Jens Lechtenboerger: > Dear reader, > > Iâm a Tor user. > > Of course, since Torâs beginning the threat model has been excluding > global passive adversaries (which are able to observe both ends of > the torified communication) but I didnât consider that a real issue. > However, now I do. > > In 2007 Murdoch and ZieliÅski [4] developed traffic analysis > techniques based on sampled data for parties monitoring Internet > eXchanges (IXes). Apparently, the parties mentioned above have > capabilities that go far beyond the paperâs sampling technique. > Thus, Iâm assuming that global adversaries are spying on me. It's also important to understand the limitations of these attacks. If the data they record is low resolution (such as Murdoch's IX sampled results), the accuracy will be poor. Murdoch didn't achieve any success at all until several megabytes were transmitted in a single connection, and even after that, the accuracy was heavily impacted by the prevalence of similar traffic elsewhere in the network (due to a phenomenon called the 'base rate fallacy'). As more people use Tor, the better this property gets. In fact, a Raccoon (when you run an anonymity network, you get all sorts of interesting characters) proved that the accuracy of dragnet correlation attacks falls proportional to 1/U^2, where U is the number of concurrent active users. This creature also pointed out the same property is visible in Murdoch's own graphs: http://archives.seul.org/or/dev/Sep-2008/msg00016.html https://lists.torproject.org/pipermail/tor-talk/2012-March/023592.html I think this property suggests that with better usability and some lightweight defenses, Tor can actually do quite well, especially for relatively small, short transmissions like website loads. I am worried about the level and duration of timing resolution that datacenters as large as the NSA one in Utah could provide (assuming that all that storage is for traffic, and not for stuff like mapping ECC curves onto Z_p). Even so, I still think protocol-level active attacks (such as RPW's hidden service Guard discovery attack, and the Raccoon's bitstomping/tagging attack) are far more likely to be how intelligence agencies and others will attack Tor: http://www.ieee-security.org/TC/SP2013/papers/4977a080.pdf https://lists.torproject.org/pipermail/tor-dev/2012-March/003347.html -- Mike Perry
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