# Hash-Chain-Based Cross-Regional Safety Authentication for Space-Air-Ground Integrated VANETs +
## 4 Cross-Regional Identity Authentication Model Based on the Hash Chain
### Authentication Process  
In the case of congested intersections or limited ground communication resources, vehicle nodes can use SAGIN to request more resource support from air SM nodes such as drones and airships. The SM nodes provide resource services for legal vehicles. The essential thing in this process is to ensure the identity security of mobile nodes such as vehicles. This paper compares and analyzes the node identity authentication process of traditional centralized network structure and distributed network structure based on the hash chain, as shown in Figure 4.  
When a vehicle node crosses from one region to another, that is an area managed by different SMs. The authentication process is as follows. Figure 4: Cross-regional authentication process under different network structures. Figure 3: Cross-regional identity information management structure based on the hash chain. The cross-regional authentication process of traditional:  
Traditional blockchain-based cross-regional activities are packaged according to the number of cross-regional nodes collected per unit time and sent to the corresponding RSU for verification. The corresponding RSU uses SM-this's public key for encryption. When SM-this receives the corresponding data packet, it decrypts and verifies it. If it is legal, the vehicle's identity materials are packaged and transmitted to CA. After CA verification, it is sent to SM-dest. The SM-dest node decrypts and verifies the vehicle identity and transmits it to the corresponding RSU to ensure the security of the vehicle nodes identity that enter across the zone. It can be seen from the above authentication process that a total of three encryption and decryption and signature authentication processes are required. At the same time, it needs to go through four times of waiting and transmission process. Therefore, the overall delay is as follows:  
\[T_{\mathit{tradition}}=n*(t_{v}+t_{d}+t_{e}+t_{s})*3+t_{\mathit{transmission}}*4 \tag{1}\]  
Among them, \(n\) is the number of cross-regional requests collected in a unit time. \(t_{e}\) represents the encryption process. \(t_{d}\) represents the decryption process, elliptic curve integrated encryption scheme [31] (ECIES) can be used in this process. \(t_{v}\) represents the verification process. In addition, \(t_{s}\) represents the signature activity, which is mainly used in the elliptic curve digital signature algorithm [32] (ECDSA). \(t_{\mathit{transmission}}\) means waiting and propagation delay. The cross-regional authentication process of hash chain-based:  
Before the vehicle nodes enter SAGIN for the first time, the CA completes the registration of the vehicle nodes. At this time, the CA is only responsible for managing the registration and periodic update or distribution of vehicle identities (\(R_{v}\)) and does not participate in the subsequent authentication process. During the registration process, we use the USRP to collect the wireless signal and then extract the RF fingerprint features of the vehicle node as its identity. For the identity authentication when the vehicle cross regions, on the one hand, the drone node needs to verify the identity information of the vehicle node, on the other hand, the authenticity of the cross-regional message must also be verified between the drone SM nodes. The specific process is shown in Figure 5, and the process is shown below. (1) The vehicle node \(V\) enters the sky-space integrated network for the first time and sends an identity authentication request to the regional SM node SM-this: \(Rv||\mathit{beacon}\). (2) The SM-this uses the onboard USRP module to collect the wireless signal of the vehicle and extracts its RF fingerprint \(R_{v}\)\({}^{\prime}\). Compare whether the collected \(R_{v}\)\({}^{\prime}\) and the \(R_{v}\) sent by the vehicle are consistent. If they match, the vehicle is judged to be a legal vehicle; otherwise, it is an illegal vehicle. Figure 5: The process of cross-regional RF fingerprint identification of vehicles based on the hash chain.