Cryptology ePrint Archive: BibTeX Citation of Report 2007/055

@misc{cryptoeprint:2007:055,
    author = {Andrey Bogdanov},
    title = {Cryptanalysis of the KeeLoq block cipher},
    howpublished = {Cryptology ePrint Archive, Report 2007/055},
    year = {2007},
    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.