@misc{cryptoeprint:2004:326,
author = {Zhengjun Cao},
title = {Cryptanalysis of Qiu-Gu-Chen Variant Group Signature Scheme},
howpublished = {Cryptology ePrint Archive, Report 2004/326},
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.