@misc{cryptoeprint:2004:127,
author = {Junghyun Nam, Jinwoo Lee, Seungjoo Kim, and Dongho Won},
title = {DDH-based Group Key Agreement in a Mobile Environment},
howpublished = {Cryptology ePrint Archive, Report 2004/127},
year = {2004},
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.