| Summary: | Quantum-key-distribution (QKD) networks can provide absolutely secure keys for the entire communication system in theory. At present, the key-distribution rate is relatively low, and the key-distribution rate decreases exponentially as the distribution distance increases. The trusted-relay scheme commonly used in existing QKD networks achieves the purpose of extending the security distance by consuming additional keys. Since the channel is unreliable, the key-relay process will accumulate system errors to a certain extent, increasing the probability of key-relay failure. In some high-bit-error-rate network scenarios such as wireless networks and disaster environments, the channel-error rate is as high as 30–50%. And in these scenarios, there are usually a large number of confidential messages that need to be delivered. However, the key-management mechanism of the current QKD system does not consider the scenario of key-relay failure. If the key is not successfully relayed, all the keys participating in the relay process will be destroyed, including the key that has been successfully relayed before. This situation causes the key to be wasted and reduces the encryption capability of the system. In this paper, we proposed the quantum-key-recycling (QKR) mechanism to increase the number of keys available in the network and introduced a secure service grading mechanism to properly reuse the recycled keys. The QKR mechanism can be regarded as a key-management mechanism acting on the point-to-point QKD system, and the mechanism is designed for a classical channel to reuse the key resources. A post-processing method for recycled keys is proposed to improve the security of the keys. Simulation results show that the QKD network using the key-recycling strategy is about 20% higher in key-utilization rate than the traditional QKD network without the QKR mechanism, and about 10% higher in-service security coverage.
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