# Cross-Platform UAV Swarm Key Management in Denied Environments
## 3 Cross-Platform Key Management of UAV Swarms in Denied Environments
### Private Key Establishment in the Regrouping and Arrival Stage
According to the performance of several mainstream UAVs analyzed in [19], \"RQ-23 Tigershark\", \"MQ-9 Reaper\", and \"MQ-1 Predator\" can fly above 24 h. In this study, we set the UAV flying time as 24 h, and the task cycle is 2 h, that is, \(m=12\). For the convenience of comparison, the transmitted data are set to be 256-bit. Table 7 shows the communication overheads during the key establishment process in different schemes. #### 4.3.3 Storage Overhead

In the ACPKMS scheme, the \(UAV_{j_{i}}\) usually needs to store \(UAV_{-}params\):

\(p_{i}=\left(K_{0},seed,UAV_{-}SK_{j_{i}},ID_{j},S_{ID},S_{UAV-ID_{j_{i}}},P_{ m}\right)\). For the sake of more convenient comparison, the size of the transmitted data is set at 256 bit; that is, the seven parameters have the same size, so the storage overhead is 7 log \(p\). Though the ACPKMS scheme has a higher storage overhead and is slightly higher than schemes in [20], the computing and communication overheads are reduced, which has sound security and is more applicable to the dynamic UAVGN featured by instability and unreliability [21]. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Schemes**} & \multicolumn{2}{c}{**Communication Overhead**} & \multirow{2}{*}{**Storage Overhead**} \\ \cline{2-3} \cline{5-5}  & **Request Node** & & **Response Node** \\ \hline ACPKMS & \(5\log p\) & \(5\log p\) & \(7\log p\) \\ Scheme in [16] & \(3\log p\) & \([(2n_{t}+3)t+2]\log p\) & \((2n_{t}l+2l+7)\log p+\) \\  & & \(2n_{t}\log p+16(n_{t}^{2}+n_{t})\) \\ Scheme in [17] & \(8\log p\) & \([(2n_{t}+3)+4]\log p\) & \((2n_{t}l+2l+7)\log p\) \\ Scheme in [18] & \(5\log p\) & \([5(t-t_{1})+3]\log p\) & \(4\log p\) \\ \hline \hline \end{tabular} \(t\) represents the stage of the task, and \(n\) is the number of UAVs.

\end{table} 
Table 7: Communication and storage overheads of nodes during key construction of different schemes. Figure 9: Computing overhead during key establishment [16; 17; 18]. ## 5 Conclusions

A simple but efficient swarm key management scheme based on the hash chain and the Chinese Remainder Theorem (CRT), termed ACPKMS, is proposed here as a solution to secure communication of UAV swarms across platforms. The proposed scheme, by combining the node update key and the session update key, achieves the local static update and the session dynamic update, which enhances the key update efficiency and reduces the communication overhead. Meanwhile, the simple swarm head selection method in the scheme further enhances the flexibility of cross-platform key establishment of UAV swarms. The scheme is compared with other schemes through experiments, and it is found that the EHGKM has good security and high efficiency: the requestor node takes 1.18 us in the key establishment stage to acquire the key establishment broadcast message and 3.36 us to recover the key; the ACPKMS scheme can meet the requirements for the UAV swarm key management. In future works, we will study the key management solutions for multiple UAV swarms and design a key management scheme for multiple UAV swarms, thereby providing more efficient and secure key management solutions to the intelligent and integrated development of UAV swarms. Conceptualization, L.Y. and Z.F. ; methodology, L.Y. and C.Z. ; software and validation, L.Y., Z.F. and C.Z. ; formal analysis, H.J., C.Z. and Z.F. ; writing--original draft preparation, L.Y. and Z.F. ; writing--review and editing, H.J. and L.Y. ; funding acquisition, Z.F. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China, grant number 61902427. Not applicable. The data are available from the corresponding author [PERSON] upon reasonable request. The authors would like to thank all the editors and reviewers who participated in the review. The authors declare no conflict of interest. ## Abbreviations

* [1][PERSON]; [PERSON] A survey on cluster-based routing protocols for unmanned aerial vehicle networks. _IEEE Access_**2018**, \(7\), 498-516.