Cryptology ePrint Archive: BibTeX Citation of Report 2006/224

@misc{cryptoeprint:2006:224,
    author = {Andr\'{e} Weimerskirch and Christof Paar},
    title = {Generalizations of the Karatsuba Algorithm for Efficient Implementations},
    howpublished = {Cryptology ePrint Archive, Report 2006/224},
    year = {2006},
    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.