@misc{cryptoeprint:2010:625,
author = {Veronique Cortier and Ben Smyth},
title = {Attacking and fixing Helios: An analysis of ballot secrecy},
howpublished = {Cryptology ePrint Archive, Report 2010/625},
year = {2010},
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.