But the downside is that exit relays can build short snapshots of user profiles based on all the streams they see coming out of a given circuit. This approach improves efficiency because we don't have to waste time and overhead making a new circuit for every tiny picture on the aol.com frontpage, and it improves anonymity because every time you build a new path through the Tor network, you increase the odds that one of the paths you've built is observable by an attacker. For efficiency, Tor puts multiple application streams over each circuit. The third attack from their paper is where things get interesting. Tor can't keep you safe if your applications leak your identity. The second answer is that if you want your Bittorrent client to actually provide privacy when using a proxy, you need to get the application and protocol developers to fix their applications and protocols. We've been saying for years not to run Bittorrent over Tor, because the Tor network can't handle the load perhaps these attacks will convince more people to listen.
The first answer is "don't run Bittorrent over Tor". So what's the fix? There are two answers here. Combined, they present a variety of reasons why running any Bittorrent traffic over Tor isn't going to get you the privacy that you might want. As a bonus, if the Bittorrent peer communications aren't encrypted, the Tor exit relay you pick can also watch the traffic and do the attack.
#Vpn or proxy on mac for bittorrent utorrent#
So if your uTorrent client picks 50344 as its port, and then anonymously (via Tor) talks to some other peer, that other peer can go to the tracker, look for everybody who published to the tracker listing port 50344 (with high probability there's only one), and voila, the other peer learns your real IP address.
Because of the first attack above, the tracker learns both your real IP address and also the random port your client chose. It turns out that the Bittorrent protocol, at least as implemented by these popular Bittorrent applications, picks a random port to listen on, and it tells that random port to the tracker as well as to each peer it interacts with. The second attack builds on the first one to go after Bittorrent users that proxy the rest of their Bittorrent traffic over Tor also: it aims to let an attacking peer (as opposed to tracker) identify you. But that probably isn't what you wanted your Bittorrent client to send. Nobody knows where you're sending your IP address from. Tor is doing its job: Tor is _anonymously_ sending your IP address to the tracker or peer. The attack is actually worse than that: apparently in some cases uTorrent, BitSpirit, and libTorrent simply write your IP address directly into the information they send to the tracker and/or to other peers. The result is that the Bittorrent applications made a different security decision than some of their users expected, and now it's biting the users. Choosing to ignore the proxy setting is understandable, since modern tracker designs use the UDP protocol for communication, and socks proxies such as Tor only support the TCP protocol - so the developers of these applications had a choice between "make it work even when the user sets a proxy that can't be used" and "make it mysteriously fail and frustrate the user". The problem is that several popular Bittorrent clients (the authors call out uTorrent in particular, and I think Vuze does it too) just ignore their socks proxy setting in this case. These people are hoping to keep their IP address secret from somebody looking over the list of peers at the tracker. The first attack is on people who configure their Bittorrent application to proxy their tracker traffic through Tor. There are three pieces to the attack (or three separate attacks that build on each other, if you prefer). This post tries to explain the attacks and what they imply. An increasing number of people are asking us about the recent paper coming out of Inria in France around Bittorrent and privacy attacks.
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