# Wide Area Key Distribution Network Based on a Quantum Key Distribution System
## 3 Public-XOR-Key Scheme for QKD Network
### The Advantages of the Public-XOR-Key Scheme  
The function of wide area key distribution network based on relay nodes is to distribute keys for multiple endpoints. The relay nodes are set up for helping the pairs of remote endpoints to share keys. Therefore, the less functions and devices the relay nodes have, the simpler the whole system is. In real implementation, our scheme has the following practical advantages.  
1. The relay nodes do not need to store the QKD keys in our scheme. In the BBN key relay scheme, each relay node recovers the plaintext with QKD keys shared with the upstream node, encrypts it with QKD keys shared with the downstream node and then transfers it to the downstream node. Once the relay nodes have been broken, the adversary can access all plaintext transferred through this relay node. The relay nodes destroy the original QKD keys securely right after the QKD keys shared with upstream node and downstream node are immediately sent to the cipher machine to do XOR operations. The adversary has no chance to access the original QKD keys or plaintext. Compared with BBN scheme, our scheme reduces the storage of QKD keys and increases the security of the system.
2. The relay nodes do not need to store the XOR results of the QKD keys secretly in our scheme. Key buffers and corresponding preventive measures are needed for the secret storage, such as anti-electromagnetic radiation devices and video surveillance system. In our scheme, the XOR results are public. The way of publishing depends on the actual network. For example, the XOR results are saved in the relay nodes without protection, sent to endpoints directly, or broadcast to the users over the network. It is difficult for the actual networks to ensure the absolute security of the data storage. The system of our scheme simplifies the defence.
3. There are no re-generation keys in our scheme. In re-encryption scheme and BBN scheme, one of the endpoints firstly generates a local random number and encrypts the random number with the help of relay nodes. Finally, the pair of endpoints shares the random number. In our scheme, the keys shared by endpoints are exactly the QKD keys and there is no need to generate random numbers from point to point.
4. In our scheme, the shared keys do not need to be encrypted by each relay node over the path. When two endpoints want to share keys in re-encryption scheme, the ciphertext of the random  
Figure 4: The QKD network scheme with three endpoints based on the public-XOR-key scheme. The solid arrow stands for the QKD process. The dotted arrow indicates the key shared by each pair of endpoints. Relay node calculates and publishes \(Qk_{A_{1}B_{1}}=Qk_{A_{1}}\oplus Qk_{B_{1}}\), \(Qk_{A_{2}C_{2}}=Qk_{A_{2}}\oplus Qk_{C_{2}}\), \(Qk_{B_{2}C_{1}}=Qk_{B_{2}}\oplus Qk_{C_{1}}\), and then dismisses the QKD keys immediately. Finally, [PERSON] and [PERSON] share the key \(Qk_{A_{1}}\), [PERSON] and [PERSON] share the key \(Qk_{C_{2}}\), [PERSON] and [PERSON] share the key \(Qk_{B_{2}}\).  
number generated by one endpoint is re-encryption and transferred by one relay node after another. There are multiple endpoints to communicate simultaneously in reality. Some relay nodes may be included in plenty of communications. The communications may hang or fail due to the heavy traffic. In our scheme, the relay nodes no longer participate in the communications after publishing XOR results. When endpoints want to share keys, either of the endpoints just calculates the key directly, which eases the traffic of the relay nodes.