applied sciences Article Quantum Key Distribution Networks: Challenges and Future Research Issues in Security Chia-Wei Tsai 1 , Chun-Wei Yang 2 , Jason Lin 3 , Yao-Chung Chang 1,* and Ruay-Shiung Chang 4 1 Department of Computer Science and Information Engineering, National Taitung University, No. 369, University Rd., Taitung 95092, Taiwan; [email protected] 2 Master Program for Digital Health Innovation, College of Humanities and Sciences, China Medical University, No. 100, Sec. 1, Jingmao Rd., Beitun Dist., Taichung 406040, Taiwan; [email protected] 3 Department of Computer Science and Engineering, National Chung Hsing University, No. 145, Xingda Rd., South District, Taichung 40227, Taiwan; [email protected] 4 Department of Institute of Information and Decision Sciences, National Taipei University of Business, No.321, Sec. 1, Jinan Rd., Jhongjheng Dist., Taipei City 100025, Taiwan; [email protected] * Correspondence: [email protected] Abstract: A quantum key distribution (QKD) network is proposed to allow QKD protocols to be the infrastructure of the Internet for distributing unconditional security keys instead of existing public-key cryptography based on computationally complex mathematical problems. Numerous countries and research institutes have invested enormous resources to execute correlation studies on QKD networks. Thus, in this study, we surveyed existing QKD network studies and practical field experiments to summarize the research results (e.g., type and architecture of QKD networks, key generating rate, maximum communication distance, and routing protocol). Furthermore, we highlight the three challenges and future research issues in the security of QKD networks and then provide some feasible resolution strategies for these challenges. Citation: Tsai, C.-W.; Yang, C.-W.; Lin, J.; Chang, Y.-C.; Chang, R.-S. Keywords: quantum key distribution; quantum key distribution network; unconditional security Quantum Key Distribution Networks: Challenges and Future Research Issues in Security. Appl. Sci. 2021, 11, 3767. https://doi.org/10.3390/ 1. Introduction app11093767 In response to the rapid development of the Internet and Internet of Things (IoT) Academic Editor: Davide Bacco 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 Received: 30 March 2021 provision of complete cryptography mechanisms for protecting the confidentiality and Accepted: 19 April 2021 integrity of data and ensuring authentication between the sender and the receiver are some Published: 22 April 2021 of the important issues in the digital age, among which establishing secure cryptography keys through untrusted networks is a fundamental cryptography task. Although existing Publisher’s Note: MDPI stays neutral public-key cryptography based on computationally complex mathematical problems (e.g., with regard to jurisdictional claims in RSA or Diffie Hellman key exchange protocol) can provide the session key distribution published maps and institutional affil- for end users/applications, the session keys distributed by these algorithms belong to iations. 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 Copyright: © 2021 by the authors. complex computational problems are secure against attacks by quantum computers to Licensee MDPI, Basel, Switzerland. design cryptographic algorithms. This article is an open access article In quantum cryptography, Bennet and Brassard [1] used the properties of quan- distributed under the terms and tum mechanics to propose the first quantum key distribution (QKD) protocol—the BB84 conditions of the Creative Commons protocol—which allows two end users/applications to distribute the session keys between Attribution (CC BY) license (https:// each other. Furthermore, some studies [2–4] have proved that the BB84 protocol is an uncon- creativecommons.org/licenses/by/ ditional security protocol; that is, the session key distributed by the BB84 protocol belongs 4.0/). Appl. Sci. 2021, 11, 3767. https://doi.org/10.3390/app11093767 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3767 2 of 15 to unconditional security keys. In classical cryptography, only a one-time pad (OTP) can conform to unconditional security ciphers. Following the BB84 protocol, various QKD pro- tocols [5–14] have been proposed. To enhance the practicality of QKD protocols under the existing quantum technologies, the semi-quantum key distribution key protocols [15–17] 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 net- works have been proposed [20–48] to enable QKD protocols to assist the remote end users/applications in distributing security keys under the existing network environment. Previous studies [20–35] indicate the architectures, quantum technologies, and experimen- tal results in the field experiments of QKD networks. Improvement methods of the routing algorithms have also been proposed [36–41] 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]. In addition, for the security issue of QKD networks, [44,45] proposed feasible resolutions for avoiding the assumption that the quantum nodes must be trusted and [46] discussed how classical end users/applications can access session keys securely from QKD networks. In addition, [47] proposed an architecture of a QKD network, a quantum access network, which allows many end users/applications (up to 64 users/applications) to simultaneously distribute the security keys with a central quan- tum node using fiber channels. Then, [48] extended the quantum access network to the n:n framework, that is, n users could distribute security keys with n central quantum nodes simultaneously. Although these studies discussed above obtained remarkable results within the key generating rate, communication distance, architecture of the QKD network, and routing algorithm, there are still some challenges that need to be overcome in the field of security. Therefore, this study aims to summarize the developmental trends and results of the QKD network first. Then, we highlight the challenges that need to be studied further to perfect QKD networks. For the proposed challenges, we have also provided some feasible solutions and strategies to allow researchers to devise complete solutions. The rest of this paper is organized as follows: Section2 reviews the background of the QKD network, and Section3 summarizes the key results in existing QKD networks. For security issues in QKD networks, we highlight the three challenges and future research issues and provide some feasible resolution strategies in Section4. Finally, a brief conclusion is provided. 2. Background to Quantum Key Distribution (QKD) Networks The QKD network is used to extend the range of the QKD protocol, and it consists of several static quantum nodes that have complete quantum capabilities (e.g., generating a single photon/entanglement state, storing qubits, and performing the quantum unitary operation). The quantum nodes execute the QKD protocol (e.g., the BB84 protocol) to distribute secure keys (also called local keys) between the neighboring nodes, and then, the hop-by-hop manner is adopted to assist the remote end users/applications to distribute unconditional security session keys. The QKD network comprises quantum nodes and quantum links, and the framework of the QKD network can be divided into three layers: a communication layer, a key Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 15 Appl. Sci. 2021, 11, 3767 3 of 15 The QKD network comprises quantum nodes and quantum links, and the framework of the QKD network can be divided into three layers: a communication layer, a key man- agement layer, and a quantum layer (shown in Figure 1). The communication layer (the management layer, and a quantum layer (shown in Figure1). The communication layer top layer) manages the routing tasks and provides the application interface (API) to allow (the top layer) manages the routing tasks and provides the application interface (API) to the end users/applications to access the secure session keys generated by QKD protocols. allow the end users/applications