# Hash-Chain-Based Cross-Regional Safety Authentication for Space-Air-Ground Integrated VANETs +
## 2 Related Work
### Blockchain-Based Security Authentication  
The high-speed mobility of nodes in SAGIN makes the network topology highly dynamic [14]. If further use, centralized key management may cause channel congestion or paralysis during key distribution. Therefore, the researchers consider adopting a distributed network structure to provide corresponding support for the development of SAGIN based on blockchain technology.  
With the rapid development of blockchain technology, new exploration based on blockchain technology has attracted extensive attention [15]. Blockchain technology originated from the Internet finance, such as the well-known Bitcoin [16], Ethereum [17], Super Ledger [18], etc. The blockchain can also be used as a decentralized distributed database, with the characteristics of sharing, non-tampering, traceability, and final consistency. Therefore, the application of blockchain in other industries is also developing rapidly. [PERSON] et al. [19] discussed the key technologies and elements that support the development of smart cities based on blockchain shared services. Moreover, the combination of blockchain technology and the Internet of Things (IoT) development has also become a popular application. [PERSON] et al. [20] summarized the challenges and solutions of blockchain in IoT applications. Based on the Peer-to-peer security and privacy of the blockchain, a new private lightweight blockchain architecture based on IoT security is proposed. To effectively eliminate the high overhead of the blockchain while maintaining its security and privacy, it provides new ideas for the development of IoT applications.  
As a special IoT with high-speed mobility, VANETs have a distributed topology structure similar to the blockchain network structure. Therefore, the applications of blockchain in the VANETs have also developed rapidly. Blockchain technology provides solutions based on security, transparency, and data management for the development of VANETs [21]. A lightweight authentication method based on blockchain for the communication security problems faced by intelligent transportation systems is proposed in [22]. The use of distributed vehicle access and revocation schemes instead of the central management digital certificate method can effectively reduce the computing cost and system security response delay. A blockchain-based architecture for Internet smart car ecosystem security and user privacy is proposed in [23]. Wireless remote software updates and other new services (such as dynamic vehicle insurance fees) illustrate the effectiveness of the architecture. We can see that the blockchain technology will have a broader application space in the future, especially in the VANETs.  
In the SAGIN, due to the security requirements such as the legitimacy of node identity and data privacy, key blockchain technologies can be combined to improve the reliability of the overall network. However, SAGIN's high-speed mobility and low latency pose new challenges to blockchain technology. Traditional blockchain technology, such as Bitcoin, takes about 10 min to generate blocks, and as the number of block nodes increases, the time and computing power required by the consensus mechanism increase exponentially. This cannot meet the requirements of SAGIN. Therefore, by simplifying the blockchain structure and building a consensus mechanism based on the hash chain, this paper provides support for SAGIN's data transmission and security.