# An On-Demand Fault-Tolerant Routing Strategy for Secure Key Distribution Network
## 4 Proposed Solution
### Analysis  
First, we analyze the security of the proposed solution. In the above path discovery phase, the RSA digital signature algorithm is used to verify the identity of the relevant information in the message to prevent the faulty node from impersonating other nodes. However, this will affect the security of the communication key. Due to the characteristics of SKD technology, the generated symmetric keys are independent of each other. Therefore, if the RSA public and private key pair are cracked at some point in the future, it will not affect the generated key and the one-time pad encryption using the key. This is called forward security. In addition, an issue that requires attention is that the possibility for an adversary to store the current so-called secure ciphertext and wait for the emergence of more advanced cryptanalysis technology to extract information from the ciphertext. This is possible in traditional symmetric key distribution scenarios. Different from the former, combining the key generated by SKD technology and the one-time pad encryption method can ensure the long-term security of ciphertext, which is of great significance for certain scenarios, such as personal medical records, business secrets, etc.  
Next, we analyze and compare the upper limit of fault tolerance and overhead of this method and existing methods in the scenario shown in Figure 3. We will now analyze the shortcomings of existing methods under this mixed fault. The method used in [15] combined with PBFT [37] ensures that in the presence of faulty nodes, any two normal nodes can reach a consensus on the same key distribution path construction scheme. Based on this, path discovery between normal nodes can resist Byzantine behavior from faulty nods, avoiding previously described traffic redirection attack. However, this method assumes that the leader node proposes a construction plan through global network information calculation, which brings huge resource consumption when implemented in dynamic SKDN [22]. In addition, the method can only tolerate most \(|F|\) faulty nodes in \(3|F|+1\) nodes simultaneously. In the multi-path communication key distribution strategy proposed by method [38], communication keys are distributed on \(|F|+1\) disjoint paths in sequence. After the destination node XORs the results, it can obtain the ITS secure communication key. This ensures the security of key distribution. However, faulty nodes can reduce the success rate of the multi-path scheme by delaying the communication key of one of the paths. In the literature [14], it has been proposed that additional \(|F|\) disjoint paths are needed to improve the key distribution liveness. Although the combination of the above methods can ensure the reliability of secure key distribution in the fault scenario shown in Figure 3, there is an upper limit to the number of faulty nodes that can be resisted, and a large number of redundant disjoint paths are required as the cost of improving reliability, which greatly reduces system availability. For example, when C, E, and G in Figure 3 become faulty, faulty nodes become the majority of network nodes, and there are not enough disjoint paths. By the path discovery and fault detection mechanism, our method can discover the path containing A, B, F, and D and distribute the key on this path. Therefore, ideally, as long as there is a path that does not contain a faulty node, communication keys can be distributed on this path without relying on local keys from other paths, which improves the availability of the system.