A Class of New Protocol-Level Attacks against Tor

Tor is a real-world, circuit-based low-latency anonymous communication network, supporting TCP applications over the Internet. In this paper, we present a new class of attacks, protocol-level attacks, against Tor. Different from existing attacks, these attacks can confirm anonymous communication relationships quickly and accurately by manipulating one single cell and pose a serious threat against Tor. In protocol-level attacks, a malicious entry onion router may duplicate, modify, insert, or delete cells of a TCP stream from a sender. The manipulated cells traverse middle onion routers and arrive at an exit onion router along a circuit. Because Tor uses the counter mode AES (AES-CTR) for encrypting cells, the manipulated cells disrupt the normal counter at exit onion routers and decryption at the exit onion router incurs cell recognition errors, which are unique to the investigated protocol-level attacks. If an accomplice of the attacker at the entry onion router also controls the exit onion router and recognizes such cell recognition errors, the communication relationship between the sender and receiver will be confirmed. Protocol-level attacks can also be used for launching the denial-of-service (DoS) attack to disrupt the operation of Tor. We systematically analyze the impact of these attacks. We have implemented these attacks on Tor and our experiments validate their feasibility and effectiveness. We also present guidelines for defending against such attacks.