## 3 System Model
### Threat Model and Assumptions  
In this section we present the threats to the system at the peer-to-peer network level, transport authentication aspects and some assumptions related to them. Figure 4: The overview of the system organisation. First of all, we make some assumptions about the system participants. Nodes in the network can join the P2P overlay network at random. They use various WAN access methods and they may be reachable nodes or unreachable nodes. Moreover, by some tests, other nodes can determine whether they are reachable nodes or unreachable nodes, and have already determined which type. Reachable and unreachable nodes together make up the entire network. They enjoy the services provided by the virtual network and also provide services to other nodes directly or indirectly, but among them there are dishonest nodes, malicious nodes, and some data eavesdroppers. There are threats that arise at the P2P overlay network level:  
1. Sybil Attack: This attack may occur in the virtual networking of second phase routing connection in Figure 4. This is an attack unique to decentralized networks, and if not prevented could lead to identity impersonation or even the entire network going down. As shown in Figure 5, there is no authentication authority in P2P networks, so it is costless for users to create nodes, which means that attackers can go to forge identities to join the network very easily. After that, they will try to obtain a large amount of node information in the network and make some malicious behaviors based on it, such as sending false node information, misleading the normal information transfer between nodes, faking the identity of normal nodes, not responding to network connection requests, etc. 2. Eclipse Attack: This attack may also occur in the virtual networking of second phase routing connection in Figure 4. The eclipse attack usually has to be coupled with a Sybil attack, where the attacker adds enough fake nodes around a certain victim by appropriating the routes of nodes in the network and finding some nodes with similar addresses, thus isolating the normal nodes outside the normal P2P network, as shown in Figure 6.
3. DDoS Attack: This attack may occur in the routing messaging of second phase routing connection in Figure 4. In P2P networks, DDoS is different from the common centralized systems. The new DDoS attack does not require the establishment of a botnet to launch a large-scale attack, which is not only low-cost and powerful, but also ensures the secrecy of the attacker. In this way, if not prevented at the connection and authentication stages, it can have more serious consequences than centralized networks. There are threats arising from data transmission and authentication:
4. Man-in-the-Middle Attack: This attack may occur in the various stages of third phase data transmission in Figure 4. In a centralized network, the authentication of the  
Figure 5: Sybil attack diagram. Figure 6: Eclipse attack diagram. communication between two parties requires the participation of an authoritative and trusted institution, which in turn ensures the security of the authentication. In a decentralized network, there are many objects that need to communicate. On the one hand, since all nodes are peer-to-peer nodes, there is no certain authoritative centralized trusted authority. On the other hand, the overhead of authentication when communicating with a large number of nodes is too high. So authentication cannot be performed in this way, making MITM attacks more likely to occur in decentralized networks. 2.