# SENTINEL: A Secure and Efficient Authentication Framework for Unmanned Aerial Vehicles
## 2 Related Work
### Authentication among Drones, Ground Stations, and Other Parties  
Besides the drone incident mentioned in Section 1, several other incidents were motivating drone authentication for detecting suspicious/unauthorized drones within fly zones. In July 2018, Greenpeace members intentionally crashed a drone into the nuclear plant facility in Lyon, France, to warn the vulnerabilities of critical infrastructures against attacks [20]. In December 2018, there was another serious incident at London Gatwick Airport, where the airport was shut down for about 30 hours due to the unknown drones around the runway, which resulted in disrupting approximately 1000 flights and 140,000 passengers [21]. In August 2016, there was a malicious attempt using a drone to secretly film a victim entering the PIN and withdrawing money from an ATM to identify the victim's PIN [22] eventually. All these incidents demonstrate that it is critical to detect and disable flights of unauthorized drones.  
Several studies focused on designing secure and efficient authentication schemes for drones, ground stations, and other smart objects (e.g., sensors). [PERSON] et al. [23] proposed a suite of cryptographic protocols to provide an authenticated key agreement between drones and smart objects in smart city environments to enable secure communications in three different communication scenarios including one-to-one, one-to-many, and many-to-one. The authors mainly focused on designing protocols without relying on public key infrastructure. [PERSON] et al. [19] proposed an authentication framework for secure communication between drones which utilizes an online/offline signature scheme and the assistance of mobile edge computing technology. Besides secure authentication, the proposed approach also considers preserving the privacy of drones' identities and locations. [PERSON] et al. [24] proposed an authentication system for IoD environments to provide mutual authentication among system entities such as drones, operators, and ground stations. To be specific, this approach relies on the coordination of the trusted authority center which issues public/private key pairs for system entities, and a ground station decides whether to approve the flight path of a drone or not through mutual authentication between the drone and the ground station as well as between the operator of the drone and the ground station. Although these approaches also use a simplified public key infrastructure for issuing keys to system entities along with digital signatures, they did not take into account the computational and communication overheads introduced by exchanging keys and signatures and performing signature operations. Unlike these approaches, our approach minimizes the use of expensive certificate-based authentication only for the initial authentication of each drone to generate a flight session key. Drones can use a more lightweight authentication scheme with the flight session key for all subsequent sessions to reduce the authentication overheads. Our approach also uses a lightweight certificate format that is more suitable for resource-constrained drones than the conventional X.509 certificate format.