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29 February 2012 On the Topic of Tor's Weaknesses
From: Chris Wheeler <grintor[at]gmail.com> I have been reading a lot about end-to-end correlation attacks on tor. I am writing a paper on the subject and have a question which I can't seem to find an answer to. I understand these attacks rely on the attacker being able to view the traffic of the first relay a client is connecting to and the exit server. At this point they could make a correlation of what exit traffic is specific to that client based on statistical analysis. My question is: since bridges are just entry-point relays, If one could be certain that they were connecting to a bridge that is not compromised (for instance, if they themselves controlled the bridge), would they then be protected from such an attack? Thank you for your help _______________________________________________
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Date: Wed, 29 Feb 2012 09:48:15 -0500 On Wed, Feb 29, 2012 at 05:17:34AM -0500, grarpamp wrote: > > The main problem, besides the overhead, is that padding doesn't work Maybe. The important thing is to understand what security is provided and what is not. Then you can make an informed decision about whether or not it's bad news. > and to a lesser extent clearnet services (since they're not as 'illegal' and thus The short but incomplete answer is yes. Generally, what you are describing we experimentally verified on the live Tor network back in 2005. See "Locating Hidden Servers" by Lasse Overlier and myself. Available at or http://www.onion-router.net/Publications.html These sorts of attacks are what motivated us to introduce guard nodes, also described in that paper. We all knew, and had seen analyzed in earlier work by Wright et al., that onion routing circuits were subject to predecessor attacks, and that what Wright et al. had called helper nodes would, well help. What Lasse and I showed was that the public Tor network as of 2005 was subject to such attacks working very quickly (minutes) using very limited resources. You could generally find a hidden server within minutes using just a single hostile Tor relay (no cooperation from evil web server required). We wanted to show what you could do with a single relay, which limited us to hidden server circuits. If you owned at least two relays, you could attack arbitrary Tor circuits. This was confirmed in simulation on PlanetLab shortly after by Bauer et al. ("Low-Resource Routing Attacks Against Tor", also available at anonbib). Entry guards don't change the asymptotic threat of such attacks, they just move it around. Since you could be screwed so quickly and easily by building random circuits, Tor was changed so that you pick just a few relays as your entry guards. If one of them is evil, it will see the entry side of (a large fraction of) your circuits and will be able to associate you with your destination whenever you go to a hostile destination or use a hostile exit. What Lasse and I showed was that you weren't really much worse off from this than when choosing circuits with random entry relays. And if none of your guards is evil, an adversary can never de-anonymize you in this way. (Never say "never". ;>) Cf. the experiments and discussion of layered guards in "Locating Hidden Servers", and our subsequent research on building trust into path selection.)
aloha,
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