Cryptology ePrint Archive: BibTeX Citation of Report 2009/631

@misc{cryptoeprint:2009:631,
    author = {DaeHun Nyang and Mun-Kyu Lee },
    title = {Improvement of Das's Two-Factor Authentication Protocol in Wireless Sensor Networks},
    howpublished = {Cryptology ePrint Archive, Report 2009/631},
    year = {2009},
    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.