@misc{cryptoeprint:2004:327,
author = {Sherman S.M. Chow and S.M. Yiu and Lucas C.K. Hui},
title = {Efficient Identity Based Ring Signature},
howpublished = {Cryptology ePrint Archive, Report 2004/327},
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.