Cryptology ePrint Archive: BibTeX Citation of Report 2011/261

@misc{cryptoeprint:2011:261,
    author = {Danilo Gligoroski and Rune Steinsmo Ødeård and Rune Erlend Jensen},
    title = {OBSERVATION: An explicit form for a class of second preimages for any message M for the SHA-3 candidate Keccak},
    howpublished = {Cryptology ePrint Archive, Report 2011/261},
    year = {2011},
    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.