This would probably require knocking on people's doors, but — couldn't the endpoint ask the exit node what the relay node was, and then ask the relay node what the entry node was, and then ask the entry node what the origin was?

Is it just not logged long enough for the NSA etc. to show up at the Tor nodes' doors?

I am looking at these diagrams...

...and trying to read the original paper, but not understanding much.

  • The answer to your question is yes, they can. But the data is there only as long as the connection is open. You'd have to knock on two people's doors and hope they both respond within a matter of hours, ifnot minutes. You can consider yourself lucky if they're even on the same continent. The Tor project exists for a while now and though there have been some high profile people using it, I never heard of this happening.
    – Luc
    Commented Jan 21, 2014 at 1:56

3 Answers 3


As others have pointed out, "knocking on people's doors" can be difficult when the said doors are spread over the World, in locations which are geographically remote and, more importantly, legally remote. If all the Tor nodes are under in scope of a single law enforcement agency (e.g. they are all in China, or all in the USA), then that agency may indeed knock on all doors and have them opened. In an international context, this becomes harder.

Further complicating the issue is that "honest" Tor nodes (those who follow the protocol faithfully) do not keep logs, so even if they want to comply to a demand for information on a past connection, they simply do not have it any more.

Breadcrumb following is not the only way to unravel Tor, though. As the metaphor goes, if Hop-o'-My-Thumb had been smarter, he would not have brought bread, but a compass. In the case of Tor, information can be obtained by correlation in time and size. When a Tor connection occurs, the data goes through a randomly chosen sequence of nodes; as the theory goes, as long as one of these nodes is honest, the original client cannot be traced back. Or can he ?

If the evil attacker runs several nodes (and I expect big agencies to actually run a lot of Tor nodes, if only to remain "in the loop"), then it may happen that both the entry node and the exit node are attacker-controlled. These two nodes can then notice the case when a data packet entered the entry node, and a packet of the same size exited the exit node a few milliseconds afterwards. A normal connection entails several successive packets back and forth, further strengthening the correlation. The exit node can see the packet contents (and, at least, their final destination), while the entry node knows the client's identity (well, at least its IP address). That's all that is needed to destroy anonymity -- and that attack works regardless of the honesty of the nodes in between, in flagrant contradiction with the overoptimistic theory alluded-to above.

("One honest node is sufficient" is the theoretical framework for Mix networks as used in electronic voting protocols -- but that works only in the context of voting, where a node can accumulate all packets and send them en masse at the end.)

  • 2
    For that attack the nodes themselves don't even need to be attacker controlled. It's enough if an attacker can observe the traffic going in and out of them. This is an important threat, considering Room 641A and stories about the NSA compromising important routers. Commented Jan 21, 2014 at 13:41
  • Thanks! As @d33tah mentioned on Lucas Kauffman's answer, is forward secrecy also a factor? Not sure how it works, but is the effect that, if nodes are compromised after the fact, they can't recover the encrypted materials? (Does that stop someone from retracing the route, or just from reading the contents?)
    – Toph
    Commented Jan 22, 2014 at 16:41
  • 1
    Forward secrecy is about the impossibility of decrypting past traffic when, at a later date, the "permanent secrets" of one of the involved parties (SSL server) are compromised. It does not change things much with regards to anonymity: if the past traffic was recorded, then its provenance (previous node in the chain) is already known.
    – Tom Leek
    Commented Jan 22, 2014 at 17:50

Imagine you pick the following route:

UK --> China --> US -->  Russia --> Netherlands --> end-point

Now if you want to follow the bread crumbs you would need to either get access to the relay nodes by asking them (to which no exit node will answer) or you would need to get a court order to get access to the relay node.

Now let's say that these guys would even keep some type of log, you would first need to notify the Dutch police. Then this goes to Russia, who's not best friends with the Netherlands so this means it might take some time. The russians find out that it went to the US, but they don't really like the US so it might take some time... and so on and so on.

The idea is that if you want a good route using tor, go through countries who hate eachothers guts. It's going to make it more expensive for law enforcement to catch, even if they get past all diplomatic barriers.

Another way of breadcrumbs, would be to inspect the packets at the exit node. Have a look at this blog post I wrote a year or so ago about analyzing Tor traffic to find out people's identity.

  • 1
    I don't like the answer. You seem to imply that the legal issue is the only problem here. Doesn't Tor use some kind of forward secrecy?
    – d33tah
    Commented Jan 21, 2014 at 1:09
  • Did you actually read the blog post I linked? Commented Jan 21, 2014 at 5:56

Yes, it is possible, but as @LucasKauffman pointed out you will have more than one problem. So the reasons that prevent following the breadcrumbs are:

  1. A Tor implementation that does not keep logs.
  2. Changing geographical problems, it will be very hard (if not impossible) to have a team in the correct Tor node in the exact time.
  3. Legal issues. International collaboration is very hard due to the long timing responses and the lack of agreements.

But even with these problems, there are some attacks that may compromise the IP of the Tor user like:

  1. One of the biggest vulnerabilities spotted against the Tor protocol is the " "traffic correlation attack" (read it here).
  2. An organization adding tons of Tor nodes so they find chances to own the full circuit.

Tor is not the "panacea", it has its problems, but it is the best option out there and it has the feature of improve the privacy when the users growth.

And above it all, it requires organizations or governments to collaborate in a way they are not willing to to compromise the Tor network.

Also, consider visiting the Tor Stack Exchange.

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