## 4 Authentication in UAV Networks  
Unmanned aerial systems (UASs) consist of one or more than one UAVs. The stated unmanned aerial vehicles are operated and controlled through a reliable communication channel by GCS [78]. Utilization of UAVs is found in commercial, civilian and military uses. From surveillance to reconnaissance, security purposes, traffic monitoring, items delivery, etc., all are applications of UAVs employing modern communication networks in the future. Swarm employment is providing promising advantages in multiple civilian applications [79]. Graceful degradation is achieved in case of any technical fault as alternate UAV can take over the mission role and task in such scenarios. Moreover, robustness as well as availability of communication with GCS is ensured beyond the line of sight through establishment of the Adhoc network [80]. The probability of mission failure is minimized as in the swarm system, multiple UAVs are employed which act as system redundancy. One of the pertinent advantages of these systems is reduced maintenance cost. Communication is of key importance in a flock of UAVs. The major reasons for communication needing to be robust and reliable in operation of such UAV networks is the high mobility of UAVs, irregular distance between each UAV nodule which results in inconsistent link quality, limited capacity of UAVs in terms of onboard available power and the ever-changing topology of the UAV network due to the mobile nature. Moreover, due to limited battery storage, unmanned aerial systems communication becomes challenging [81]. Secure networks are an essential requirement of worldwide users in connection with different applications and have been an unvarying challenge for researchers in ever-evolutionary modern communication models. Similarly, it has been a growing concern in unmanned aerial vehicle systems. It is a significant consideration in wireless networks that they are intrinsically insecure [82]. Figure 11: UAV regulations domains. Wireless networks can be victim of sniffing, eavesdropping and other related wireless network attacks that include MitM, impersonation attacks, DoS [83] and Sybli. These attacks are vulnerable as they compromise privacy; moreover, they can result in major denial of the overall system by exhausting system bandwidth, memory, power, etc. [84]. The jamming of wireless communication between unmanned aerial vehicle system elements can be devastating. Functional as well as operational control of the unmanned vehicle can be lost through such attacks, causing overall system hacking by the enemy [85]. The classic example of eavesdropping is through man in the middle attacks as malicious element records and the transport of information is through passive means to attacker. Each eavesdropped information packet can be altered in Man in the middle active attack to take over the command and control of the UAV system or to inject manipulated data into the network system [86]. In the same way, such MitM attacks can be excessively destructive as the same can cause a complete crash of UASs by failing the complete authentication mechanism of the UAV network [87]. Replay attacks can result in overall compromise of the system as they target authentication systems by replay technique of already captured data to gain unauthorized access from the legitimate system server [88]. It is quite a successful approach by the attackers to obtain unauthorized system access. One of the techniques used by attackers is Sybil, in which a malevolent node plays to be numerous different people in the system. This attack allows for the injection of fake data and routing disruption [89; 90]. This section covers elements broadly categorized in second category of Specification Group (SG). UASs work with a minimum or no human interaction and the authentication process in such system is node to node. Approaching GCS by any node in UAS, it is vital that all nodes are authenticated. However, the limited computing and power resources of UAVs make off-the-shelf security solutions impractical [91].