NSAはどのようにしてTorによる通信を解析しているのか？

下記の記事が出ている。

　New report says the NSA is checking who visits Tor's website
　Verge: July 3, 2014
　http://www.theverge.com/2014/7/3/5868159/new-report-says-the-nsa-is-checking-who-visits-tors-website

　XKeyscore exposed: How NSA tracks all German Tor users as 'extremists'
　RT: July 3, 2014
　http://rt.com/news/170208-nsa-spies-tor-users/

　Report: Rare leaked NSA source code reveals Tor servers targeted
　ars technica: July 4,2014
　http://arstechnica.com/tech-policy/2014/07/report-rare-leaked-nsa-source-code-reveals-tor-servers-targeted/