# Wide Area Key Distribution Network Based on a Quantum Key Distribution System
## 1 Introduction  
To ensure the security of communications on the internet, an effective solution is to use cryptographic techniques. The security of the most classical cryptosystems depends on the computational complexity of the algorithms. With the fast development of quantum computation and quantum algorithms, communication security based on the above algorithms is greatly compromised. In conventional cryptography, a typical cryptosystem not based on the computational assumptions is the one-time-pad, which is unconditionally secure and free from the threat of quantum computing. In one-time-pad, the length of the keys and the messages is the same, and the keys can only be used once. Both communicating parties need to share the keys in advance. The security of one-time-pad should be completely dependent on the confidentiality of the pre-shared keys. It is too expensive to realize and too difficult to apply in practice. Thus, how to realize the unconditionally secure key distribution becomes a critical problem, which motivates the development of quantum key distribution (QKD). In 1984, [PERSON] and [PERSON] proposed the concept of QKD [1], which achieves the unconditionally secure key distribution based on quantum mechanics. It enables both parties of communication to generate and share a random and secure key. Then a series of QKD protocols have been proposed [2; 3; 4; 5]. More importantly, the unconditional security of QKD protocol has been strictly proved [6; 7; 8; 9]. The early implementations of QKD systems mainly focus on the communication between two endpoints. The point-to-point QKD system is limited by the transmission distance, which is limited by the key rate. So, the development of a wide area QKD network is a challenging issue. Some countries and regions have deployed QKD networks for last years. The first experiment of QKD network was designed by [PERSON] [10]. Some QKD network projects [11; 12; 13; 14; 15; 16; 17; 18] have been completed successfully, such as the defense advanced research projects agency (DARPA) quantum network [11] set up by the USA in 2004, secure communication based on quantum cryptography (SECOQC) designed by the European project [13], the commercial telecommunication fiber network in China [12], the SwissQuantum QKD network project launched in Geneva [14], and the live video conference through a high-speed QKD network in Tokyo [16]. Satellite communications are also building QKD networks, such as the international QKD channel created by the QUESS space mission [19; 20] and the world's first space-ground quantum network [21; 22]. These projects supported by different underlying devices can be divided into three categories: (1) satellite communications; (2) quantum relaying; (3) classical trusted relay. In the light of the present situation, the satellite-based method is not realistic, and the research of quantum relay equipment is not perfect. Thus, determining how to establish a practical and secure QKD network based on the trusted relay is the key problem. The approach of using trusted relays in QKD networks was proposed in 2002 [23]. This allows both endpoints to access a series of trusted relays to expand the arbitrary distance. The BBN key relay protocol has been operating continuously in the DARPA quantum network since 2003 [24]. In the protocol, the endpoints create a new random number \(R\) and send \(R\) through the trusted relay nodes by one-time-pad encryption. Each trusted relay node of the chosen path decrypts the ciphertext with the QKD keys shared with the upstream node and encrypts \(R\) with the QKD keys shared with the downstream node. In 2012, Los Alamos National Laboratory set up a trusted hub-and-spoke QKD network [18]. To communicate, each node sends a one-time pad to the hub, which it then uses to communicate securely over a classical link. The entire network is secure only if the central hub is secure. In these relay nodes-based key distribution schemes, if one of the relay nodes gets compromised, the whole network is insecure.