## 1 Introduction
### Blockchain and Related Work  
The security of the blockchain relies on a consensus mechanism rather than a trusted third party [35]. For the Bitcoin system, as long as more than 25% of the miners are honest, its security and trust can be guaranteed [36]. Each block is linked by a hash value, and the content of previous blocks can be traced from the last block. All stored information is managed by all participating nodes or several authoritative nodes. More importantly, any change in block data will affect all subsequent blocks [37]. Due to the tamper-proof, distributed, and trusted advantages of the blockchain, it is combined with UAV in the military, aiming to solve some risks faced by UAV systems [38; 39]. So far, the application of the blockchain in UAV systems was extensively explored by researchers. To improve the privacy and security of UAV equipment, [PERSON] et al. [8] proposed a blockchain-based solution that stores some data on Ethereum, but there is no mention of how to manage the drone's identity and how to transmit data securely. Blockchain-based federated learning in UAVs beyond 5G networks was studied by [PERSON] et al. [14], who focused on the privacy and security of UAVs in open channels. However, the identity management and authentication of UAVs were not studied in depth. For secure communication and network management, [PERSON] et al. [40] proposed a software architecture of UAV supporting blockchain, which makes use of the distributed and tamper-proof advantages of blockchain to store data. A method to realize the privacy and security of drones using blockchain is proposed by [PERSON] et al. [41], in which interactions between UAVs and other entities are secured only by encryption. To solve the security and privacy issues of UAV data, [PERSON] et al. [42] studied a privacy protection method for UAV big data based on blockchain technology, but ignored how to ensure that the identities between UAVs are trustworthy to each other. [PERSON] et al. [43] studied a method for UAV identity authentication, issuing a set of x.509 certificates to each drone and storing these certificates on the blockchain. [PERSON] et al. [44] explored the combination of drones, blockchain, and cloud services, where complex computations are performed by cloud services, and the blockchain stores some key information. However, there is no mention of how to ensure that cloud services, drones, and blockchain can be trusted with each other. [PERSON] et al. [45] proposed a method of secure data transmission based on a blockchain but did not study how to implement UAV identity management and authentication. A blockchain-based secure system architecture was studied by [PERSON] and [PERSON] [46], in which drones act as nodes in a blockchain network. [PERSON] et al. [47] designed a distributed scheme of UAVs based on blockchain technology, but it was only limited to the design of a lightweight structure of blockchain and the secure storage of data. Although the above studies proposed some constructive methods, some problems are still ignored. The nodes in the blockchain network need to frequently synchronize transaction data while using UAV with the limited computing power as a node is currently impractical. In addition, most of the above studies only focused on the security of UAV data storage, and rarely studied the security of data transmission. More importantly, there is a lack of a safe identity management model and reliable identity authentication for drone swarms belonging to different institutions. It was also proposed by [PERSON] et al. [48] that identity authentication will play an important role in future 5G communication. Although [PERSON] et al. [43]. proposed an identity authentication method, it is still limited to the public key infrastructure (PKI) system and lacks a set of secure and reliable communication protocols. The following Table 1 provides a summary of recent studies conducted using blockchain technologies.