TOR is an anonymizing service that uses multiple hops to obfuscate both origination and endpoint of a particular internet communication. So understanding how TOR works is paramount to understanding how it is attacked.

You can hardly attack a system you don't understand. As I have a fondness for probability and computers, I thought I would try to help explain how a correlation attack works, how long it takes for a positive identification, what other types of attacks exist, and finally, how to defend yourself. This is hardly the first attack against TOR, but the question many may wonder is how this attack actually works, and better yet, what methods exist that do not require a billion dollar budget and a horde of computer specialists. Once both are compromised, it is much easier to correlate traffic with a particular individual. The purpose is presumably to trace the origin of a communication by compromising some entrance and exit nodes. In an attempt to reduce the anonymity granted by the service, the NSA has opened a great many nodes of their own. As was mentioned by the great OTW last week, TOR, aka The Onion Router, has had its integrity attacked by the NSA.

How to use tor correctly