# TOAR: Toward Resisting AS-Level Adversary Correlation Attacks Optimal Anonymous Routing
## 2 Background and Motivation  
The strong pursuit of privacy preservation motivates the emergence of anonymous communication systems. Tor is the most widely used among the deployed anonymous systems. Latency is a major factor contributing to a positive user experience. Tor aims to provide low-latency anonymous communication, and therefore does not employ traffic obfuscation, making it difficult to resist correlation attacks. Correlation attacks utilize traffic features, such as packet timing, packet sizes, and inter-packet delays, to statistically correlate and link network flows [3]. To perform the correlation attack, an adversary must monitor the traffic entering and exiting the Tor network. An adversary can operate a large number of Tor relays to increase the likelihood of monitoring the connections at both ends of communication [14]. Alternatively, they can take control of Autonomous Systems and manipulate the underlying network communication to place themselves on the forwarding path of Tor traffic, thereby increasing the likelihood of successfully executing a correlation attack [15; 16; 17]. Technically, Tor offers anonymity by establishing tunnels between the client and destination through cascades of onion routing relays. A significant portion of the latency overhead is due to the underlying structure of the connections between Tor relays. The Tor client randomly selects an entry relay, a middle relay, and an exit relay to establish an anonymous channel. The onion routing established by Tor is an overlay anonymous circuit network based on transport layer protocols. Creating onion routes is independent of Internet routing and not constrained by the underlying network topology, and often involves detours. This routing detour causes more than 90% of connections in the Tor network to experience delays exceeding five times that of direct Internet connections [18]. Furthermore, an Autonomous System (AS) may be traversed multiple times. The AS may be on the route between the client and the entry relay, as well as on the route between the exit relay and the destination. In this case, an adversary controlling only one AS may carry out traffic correlation attacks and degrade the anonymity of an anonymous system by monitoring traffic in both the entry and exit routing segments. Figure 1 shows the scenarios of traffic correlation attacks by a single AS-level adversary, where S is the message source and D is the message destination; the anonymous communication between S and D is carried out using multi-hop encrypted forwarding; and the forwarding relays are R1, R2, and R3. Due to routing detours, an AS controlled by an adversary, AS2, simultaneously appears on the routing segment from the sender S to the entry relay R1, as well as on the routing segment from the exit relay R3 to the destination D. Even though the communication between the user and the entry relay is encrypted, AS2 can still observe the user's IP address by analyzing the headers of the packets sent by the user to the entry relay. Additionally, by analyzing the headers of the packets sent by the exit relay to the destination, AS2 can observe the destination's IP address. Then, through traffic correlation and statistical methods, the adversary at AS2 can infer that [PERSON] is communicating with D.  
Additionally, Internet routing is asymmetric, meaning the path from the client to the entry relay may differ from the path returning from the entry relay to the client; similarly, the path from the exit relay to the destination (e.g., a web server) may differ from the path returning from the web server to the exit relay.