@misc{cryptoeprint:2005:427,
author = {Joseph K. Liu and Duncan S. Wong},
title = {Solutions to Key Exposure Problem in Ring Signature},
howpublished = {Cryptology ePrint Archive, Report 2005/427},
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.