Cryptology ePrint Archive: BibTeX Citation of Report 2007/220

@misc{cryptoeprint:2007:220,
    author = {Fengjiao WANG and Yuqing ZHANG},
    title = {A New Provably Secure Authentication and Key Agreement Mechanism for SIP Using Certificateless Public-key Cryptography},
    howpublished = {Cryptology ePrint Archive, Report 2007/220},
    year = {2007},
    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.