@misc{cryptoeprint:2005:157,
author = {Colm O'Keeffe and Emanuel Popovici},
title = {FOX Algorithm Implementation: a hardware design approach},
howpublished = {Cryptology ePrint Archive, Report 2005/157},
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.