# Quantum Key Distribution Networks: Challenges and Future Research Issues in Security
## 4 Challenges and Research Issues
### Numerous Quantum Node Resources Are Consumed by the Multiple-Path Strategy  
The QKD network must use the hop-by-hop method to distribute the security session key owing to the limitation of the qubit communication distance. Any node on the routing path can know the session keys distributed from the source node to the destination node. In the DAPRA QKD network, after the routing path is decided, the source node (node N1 in Figure 5) sends reservation requests to all nodes (N2, N3, and N4 in Figure 5) in the routing path and the destination node (N5 in Figure 5). Then, these nodes use the XOR operation to encrypt the corresponding local key or session key to assist the source, and the distance nodes share the session key. By observing this session key transmission method of the DAPRA QKD network, we can determine that the session key may be leaked if the part nodes (e.g., N4 or N2) are compromised. Taking the situation in Figure 5 as an example, N4 has the local key \(K_{(4,5)}\); thus, N4 can intercept the ciphertext \(K_{(4,5)}\oplus SK\) that is sent from N5 to N1 and then perform \(K_{(4,5)}\oplus K_{(4,5)}\oplus SK\) to obtain \(SK\); likewise, N2 can intercept the ciphertexts \(K_{(4,5)}\oplus SK\), \(K_{(3,4)}\oplus K_{(4,5)}\), and \(K_{(2,3)}\oplus K_{(3,4)}\) sent from N5, N4, and N3 to N1, respectively. It can then perform the calculation as shown in the following equation:  
\[K_{(1,2)}\oplus(K_{(1,2)}\oplus K_{(2,3)})\oplus(K_{(2,3)}\oplus K_{(3,A)}) \oplus(K_{(3,A)}\oplus K_{(4,5)})\oplus(K_{(4,5)}\oplus SK).\]  
Because the associative properties of the XOR function, \(A\oplus(B\oplus\mathbb{C})=(A\oplus B)\oplus\mathbb{C}\) and N2 owns the local keys, \(K_{(1,2)}\) and \(K_{(2,3)}\), in which it can extract \(SK\). The SECOQC QKD network adopts a similar method to transmit the session key, as shown in Figure 6. This session key transmission method allows any node in the routing path to decrypt the ciphertext of the session key, re-encrypts the session key with the local key shared between it and the next node and, then, sends fresh ciphertext to the next node. In other words, any node in the routing path can obtain the session key directly; thus, the session key will be revealed if any node among the routing path is compromised. Although [PERSON] et al. [44] used the multiple-path strategy to prevent the aforementioned problem--the partial quantum nodes were compromised, and the multiple-path  
Figure 5: Session key transmission method in the DAPRA QKD network. Figure 6: Session key transmission method in the SECOQC QKD network. strategy consumed numerous resources of quantum nodes, in which the consumption of local keys was the most important because of the high cost of generating local keys. Let us take an example to explain this. [PERSON] wants to distribute a session key to [PERSON] with the help of the source node NS. If NS selects m paths for transmitting the session key to the destination node ND, m times the number of local keys will be used to achieve this task (also shown in Figure 7). Therefore, reducing the consumption of the local key and avoiding the unrealistic assumption of the trustworthiness level of quantum nodes is an interesting research issue. To address this issue, we propose a feasible solution: each node in the routing path can share the secret shadow using the QSS protocol [72; 73; 74; 75]. Only nodes in the routing path cooperate here, and the session key can be extracted; otherwise, no information regarding the session key can be revealed to anyone. Taking an example (shown in Figure 8) to explain the resolution, [PERSON] wants to distribute a session key with [PERSON] with the help of source node N1. All nodes in the routing path (i.e., N4, N8, and N9) share the secret shadows (i.e., \(SS_{1}\), \(SS_{2}\), and \(SS_{3}\)) with N1, respectively, using a secure QSS protocol. After N4, N8, and N9 perform XOR operation on their secret shadows and the ciphertext sent from the previous node, the session key \(SK\) can be extracted.