# SENTINEL: A Secure and Efficient Authentication Framework for Unmanned Aerial Vehicles
## 1 Introduction
Therefore, we should avoid doing the certificate exchange process as many as possible. Second, asymmetric cryptography used in certificate-based schemes is significantly slower and requires more power consumption than symmetric cryptography [13]. Therefore, it seems better to use symmetric cryptography instead of using asymmetric cryptography. Third, since the size of a digital certificate is quite large, it could be a burden for resource-constrained drones. Currently, X.509 v3 [14] is most popularly used as the de facto standard of digital certificates, and its size is about 1 KB-1.5 KB on average [15]. However, because the X.509 v3 certificate was initially designed for  
Figure 1: Communication channels between drones and ground stations in IoD. computers and servers with sufficient resources, thus it is desirable to develop a lightweight certificate format that is viable for IoD. In this paper, we propose an authentication framework named SENTINEL (Secure and Efficient autheNTIcation for uNmanned aErial vehicleLes) for IoD to provide mutual authentication between drones and ground stations. To avoid the computational and traffic overheads caused by certificate exchanges and asymmetric cryptography operations, we introduce the idea about the _flight session key_ between a drone and a ground station created by the key agreement protocol between them once the drone takes off. The flight session key is used as a MAC (message authentication code) key for efficiently authenticating drones in subsequent sessions whenever authentication is needed. To provide a secure key agreement protocol, we build public key infrastructure (PKI) using a lightweight certificate format that is more suitable for the IoD environment than the conventional X.509 certificate format. Our main contributions are summarized as follows:  
* We propose SENTINEL (Secure and Efficient autheNTIcation for uNmanned aErial vehicleLes) to provide mutual authentication between drones and ground stations. SENTINEL initially generates a _flight session key_ for a drone having a flight plan and registers the flight session key and its flight plan into a centralized database that can be accessed by all ground stations. The registered flight session key is used to authenticate the drone by ground stations as the MAC key while it is flying (see Sections 4 and 5). * We designed a lightweight certificate format that is suitable for IoD environments. Compared to the conventional X.509 v3 certificate, the proposed certificate is designed to contain only minimal information required to build public key infrastructure in IoD environments. To further reduce the size of certificates, we adopt a binary format in the proposed certificate, instead of a human-readable text format adopted in X.509 v3 certificates. Our results show that the size of the proposed certificate was about 3.7 times smaller than the size of a typical X.509 v3 certificate (see Section 6). * We implement a prototype of SENTINEL to show its feasibility and efficiency. Our results demonstrate that the execution time of the authentication protocol in SENTINEL is 3.1 times faster than the \"TLS for IoT\" protocol on average (see Section 7). We also formally verify the security of SENTINEL using ProVeif, which is a formal verification tool (see Section 8). The rest of this paper is organized as follows. In Section 2, we give an overview of the related work. In Section 3, we provide background knowledge such as terminology, notations, and present the threat model considered in this paper. In Section 4, we describe the overview of SENTINEL. In Section 5, we present the protocols for key generation and authentication. In Section 6, we present the digital certificate structure used in SENTINEL. In Section 7, we present the experimental results to show the efficiency of SENTINEL. In Section 8, we formally verify the proposed framework using a verification tool called ProVerif. Finally, in Section 9, we discuss conclusions.