Cryptology ePrint Archive: BibTeX Citation of Report 2009/223

@misc{cryptoeprint:2009:223,
    author = {Tao Xie and Dengguo Feng},
    title = {How To Find Weak Input Differences For MD5 Collision Attacks},
    howpublished = {Cryptology ePrint Archive, Report 2009/223},
    year = {2009},
    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.