@misc{cryptoeprint:2005:381,
author = {Xavier Boyen and Brent Waters},
title = {Compact Group Signatures Without Random Oracles},
howpublished = {Cryptology ePrint Archive, Report 2005/381},
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.