@misc{cryptoeprint:2004:206,
author = {Jing Xu and Zhenfeng Zhang and Dengguo Feng},
title = {ID-Based Proxy Signature Using Bilinear Pairings},
howpublished = {Cryptology ePrint Archive, Report 2004/206},
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.