@misc{cryptoeprint:2005:178,
author = {Zhou Sujing and Lin Dongdai},
title = {Analyzing Unlinkability of Some Group Signatures},
howpublished = {Cryptology ePrint Archive, Report 2005/178},
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.