Cryptology ePrint Archive: BibTeX Citation of Report 2008/281

@misc{cryptoeprint:2008:281,
    author = {Taehyun Kim, Jongsung Kim, Seokhie Hong and Jaechul Sung},
    title = {Linear and Differential Cryptanalysis of Reduced SMS4 Block Cipher},
    howpublished = {Cryptology ePrint Archive, Report 2008/281},
    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.