## 1 Introduction
### Our Contribution  
In this work, a registration protocol is proposed, where a new drone joins the IoD system and exchanges secret keys. During the design, we considered that most of the drones are resource-constrained; hence lightweight cryptographic algorithms are applied. An important innovation in the registration scheme is the use of both PUF and blockchain technology. PUF technology provides a secure authentication, even for physical attacks (e.g., drone capture attack), and to verify the correctness of the PUF values securely, they are stored on the blockchain with only base station access. Besides the formal and informal security analysis, an efficiency evaluation is provided. When registering a new drone, it is essential that the new drone is authenticated and exchanges a key between the participants to ensure confidential and authenticated communication later. If PUF-based authentication is used, it is necessary to ensure that other parties in the IoD network know this value; therefore, the secrecy of this value must be preserved. As drones can be intercepted or hijacked, we restrict registration so that only base stations have access to this PUF value. However, with traditional central data storage solutions, several problems can arise. Such concerns include single points of failure, compromise of the central database, synchronization of the participants, and the immutability of providing data. Therefore, we propose a private blockchain, where only the base stations have access to the distributed ledger containing the PUF values of the drones. In this way, we can take advantage of the distributed nature of the blockchain, which fits the IoD architecture, share these data between base stations, manage or restrict access to this sensitive information, and synchronize and make these PUF values available with higher availability. Furthermore, the blockchain provides an immutable property, i.e., the data origin is ensured, and it can be seen that the manufacturer sets the value, and it has not been changed. Another major problem with the IoD network is the lack of scalability. When designing our protocol, we allowed any number of drones to be registered and added to the network. In addition to scalability, efficiency is another critical consideration, and we prefer using operations that are not resource-intensive (message authentication code, scalar multiplication) regarding the computational resource limitations of most of the drones. Lightweight cryptographic algorithms are designed to consume minimal computational resources. The NIST report [19] provides an overview of their lightweight cryptography project, where NIST approves the AES-GCM block cypher to provide the confidentiality and integrity of messages and standalone MACs, like CMAC, GMAC, and HMAC, for authentication. However, the use of lightweight cryptography is not only valuable for IoD, but it is also a frequently proposed solution for other areas of IoT [20; 21]. Our scheme applies these cryptographic primitives to correspond to the NIST. After the successful registration, the drones can send authenticated and confidential messages to every participant (registered drones or the base station) in the IoD network. It is vital to prove the security of the proposed protocol to ensure that it meets the security requirements and that attackers cannot compromise the registration. The security analysis of our scheme is carried out by ProVerif, and we check that an attacker cannot impersonate the participants, and the confidentiality of the key exchange is also provided. Besides examining the security requirements specific to traditional key exchange protocols, we also provide an informal security analysis in which we discuss other aspects as well. Such aspects include denial of service, jamming attacks, blockchain manipulability, drone capture attacks, man-in-the-middle, and replay attacks. Finally, we were careful in the practical implementation of the designed scheme.