, which is protected with the \(K_{AF}\), to establish their secure channel. If the request for \(K_{AF}\) fails, the AF may reject the session, prompting the UE to re-initiate the request with updated parameters. **AKMA key request and change:** The AKMA framework incorporates procedures to manage the lifecycle of keys, ensuring they remain secure and up-to-date. Key changes within the AKMA system are necessary to maintain the integrity and confidentiality of communications. The primary components involved in key changes include the two AKMA keys, \(K_{AKMA}\) and \(K_{AF}\). These keys are subject to specific procedures for re-keying and refresh based on their lifetimes and usage scenarios. On one hand, the AKMA anchor Key \(K_{AKMA}\) is re-keyed through the primary authentication process. Whenever a new primary authentication occurs, a new \(K_{AUSF}\) is generated, from which a new \(K_{AKMA}\) is derived. This ensures that the \(K_{AKMA}\) remains secure and is periodically refreshed to prevent potential security breaches. The re-keying process effectively replaces the old \(K_{AKMA}\) with the newly derived key. On the other hand, the AKMA application key \(K_{AF}\) has a predefined lifetime based on the operator's policy or configuration parameters. When the \(K_{AF}\)'s lifetime expires, it needs to be re-keyed. The AF can initiate a request to the AAnF for a new \(K_{AF}\). The UE may also trigger this re-keying by attempting to re-establish a session with the AF. The \(K_{AF}\) can be refreshed without necessarily waiting for the \(K_{AKMA}\) to be renewed.