Deanonymizing Tor in a Stealthy Way

The Tor network is one of the largest anonymity networks and has attracted millions of users worldwide. Since deployed in October 2002, many forms of attacks against Tor had been proposed, aiming to deanonymize the network. The protocol-level attacks, which can deanonymize users by manipulating a cell, are effective to compromise the anonymity of Tor by controlling the entry node and the exit node in a circuit. However, due to the absence of stealthiness, it may be noticed by the victim since the connection will be released. To address this issue, we present two types of attacks. The Type-I is based on protocol-level strategy but attempts to keep the connections alive by fixing damaged cells in the network, thereby making it much stealthier in some cases and it is quite difficult to be defended. In the Type-II attack, the entry node sends a signal to the accomplice exit node via a special type of outbound cells to prevent the connection from being closed, thus the attack can keep stealthy in more general situations. We also propose some countermeasures to keep the network away from the Type- II attack. An evaluation of the two types of attacks has been performed. And the results showed that both the two types of attacks are effective and pose serious threats to the Tor network.