Cryptology ePrint Archive: BibTeX Citation of Report 2009/242

@misc{cryptoeprint:2009:242,
    author = {Alex Biryukov and Dmitry Khovratovich and Ivica Nikoli\'{c}},
    title = {Examples of differential multicollisions for 13 and 14 rounds of AES-256},
    howpublished = {Cryptology ePrint Archive, Report 2009/242},
    year = {2009},
    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.