Cryptology ePrint Archive: BibTeX Citation of Report 2004/304

@misc{cryptoeprint:2004:304,
    author = {John Kelsey and Bruce Schneier},
    title = {Second Preimages on n-bit Hash Functions for Much Less than 2^n Work},
    howpublished = {Cryptology ePrint Archive, Report 2004/304},
    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.