I encourage everyone in this thread to read up on convergent encryption.
http://research.microsoft.com/apps/pubs/default.aspx?id=7421... http://www.ssrc.ucsc.edu/Papers/storer-storagess08.pdf
http://research.microsoft.com/apps/pubs/default.aspx?id=7421... http://www.ssrc.ucsc.edu/Papers/storer-storagess08.pdf
(tldr; there are serious security/privacy vulnerabilities with convergent encryption)