# An On-Demand Fault-Tolerant Routing Strategy for Secure Key Distribution Network
## 3 System and Problem Model
### System Model  
SKDN consists of SKD nodes, twinning SSL devices, and public channels; it is used to extend the service scope of point-to-point security key distribution and complete security key distribution between any SKD nodes.  
In the layered architecture of the SKDN, the link layer performs real-time point-to-point secure key distribution; the service layer receives the user's confidential communication request, waits for the network layer to provide the communication key, and then returns it to the user. The network layer receives requests from the service layer, while the communication key is reconstructed through the local key generated by the link layer and returned to the service layer. The SKDN model is shown in Figure 1.  
The link layer is composed of SKD nodes and logical links. The logical link consists of SKD nodes and public channels connecting SKD nodes. Each SSL device on the node matches an SSL device on another node within the SKDN. The link layer performs point-to-point secure key distribution between nodes containing twinning SSL devices based on the logical link. The node sends the driving signal to the other node through the logical link. Both parties convert the signal from digital to analog and then input it into the SSL device to generate a local pre-key. Twinning SSL devices produced on the same wafer have similar input and output characteristics [9], but it is still necessary to exchange auxiliary information derived from the local pre-key on the logical link, and then use the fuzzy extractor to perform information harmonization and privacy amplification to generate an unconditionally secure and consistent local key [5]. The locally generated key is cached in memory and used to establishing the communication key within the SKD network layer.  
The network layer is responsible for the control and management of local key resources and the execution of routing strategies for generating communication keys. The routing strategy is divided into two steps: path discovery and communication key distribution. Path discovery determines the set of relays used for subsequent communication key distribution.  
Figure 1: SKDN model. Yellow, orange and pink region represent service, network and link layer in SKDN respectively.  
The selected trusted relay needs to cache enough local key resources to support the forwarding of communication key. The communication key distribution process is shown in Figure 2.  
Assume that a path including A, B, C, and D is selected through the path discovery mechanism, and A generates a certain random number. Then the random number and the local key generated by A and the next hop B can be XORed to generate ciphertext. The ciphertext is forwarded to the next hop on the public channel. After decryption, the next hop continues to forward according to the rules until D obtains the random number. Since the key is sent in the form of ciphertext through a one-time pad on the classic channel, the information cannot be obtained by intercepting the ciphertext. Furthermore, assuming the relay is trustworthy, it can be proved that the communication key represented by the random number is unconditionally secure [3].  
The service layer obtains the communication key generated by the network layer. Therefore, SKDN provides a secure symmetric key distribution service for the application data network, which is of great significance to ensuring long-term security of many application data. There are existing symmetric key distribution algorithms on the protocol stack, such as IPSec, VPN, TLS, etc., to strengthen the security of user data [10]; as a basic module, it is also suitable for many application scenarios [9; 30], such as the information-centric network, Internet of Things, Space-Air-Ground Integrated Network, etc.