@misc{cryptoeprint:2010:494,
author = {Jianjie Zhao, Dawu Gu and M. Choudary Gorantla},
title = {Stronger Security Model of Group Key Agreement},
howpublished = {Cryptology ePrint Archive, Report 2010/494},
year = {2010},
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.