# A Blockchain-Assisted Security Protocol for Group Handover of MTC Devices in 5G Wireless Networks
## 4 System Background  
### System Model  
The system under study follows the structure of the 5G wireless cellular network specified in the 3 GPP TS 23.501 R16 [21]. As depicted in Figure 1, the 5G wireless network consists of a core network (CN) and radio access networks (RANs). The devices involved in the CN mainly include the Access and Mobility Management Function (AMF), Authentication Server Function (AUSF), and Unified Data Management (UDM), while Next Generation Node B (gNB) and user equipment or MTCDs exist in the RANs. The entities primarily involved in the handover process are MTCDs, gNB, AMF, AUSF, and UDM. **AMF:** AMF is responsible for managing access and mobility-related tasks of user devices, ensuring network efficiency, security, and reliability. In the system, in our group handover authentication phase, the AMF does not need to forward authentication information anymore. **AUSF:** AUSF is responsible for handling authentication and security-related tasks for MTCDs. It verifies the identity of MTCDs and the security credentials they provide. In our system, the AUSF primarily generates anonymous identities for both MTCDs and gNBs. **UDM:** UDM is responsible for managing and accessing user data. In our system, the UDM stores the permanent identity identifiers of MTCDs and gNBs. Figure 1: System model. **MTCD:** In our system, an MTCD, which is a user in the 5G network, initially needs to send a request for registering its identity to the AMF. The MTCD undergoes identity authentication with the gNB upon accessing the network. **gNB:** gNB is the base station in the 5G wireless network. It is responsible for MTCD's access and connection, as well as reasonably allocating wireless communication resources. **Blockchain:** Blockchain is a distributed database that ensures secure storage and sharing of data by creating a continuously growing and tamper-resistant chain of data records. In our system, a private blockchain is utilized as the secure data structure consisting of registered MTCDs. Each gNB maintains a backup copy of the blockchain. In the 5G wireless networks specified by the 3 GPP standards, when an MTCD enters the signal range of a gNB, it sends an authentication request to the gNB. This request is then forwarded by the gNB to the AMF, which is in turn forwarded to the AUSF. After the AUSF retrieves the necessary key information from the UDM, it executes the authentication process and returns the authentication response to the MTCD through the AMF and the gNB, completing further authentication procedures. In a typical 5G core network, one AUSF typically serves multiple AMF, and each AMF manages connections with multiple gNBs. By the proposed scheme, decentralization of the authentication entities is achieved by integrating blockchain technology with the 5G system model specified by the 3 GPP standard. In this system, each gNB is part of a private blockchain network and holds a copy of the entire blockchain. Once an MTCD completes the initial registration, there is no need to forward the authentication requests to the AUSF and UDM again. When an MTCD needs to undergo a handover authentication, the AMF only needs to forward the information of the service-gNB (s-gNB) to the target-gNB (t-gNB). At this point, the gNB can directly verify the identity of the MTCD using the information stored on the blockchain, without further forwarding to the AMF, AUSF, or UDM. To simplify the design, the proposed scheme focuses on the most common scenario, where all MTCDs connect to the gNB in their home network via 3 GPP standard access technologies. Moreover, the connection between the gNB and the 5G core network is provided by a wired connection that is safeguarded by an Internet Protocol Security (IPSec) tunnel. If the MTCD and the gNB successfully achieve mutual authentication, the MTCD can be considered to have secure access to the legitimate 5G network.