We argue that it is possible to continuously monitor from a single machine most BitTorrent users and to identify the content providers (also called initial seeds) [LLL_LEET10, LLL_TR10]. This is a major privacy threat as it is possible for anybody in the Internet to reconstruct all the download and upload history of most BitTorrent users.
To circumvent this kind of monitoring, BitTorrent users are increasingly using anonymizing networks such as Tor to hide their IP address from the tracker and, possibly, from other peers. However, we showed that it is possible to retrieve the IP address for more than 70% of BitTorrent users on top of Tor [LMC_POST10]. Moreover, once the IP address of a peer is retrieved, it is possible to link to the IP address other applications used by this peer on top of Tor.