@misc{cryptoeprint:2005:240,
author = {Fangguo Zhang and Xiaofeng Chen},
title = {Attack on Okamoto et al.'s New Short Signature Schemes},
howpublished = {Cryptology ePrint Archive, Report 2005/240},
year = {2005},
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.