@misc{cryptoeprint:2008:380,
author = {Andreas Noack and Stefan Spitz},
title = {Dynamic Threshold Cryptosystem without Group Manager},
howpublished = {Cryptology ePrint Archive, Report 2008/380},
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.