Cryptology ePrint Archive: BibTeX Citation of Report 2013/328

@misc{cryptoeprint:2013:328,
    author = {Nicky Mouha and Bart Preneel},
    title = {Towards Finding Optimal Differential Characteristics for ARX: Application to Salsa20},
    howpublished = {Cryptology ePrint Archive, Report 2013/328},
    year = {2013},
    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.