Cryptology ePrint Archive: BibTeX Citation of Report 2007/309

@misc{cryptoeprint:2007:309,
    author = {Subhamoy Maitra},
    title = {Balanced Boolean Function on 13-variables having Nonlinearity strictly greater than the Bent Concatenation Bound},
    howpublished = {Cryptology ePrint Archive, Report 2007/309},
    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.