Cryptology ePrint Archive: BibTeX Citation of Report 2005/223

@misc{cryptoeprint:2005:223,
    author = {Ayan Mahalanobis},
    title = {Diffie-Hellman Key Exchange Protocol, Its Generalization and Nilpotent Groups},
    howpublished = {Cryptology ePrint Archive, Report 2005/223},
    year = {2005},
    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.