# SENTINEL: A Secure and Efficient Authentication Framework for Unmanned Aerial Vehicles
## 4 Overview of Sentinel  
In this section, we describe an overview of SENTINEL whose objective is to verify the authenticity of drones for detecting unauthorized drones in each fly zone. Figure 2 shows how SENTINEL works with the four entities--drone, ground station, certificate authority (CA), and operator (described in Section 3.1)--to achieve the objective of SENTINEL eventually. First of all, the operator obtains a certificate for the drone he/she owns from the CA (step 'a. Register' and 'b. Certificate') and installs the certificate on the drone (step 'c. Install Certificate'). The ground station also obtains its certificate from the CA (step 'a. Register' and 'b. Certificate'). After these steps, the drone, its operator, and the ground station are all registered to SENTINEL. That is, the drone and the ground station can be identified using the certificates issued by the CA.  
Each drone must initially obtain approval of its flight plan about which fly zones it will fly in from SENTINEL before its flight. This approval procedure is carried out based on the two-party mutual authentication and key agreement protocol between the drone and the ground station (For more details of the mutual authentication and key agreement protocol provided by SENTINEL, please refer to Section 5.1). As shown in step 1 in Figure 2, the drone and the ground station performs the mutual authentication and key agreement protocol. During this process, the drone also requests an approval of its flight plan to the ground station. Then the ground station decides whether or not to approve the flight plan requested from the drone. Consequently, a _flight session key_ for the drone is uniquely generated through the key agreement protocol between the drone and the ground station. As shown in step 2 in Figure 2, the approved flight plan, along with the associated drone's identity and the flight session key, is stored in the flight information database ('Flight Info. DB' in Figure 2), which is shared by all the ground stations in SENTINEL. By referring to this flight information database, a ground station decides on whether or not to allow the flight of each drone entering its fly zone. That is, when the drone moves from \"fly zone i\" to \"fly zone j\", a ground station located in \"fly zone j\" can identify and authenticate the drone by accessing the flight information database.  
Whenever the drone enters a new fly zone during its flight, it performs an authentication protocol with the ground station associated with the new fly zone by using the flight session key (see Section 5.2). For the authentication protocol, the drone first sends a message, including the drone's identity and authenticated code value computed with the flight session key to the ground station. After receiving the message, the ground station retrieves the drone's flight information (the flight session key and flight plan approved for the drone) with the drone's identity from the flight information database. The ground station then verifies the authenticity of the received authenticated code value with the flight session key and decides to allow the drone to fly in the current fly zone by checking whether that fly zone is contained in the approved flight plan.