Cryptology ePrint Archive: BibTeX Citation of Report 2004/199

@misc{cryptoeprint:2004:199,
    author = {Xiaoyun Wang  and Dengguo Feng and Xuejia Lai and Hongbo Yu},
    title = {Collisions for Hash Functions MD4, MD5, HAVAL-128 and RIPEMD},
    howpublished = {Cryptology ePrint Archive, Report 2004/199},
    year = {2004},
    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.