Cryptology ePrint Archive: BibTeX Citation of Report 2008/496

@misc{cryptoeprint:2008:496,
    author = {Debra L. Cook and Moti Yung and Angelos Keromytis},
    title = {Constructing Variable-Length PRPs and SPRPs from Fixed-Length PRPs},
    howpublished = {Cryptology ePrint Archive, Report 2008/496},
    year = {2008},
    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.