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BitThief is a Java-based BitTorrent client designed to download from, but not upload to, BitTorrent networks. It is freeware and cross-platform. According to the BitThief homepage, it is intended as a proof of concept that there are weaknesses in the current BitTorrent protocol, which can be exploited by "cheating" BitTorrent clients (such as BitThief itself).
WiredWebmonkey, a website operated by Wired Digital, called BitThief "straight-up nefarious and wrong." It has also attracted controversy due to its now documented feature whereby it collects information about the torrent files being downloaded by the client, such as total number of pieces available, size of the pieces and a hash of the torrent hash. The makers of BitThief have posted a paragraph on their main page in response to such claims. Stated directly from the BitThief website:
As it is a scientific project, we require measurements of the performance of our exploits. For that purpose, the client occasionally transmits data to our webserver. The transmitted data merely contains information about the time required to download files of different size. In particular, only a hash of the info hash is transmitted, which means that it is not feasible to determine what has actually been downloaded. However, we advise you not to download any copyrighted material.