Cryptology ePrint Archive: BibTeX Citation of Report 2009/438

@misc{cryptoeprint:2009:438,
    author = {Jean-Philippe Aumasson and Cagdas Calik and Willi Meier and Onur Ozen and Raphael C.-W. Phan and Kerem Varici},
    title = {Improved Cryptanalysis of Skein},
    howpublished = {Cryptology ePrint Archive, Report 2009/438},
    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.