## 5 Challenges and Future Research Directions
### Security Risks  
As per our survey, we discussed various emerging technologies that can be employed for secure authentication in next generation UAV communication such as blockchain. However, there are still several security risks associated with these technologies which require further research.  
Blockchain relies in part on transparent transactions. Hence, user privacy may be jeopardized in blockchain-based systems. Further, other attacks against blockchain include selfish mining [127] and Sybil attacks [128]. In a Proof-of-Work consensus mechanism,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Ref.** & **Concept** & **Description** \\ \hline
[110] & Deployment of BCT schemes & Convergence of IoT in blockchain to enable intelligent distribution in the technical model of 6G networks. \\
[111] & Four key aspects of 6G networks & Real-time intelligent edge computing, distributed artificial intelligence, intelligent radio and 3D intercoms. \\
[112] & DM privacy requirements & Research issues and potential solutions for blockchain-based DM toward 6G. \\
[113] & Edge-enabled UAVs & a Software Defined Network (SDN) model for data transfer the within the edge and cloud layers. \\
[114] & P4C algorithm & Protection of the privacy of vehicles and accelerate the authentication process. \\
[115] & Formal informal security & Authentication cycle with a minor increase in computation cost but provides all security goals along with privacy. \\
[116] & B-RAN & Secure paradigm for 6G networking by utilizing blockchain technologies. \\
[117] & zkLedger & Blockchain distributed ledgers solution based on zero knowledge-based cryptography. \\
[119] & Network intelligence and & AI toward a future 6G vehicular network. \\  & self-learning & Blockchain technology to manage the identity certificates of entities in the network and superwise the authority. \\
[120] & Unified identity authentication & AI and blockchain to evaluate and optimize the quality of intelligent service \\
[121] & Indoor navigation system & blockchain application for key generation in IoV. \\
[122] & key enabler of ITS & \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._selfish mining is a means to obtain more reward while wasting honest miners' resources and energy, whereas a Sybil attack is when a user creates several blockchain identities in order to control it. Finally, smart contracts on the blockchain have considerably increased the application of the blockchain by allowing software-defined contracts between participants to be transacted as transactions [129]. Smart contracts, however, as versatile as they are, may inject a number of new attack surfaces into the system. Vulnerabilities in the blockchain, the smart contract and the virtual machine executing code are the three main attack surfaces for blockchain-based smart contracts in 6G enabled UAV communication [130]. These above discussed aspects must attract future researchers' attention for comprehensive research, in order to achieve blockchain-based light weight authentication model resistant to vital threats as highlighted above. The most optimum and finest blockchain model addressing existing limitations can be a reliable and functional solution for authentication of future UAV networks in terms of lightweightness as well as robust security [131].