## 1 Introduction
### Security Issues Faced by UAV Swarm  
Drones were originally used in the commercial field and have already achieved many good results, for example, Amazon has announced the benefits of integrating drones and related technologies into their existing delivery system, which has enhanced its rate of delivery [8, 9]. In recent years, the application of UAVs in the military has become an important research topic. Many UAV research projects were launched by American technology institutes, such as the Defense Advanced Research Projects Agency's \"Gremlins\" UAV Swarm Project and the US Naval Research Office's \"Low-Cost UAV Swarm\" project. Moreover, the European Defense Agency launched the European Swarm project in November 2016 to develop key technologies, such as autonomous mission decision-making and collaborative navigation for UAV swarms [10]. While drones have excellent combat capabilities, they also attract hackers attempting to break into them. In UAV swarm warfare, communication between UAVs is the main target of attack [11; 12]. A study by [13] indicates that many IoT (Internet of Things) devices lack basic security considerations, and the UAV swarm is no exception. In general, the risks faced by a UAV swarm mainly include the following.  
* Single point of risk brought by centralized management. At present, numerous UAV systems mainly adopt aerial- or ground-layered methods to control UAVs, which are highly centralized. When the management node is destroyed or taken over by the enemy, the control of the entire UAV network will become invalid, affecting the flight and operation of UAVs [14; 15; 16];
* The lack of reliable authentication mechanisms. Some software bugs or insecure settings may be exploited by enemy drones to invade [17; 18]. However, the current drone systems lack reliable identity authentication, which makes it difficult to ensure the authentication and data reliability between UAV nodes;
* Communication link interference and data tampering risk. UAV swarm generally transmits data through wireless channels. The unreliability of the wireless communication environment will seriously affect the interaction between nodes [19; 20; 21]. At the same time, the UAV terminal capacity is insufficient, and traditional encryption technology and data storage are difficult to use on drone equipment. So, UAVs are particularly vulnerable and tamper-proof, and there is a risk of wireless link monitoring and data theft;
* Difficulty managing drones belonging to different agencies. In typical military applications, it is sometimes necessary for multiple agencies to jointly execute a combat mission. Due to the different management methods, management platforms, and identity authentication methods of different parts, the management of UAVs belonging to different agencies is difficult.  
In summary, it is crucial to maintain the communication security between UAVs, realize the reliable identity management of UAVs, and the identity authentication of UAVs between different agencies. The emergence of blockchain technology provides a possible solution to the above problems [22; 23; 24; 25; 26]. Blockchain was first applied in decentralized virtual currency [27]. After several years of development, it plays an essential role in many fields, for example, finance, trade, the Internet of Things, medical care, renewable energy, smart cities, social media, copyright protection, etc. [28; 29; 30; 31; 32; 33; 34].