# Quantum Key Distribution Networks: Challenges and Future Research Issues in Security
## 1 Introduction  
In response to the rapid development of the Internet and Internet of Things (IoT) technologies, digital applications/services have become mainstream in today's world. This trend allows more information and data to be transmitted over the Internet. Therefore, the provision of complete cryptography mechanisms for protecting the confidentiality and integrity of data and ensuring authentication between the sender and the receiver are some of the important issues in the digital age, among which establishing secure cryptography keys through untrusted networks is a fundamental cryptography task. Although existing public-key cryptography based on computationally complex mathematical problems (e.g., RSA or Diffie Hellman key exchange protocol) can provide the session key distribution for end users/applications, the session keys distributed by these algorithms belong to theoretical computational security. That is, the computational security keys can be broken using quantum computation. To overcome this issue, some feasible solutions have been proposed, including quantum cryptography, which uses quantum mechanics to design secure communication protocols, and post-quantum cryptography, which shows that complex computational problems are secure against attacks by quantum computers to design cryptographic algorithms. In quantum cryptography, [PERSON] and [PERSON] [1] used the properties of quantum mechanics to propose the first quantum key distribution (QKD) protocol--the BB84 protocol--which allows two end users/applications to distribute the session keys between each other. Furthermore, some studies [2; 3; 4] have proved that the BB84 protocol is an unconditional security protocol; that is, the session key distributed by the BB84 protocol belongsto unconditional security keys. In classical cryptography, only a one-time pad (OTP) can conform to unconditional security ciphers. Following the BB84 protocol, various QKD protocols [5; 6; 7; 8; 9; 10; 11; 12; 13; 14] have been proposed. To enhance the practicality of QKD protocols under the existing quantum technologies, the semi-quantum key distribution key protocols [15; 16; 17] and measurement device independent (MDI) QKD key protocols [13; 18; 19] have also been proposed. In addition, various types of quantum communication protocol have been proposed for different applications, including quantum secure direct communication, quantum secret sharing (QSS), quantum private comparison, and quantum information splitting. However, QKD protocols cannot provide the service of key distribution as the fundamental infrastructure instead of existing public-key cryptography in the existing network environment immediately because of limitations of quantum technology (e.g., the reachable distance of qubit transmission and the accuracy of the qubit detector). Moreover, QKD protocols cannot be integrated with existing network protocols immediately because of the specificity of quantum links and network organization. To address the aforementioned problem, the concept and framework of QKD networks have been proposed [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35] to enable QKD protocols to assist the remote end users/applications in distributing security keys under the existing network environment. Previous studies [20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35] indicate the architectures, quantum technologies, and experimental results in the field experiments of QKD networks. Improvement methods of the routing algorithms have also been proposed [36; 37; 38; 39; 40; 41] along with evaluation metrics of quantum links, in which notably the first quality of service (QoS) mechanism for QKD networks was designed in [39]. Another study [41] proposed a routing algorithm for hybrid QKD networks. A method that uses a QKD network to protect the security of power microgrids was proposed in [42; 43].