Cryptology ePrint Archive: BibTeX Citation of Report 2011/551

@misc{cryptoeprint:2011:551,
    author = {Shengbao Wu and Mingsheng Wang},
    title = {Security Evaluation against Differential Cryptanalysis for Block Cipher Structures},
    howpublished = {Cryptology ePrint Archive, Report 2011/551},
    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.