@misc{cryptoeprint:2003:260,
author = {Xinjun Du, Ying Wang, Jianhua Ge, Yumin Wang},
title = {An Improved ID-based Authenticated Group Key Agreement Scheme},
howpublished = {Cryptology ePrint Archive, Report 2003/260},
year = {2003},
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.