## 2 Blockchain-Based UAV Swarm Identity Management Model  
At present, the management of drones is centralized by the owner of the drones. This centralized management often brings some problems, such as a single point of failure. Once the management node fails, all UAVs belonging to it may be invaded. Moreover, each agency has its own management scheme, which leads to great differences in the management systems. As the swarms of drones that perform tasks collaboratively by multiple agencies continue to expand, which greatly increases the difficulty of drone management and the authentication of drones from different agencies to each other. The identity authentications of almost all systems adopt PKI systems. A centralized third-party organization issues digital certificates for participating nodes, which contain the entity's public key and related identity information. These certificates establish the association between entity identity information and its public key and are transmitted publicly in the network. Although the PKI system can complete identity authentication, the following problems still exist. * The authentication process of the digital certificate is cumbersome. The OCSP (online certificate status protocol) must be asked for each verification of digital certificates, and it needs to update the CRL (certificate revocation list) signed by the CA (certificate authority);
* The identity of the UAV is centrally managed by the owners of the drones or some trusted third parties, which often has a single point of failure and leads to the disclosure of UAV identity information. Therefore, a blockchain-based UAV identity management model (B-UIM-M) is proposed. Using blockchain to manage UAV identity information avoids the single point of failure caused by centralized management and provides distributed identity authentication for UAVs of different agencies so that their identity authentications are no longer dependent on trusted third parties. This model is mainly divided into two layers. The first layer manages the UAV-owning organizations to ensure mutual trust between them, and the second layer manages UAVs to ensure that UAVs belonging to different organizations can verify each other's identities. The details are shown in Figure 1.  
In Figure 1, \(DID_{O}\) and \(DID_{U}\) are the DIDs of the UAV owners and the UAVs, respectively. Moreover, \(SK_{O}\) and \(SK_{U}\) separately represent the private keys of UAV owners and UAVs, \(O=1,2,\ldots,n\), \(U=1,2,\ldots\ldots,k\). The DID document is mainly used to store the identity information of UAV owners and UAVs. Identity information distributed to them can be transmitted through physical media, and identity information stored on the blockchain can be transmitted through secure protocols. In B-UIM-M, a blockchain-based identity information distribution process is proposed. First, the government management department generates identity information for the UAV owners uniformly, and this identity information is distributed to the corresponding UAV owners after tamper-proof storage by blockchain. The identity information of government management departments must be authenticated and stored by blockchain. The drone owner generates identity information for the drone when it is registered. Similarly, the identity information is distributed to the corresponding drone after tamper-proof storage by the blockchain. The specific process of identity information distribution is shown in Figure 2.  
Figure 1: Blockchain-based UAV identity management model. Figure 2: Blockchain-based identity information distribution processing. To ensure the authenticity and credibility of the identity information, after the drone owner and the drone respectively obtain the identity information, they will request the blockchain for the following verification. * Whether the identity information has been successfully stored in the blockchain;
* Whether the identity information distribution agency is credible, i.e., whether its identity information has been tamper-proof-stored by the blockchain.