De-anonymizing BitTorrent Users on Tor

Privacy of users in Peer-to-peer (P2P) networks goes far beyond their current usage and is a fundamental requirement to the adoption of P2P protocols for legal usage. In a climate of cold war between P2P filesharing users and anti-piracy groups, more and more users are moving to anonymizing networks in an attempt to hide their identity. However, when not designed to protect users information, a P2P protocol often leaks information that compromises the identity of its users. BitTorrent is a P2P filesharing protocol that is daily used by millions of users but that has not been designed to protect the anonymity of its users. Indeed, it has recently been shown that an adversary can continuously spy, i.e., collect the IP-to-contents mapping, on most BitTorrent users of the Internet and from a single machine [1]. In addition to spy on BitTorrent users, an attacker might be able to exploit BitTorrent control messages to de-anonymize a user behind an anonymizing network such as Tor. Tor relies on onion routing over an overlay network maintained by volunteers to anonymize TCP applications such as web browsing, P2P filesharing, etc. To reach the Internet via Tor, an application selects 3 Tor nodes at random and then first encrypts its messages with the key shared with the last node (exit node), then with the key of the 2nd node, and finally with the key of the 1st node. The 3 Tor nodes that route a user’s messages form a circuit and all TCP streams created by that user during a 10-minutes period will be multiplexed into one, or a few circuits. Each Tor node in a circuit then decrypts/encrypts the messages after routing them to/from the Internet. Onion routing thus guarantees that no Tor node knows both the source IP address and the payload of a message. A BitTorrent user may use Tor to (1) connect to a server (tracker) to collect lists of peers sharing a file, (2) connect to other peers to distribute a file, or (3) both. In this proposal, we instrument 6 exit nodes for a period of 23 days to demonstrate that an attacker can deanonymize BitTorrent users for any of the 3 aforementioned usages by volunteering to maintain an exit node and eavesdropping appropriate BitTorrent control messages. In addition, as all streams are multiplexed into the same circuit, we show that de-anonymizing one Bit-