# Efficient Security Scheme for Disaster Surveillance UAV Communication Networks
## 7 Performance Evaluation
### Computation Complexity  
The monitoring drones' main task is scanning the area and forwarding photos or videos about the situation in the disaster area to \(CS\). The drones are not capable of performing complex cryptographic operations to preserve their energy. Our proposed scheme implements the efficient Ring-LWE crypto-system [33] on the drones to guarantee the security requirements without increasing the computation overhead on drones; the monitoring drones need to perform one encryption processes only per round. During the mission time, the monitoring drones encrypt and then send their periodic reports to the two cluster heads every second, which equals 1800 messages in total but only 900 encryption operations. \(C_{D}=900*T_{e}\), where \(T_{e}\) is the encryption time; the computation overhead per drone is trivial and does not consider a load on the limited-computation capabilities units. For the whole subarea, the total computation delay equals \(C_{T}=900*T_{e}*I*J\), where \(I\) is the number of groups in the subarea and \(J\) is the current number of participated drones in each group, i.e., \(J\) varies from one group to another and varies at the same group from time to another, while the computation load for the join/leave process is neglected as the process utilizes symmetric key encryption with tiny computation overhead. Figure 9 shows the total computation overhead for the subarea during the collecting the area information phase as the number of drones and groups vary. As shown, the total load for the subarea is linearly increased as the number of drones and groups increase, as the number of encrypted messages by the drone during the reading round is constant. However, the overhead is restricted by the maximum number of groups and drones in the area, which is a limited number. Then, the computation burden for the subarea is small and tolerable by the network.  
In addition, we have studied the performance of our proposed scheme utilizing Ring-LWE versus using RSA 2048 crypto-system (which is widely used for securing data transmission in practice) implementing the ARM Cortex M4F platform (Cambridge, UK). Figure 10 shows the computation overhead comparison per group as the included drones' number changes. It can be seen that the overhead in the RSA-based scheme is rapidly increased while the number of computation delays for Ring-LWE based proposed scheme remains very few. The huge gap between the overhead in the two cases is clear, because of the simple ARM Cortex M4F processing abilities; the platform is suitable for the limited-computation capabilities drones in the system. As the lightweight Ring-LWE encryption/decryption operations do not require complex processing units, the proposed scheme performance is more efficient in the Ring-LWE-based case. According to the total computation overhead  
Figure 9: Computation overhead for monitoring operation.  
of the subarea, our proposed scheme utilizing Ring-LWE crypto-system overhead increases as the number of groups increases from 900 msec in one group that includes a one-drone case to 18,000 msec in the case of 10 groups with 20 drones each. While the computation overhead for the RSA-based scenario is ranging between 79,200 msec until 1,584,000 msec as the number of groups and included drones increases. In summary, implementing our proposed scheme using a Ring-LWE crypto-system will not only protect it against the post-quantum computer attacks but also achieve the task with low computation overhead even on the simple Cortex M4F platform.