Cryptology ePrint Archive: BibTeX Citation of Report 2015/666

@misc{cryptoeprint:2015:666,
    author = {Huaifeng Chen and Xiaoyun Wang},
    title = {Improved Linear Hull Attack on Round-Reduced Simon with Dynamic Key-guessing Techniques},
    howpublished = {Cryptology ePrint Archive, Report 2015/666},
    year = {2015},
    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.