On Thu, Nov 18, 2010 at 06:19:03PM -0800, Theodore Bagwell wrote: > Some of you may be aware of the paper,"Cyber Crime Scene Investigations > (C2SI) through Cloud Computing" > (http://www.cs.uml.edu/~xinwenfu/paper/SPCC10_Fu.pdf) which illustrates > a feasible method of invalidating the anonymity afforded by Tor.
I just took a brief look through it. I wish they'd included analysis of guard nodes in their equations -- because relays take several days or more to get the Guard flag, and clients only rotate their guards monthly, the equations in this paper are misleading and their conclusions like "99% if the user connects three times" and "the network forensics section may last for a few hours [and still be effective]" are also misleading. That isn't to say that the general point is wrong -- I think with the current size of the Tor network, a well-funded adversary could run enough relays that he will have a high probability of deanonymizing users. We sure do need to get a larger network if we want to raise the cost of these attacks. But at some point somebody should run the numbers to find out how much it would cost in practice. (These numbers might also convince us to change the parameters like "3 guards" and "30 days".) We should also take the next step in our bandwidth measurement authorities at some point -- right now the directory authorities put in a better estimate for your bandwidth _once we have a better estimate_, and use the self-advertised bandwidth until that point. I think that's a security flaw. We could cap the believed self-advertised bandwidth at something like 100KB. It would mean that newly volunteering relays would take even longer before they're usefully contributing. The step after that would be to accelerate the initial measurements on new relays, to narrow the window where we don't have an opinion on bandwidth weight. There's also an open research question on how to combine Mike Perry's measurements (which are more accurate at high bandwidths) with Robin Snyder's measurements (which are more accurate at low bandwidths). I know Mike would love to have some help there. > I nominate this paper as a founding reason why Tor should permit users > to increase the number of relay nodes used in each circuit above the > current value of 3... No, that won't work. The key vulnerability is the first-last correlation attack, which doesn't care how many hops your path has (as long as it's at least two). You can read more about it from the various freehaven.net/anonbib/ links in this blog post about a related topic: https://blog.torproject.org/blog/one-cell-enough --Roger *********************************************************************** To unsubscribe, send an e-mail to majord...@torproject.org with unsubscribe or-talk in the body. http://archives.seul.org/or/talk/