# Quantum Key Distribution Networks: Challenges and Future Research Issues in Security
## 4 Challenges and Research Issues
### No Suitable Security Interface between the Classical End Users/Applications and Quantum Nodes  
Because the construction cost of quantum nodes is still very high, it is not feasible to let each end user/application have a dedicated quantum node for accessing the service of the QKD network. Therefore, several end users/applications must share one quantum node (i.e., the access node mentioned in Section 2) in a real time environment. Therefore, the end users/applications still use the classical network to link the quantum nodes (shown in Figure 9); that is, designing a complete security mechanism for the communication between the end users/applications and the quantum nodes is an important issue. Although [PERSON] et al. [46] proposed a strategy using OpenSSL for this issue, the strategy is still not perfect within the quantum computing environment because OpenSSL is a computing-based security method.  
To address this issue, we believe that post-quantum cryptography (PQC) is the optimal strategy for designing a security mechanism for the interface between the quantum node and the end users/applications. Here, PQC can be used to complete the authentication and session key transmission between the quantum and classical end user/application. After the end users/applications obtain the session keys with the quantum node using post-quantum public-key cryptography, they can use the OTP or symmetric key cryptography to communicate securely. Although using an OTP to encrypt the messages transmitted between the quantum node and the end users/applications can provide unconditional security, this method has a higher cost than using symmetric key cryptography because of the heavy demand for encryption/decryption keys. However, using symmetric key cryptography cannot provide unconditional security for the end users/application to access the session keys from the quantum node; however, the security of this method is better than that of other currently existing methods (e.g., SSL protocol) because the encryption/decryption keys are obtained from algorithms that cannot be broken by a  
Figure 9: Security interface between the quantum node and end users/applications.  
quantum computer. We can select the suitable encryption/decryption methods depending on the practical context; that is, if the application's security requirement is not high but the communication traffic is heavy, symmetric key cryptography will be applicable; otherwise, an OTP can be used to provide the best security protection. For demonstrating the feasibility of the integrating PQC and QDK protocol, [PERSON] et al. [76] used an experimental approach to verify the efficiency and stability of the PQC algorithm in QKD authentication. However, the National Institute of Standards and Technology (NIST) is currently working on the new generation of quantum-resistant key encapsulation and authentication schemes, the implementation of integrating the PQC key exchange and the authentication into the standard cryptographic protocols of the classical network (e.g., SSL, TLS and so on) will need to be studied further to evaluate and optimize its performance.