Dear Tor developers,
We’ve been working on an MSc Thesis project looking into an attack that links different Tor circuits back to the same user, using only information available to a Tor middle node. We’ve discovered that an adversary can quite accurately determine if two consecutive circuits are used by the same user just by looking at when these circuits are built and destroyed or when they begin and stop relaying traffic. This is mainly the result of the fixed 10 minute circuit lifetime and the fact that the transition to using a new circuit is quite sharp. According to our measurements, circuits belonging to the same user are built at very close to 10 minute intervals. Also, when one circuit stops relaying traffic another one suddenly becomes active, at almost the very same moment.
Thanks to these properties we could train a classifier to quite efficiently identify matching pairs of circuits (9% EER). We looked into the threat posed by this attack to Tor users and concluded that because of entry guards the threat is real, given that an adversary can control a significant portion of middle node traffic. Finding matches among all possible Tor circuits would result in too many false positives but thanks to entry guards an adversary can focus only on circuits built through these specific nodes and quite efficiently determine if two circuits belong to the same user. Possible plans of moving to using only a single guard node make the situation much more severe: we concluded that for a user picking a slow guard node, the adversary can find almost all circuits belonging to a same user (again assuming control of a large portion of middle node bandwidth). We thus see that before moving on with any such plans, one should consider the effect of our findings on the threat posed by a middle node linking user circuits.
As a final note, we considered the effects of randomizing the circuit lifetime (e.g. between 5-15 minutes) to mitigate this threat but concluded that it would not necessarily solve the problem. The most prominent feature linking two circuits together seems to be the time difference between traffic ending on one circuit and beginning on another circuit, and this link cannot be broken by varying the circuit lifetime. However, if someone were to devise such a modification, we would be happy to re-run our tests to see what kind of an effect this would have on the success rate of an adversary.
For those interested in our findings, the full thesis Linking Tor Circuits can be found at:
http://www0.cs.ucl.ac.uk/staff/G.Danezis/students/Huhta14-UCL-Msc.pdf
Best regards,
Otto Huhta, UCL