The the discussion of the paper at the tor project or ars technica is much more informative for the HN crowd; if for no other reason than the simple fact that they include a link to the paper:

Paper: http://people.csail.mit.edu/devadas/pubs/circuit_finger.pdf

TorProject: https://blog.torproject.org/blog/technical-summary-usenix-fi...

Ars Technica: http://arstechnica.com/security/2015/07/new-attack-on-tor-ca...