@misc{cryptoeprint:2008:045,
author = {Rosario Gennaro and Shai Halevi and Hugo Krawczyk and Tal Rabin},
title = {Threshold RSA for Dynamic and Ad-Hoc Groups},
howpublished = {Cryptology ePrint Archive, Report 2008/045},
year = {2008},
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.