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I'm doing some research on Tor for a paper and stumbled upon this section on the Tor FAQ page, where it says how to choose or exclude certain nodes from your circuit. Below it says that it is not recommended to do this, but not explicitly why. Here's the link: https://www.torproject.org/docs/faq.html.en#ChooseEntryExit

My guess is that you can be distinguished from other users more easily if certain relays are constantly avoided. But that would require extensive statistical traffic analysis, wouldn't it?

Or is there another way that I fail to see?

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The idea is that you're subjecting yourself to a partitioning attack where your path selection is different than most other Tor clients and one of the anonymity benefits of the Tor Network is the disparate potential network locations. In a long enough time spectrum and a consistent enough black list configuration, it may be possible to correlate your circuit choices if the attacker is able to monitor the connections. The attack (where the client partitions herself resulting in a compromised), AFAIK, has never been shown but the warning is there because there is an unknown potential.

It would not be a simple attack to employ but you can imagine an extreme case where you've configured your client to only use the same 3 nodes for all circuit attempts. In this case if your exit node was monitored, they may be able to fingerprint your personal traffic patterns and daily habits. This would be much more difficult if you used the full Tor Network.

Or you could imagine a scenario where someone wanting to target you can see your activity on the exit, compromise your middle node by attacking it, and in-turn, uncover your entry. An attacker wouldn't take the effort to do this if the entire Tor Network was used because it would be statistically unlikely that you'd ever use that same set of circuits again.

  • Thank you for your answer. This makes sense and confirms my own thoughts on the matter (except for the fact that I didn't know the term partitioning attack before so thanks for that as well :) ) – Curtis Jun 10 '15 at 17:38

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