@misc{cryptoeprint:2003:259,
author = {Fangguo Zhang and Xiaofeng Chen},
title = {Attack on Two ID-based Authenticated Group Key Agreement Schemes},
howpublished = {Cryptology ePrint Archive, Report 2003/259},
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.