Cryptology ePrint Archive: BibTeX Citation of Report 2011/640

@misc{cryptoeprint:2011:640,
    author = {Emiliano De Cristofaro, Claudio Soriente, Gene Tsudik, Andrew Williams},
    title = {Hummingbird: Privacy at the time of Twitter},
    howpublished = {Cryptology ePrint Archive, Report 2011/640},
    year = {2011},
    note = {\url{http://eprint.iacr.org/}},
}

You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.