It's easier to identify TOR users than they believe, according to research published by a group of researchers from Georgetown University and the US Naval Research Laboratory (USNRL).
Their paper, Users Get Routed: Traffic Correlation on Tor by Realistic Adversaries, is to be presented in November at November's Conference on Computer and Communications Security (CCS) in Berlin. While it's been published at the personal page of lead author Aaron Johnson of the NRL, it remained under the radar until someone posted a copy to Cryptome.
The paper states simply that “Tor users are far more susceptible to compromise than indicated by prior work”. That prior work provided the framework for what Johnson's group has accomplished: using traffic correlation in the live TOR network to compromise users' anonymity.
“To quantify the anonymity offered by Tor, we examine path compromise rates and how quickly extended use of the anonymity network results in compromised paths”, they write. In some cases, they found that for the patient attacker, some users can be identified with 95 percent certainty.
The compromise isn't something available to the trivial attacker. The models that Johnson developed assume that an adversary has access either to Internet exchange ports, or controls a number of Autonomous Systems (for example an ISP). However, it's probably reasonable to assume that the instruments of the state could deploy sufficient resources to replicate Johnson's work.
At the core of Johnson's work is a Tor path simulator that he's published at github. The TorPS simulator helps provide accurate AS path inference from TOR traffic.
“An adversary that provides no more bandwidth than some volunteers do today can deanonymize any given user within three months of regular Tor use with over 50 percent probability and within six months with over 80 percent probability. We observe that use of BitTorrent is particularly unsafe, and we show that long-lived ports bear a large security cost for their performance needs. We also observe that the Congestion-Aware Tor proposal exacerbates these vulnerabilities,” the paper states.
If the adversary controls an AS or has access to Internet exchange point (IXP) traffic, things are even worse. While the results of their tests depended on factors such as AS or IXP location, “some users experience over 95 percent chance of compromise within three months against a single AS or IXP.”
The researchers also note that different user behaviours change the risk of compromise. Sorry, BitTorrent fans, your traffic is extremely vulnerable over time. ®