@misc{cryptoeprint:2004:316,
author = {Zhengjun Cao},
title = {On a Threshold Group Signature Scheme and a Fair Blind Signature Scheme},
howpublished = {Cryptology ePrint Archive, Report 2004/316},
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.