## 3 Network Model
### QKD Protocol, Model and Related Definitions
After obtaining the gain and error rate, classical error correction techniques can be applied to correct the erroneous bits, and privacy amplification techniques can be used to improve the security of the key. Finally, [PERSON] and [PERSON] obtain a set of secure keys that can be used for secure communication. Through steps such as phase randomization, Bell state measurement, decoy-state method, classical error correction, and privacy amplification, the security and reliability of the key can be guaranteed [32]. The BB84 protocol is based on the principle of the unclonability of quantum states, which ensures that the key information cannot be obtained by a middleman due to the characteristic that quantum states cannot be copied or stolen [33]. The security of the MDI-QKD protocol is based on the impossibility of Bell state measurements, which means that a middleman cannot simultaneously interfere with and eavesdrop on the two quantum states sent by [PERSON] and [PERSON]. In contrast, in the BB84 protocol, a middleman can more easily steal communication between [PERSON] and [PERSON] and tamper with data. By utilizing the quantum entanglement properties of Bell states, the MDI-QKD protocol effectively avoids  
Figure 3: Basic principle diagram of using the MDI-QKD protocol in the QKD link. middleman attacks and eavesdropping attacks, thus providing higher attack resistance and security. Based on these principles, the MDI-QKD protocol can achieve higher security and confidentiality in quantum communication. On the contrary, due to greater instability, higher deployment costs, and lower key generation rates under the same link deployment length conditions, the keys generated by the MDI-QKD protocol are more valuable than those generated by the BB84 protocol [34]. The CPSR-HQKDN scheme is suitable for scenarios with varying security requirements for the data packets to be encrypted and significant differences in key demands. It establishes priority criteria for different requests. Therefore, this paper employs a QKD network model to define routing strategies for packet encryption. Additionally, the total key demand and key update rate of the QKD network are evaluated. The following terms are defined in the model:  
1. QKD Link: A QKD link refers to the physical components involved in quantum key distribution, including optical transmitters, optical receivers, and the transmission medium. It is a virtual link abstracted between two quantum nodes, where the transmission and measurement of quantum bits based on QKD technology take place on the channel. 2. Trusted Relay/Untrusted Relay: A trusted relay is an entity trusted by [PERSON] and [PERSON], capable of establishing a secure communication link between them. Its functions mainly include channel enhancement, relay authentication, and key generation and distribution. Therefore, a trusted relay involves operations such as key storage and key utilization with the key pool. In MDI-QKD, an untrusted relay is considered an entity that cannot be trusted. Its functions mainly include the forwarding of quantum states, the measurement of Bell states, and noise filtering. 3. Quantum Key Pool: The quantum key pool is a virtual concept that represents a series of quantum keys generated between a pair of quantum nodes in time slots. It also involves the issue of how to allocate keys to different data packets within the same time slot. 4. Data Packet Security Requirement Levels: During the process of data transmission, not all data packets have the same security requirements. Some data packets may only require traditional encryption methods or even no encryption at all. Some data packets may be encrypted using quantum keys generated by the BB84 protocol. For the data packets with the highest security requirements, they will be encrypted using more valuable and secure quantum keys generated by the MDI-QKD protocol. 5. Path Constraints: Different path selection constraints exist for data packets with different security requirements.