Cryptology ePrint Archive: BibTeX Citation of Report 2008/435

@misc{cryptoeprint:2008:435,
    author = {Stanislav Bulygin and Michael Brickenstein},
    title = {Obtaining and solving systems of equations in key variables only for the small variants of AES},
    howpublished = {Cryptology ePrint Archive, Report 2008/435},
    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.