@misc{cryptoeprint:2010:260,
author = {Bernhard Jungk and Steffen Reith},
title = {On FPGA-based implementations of Gr\{o}stl},
howpublished = {Cryptology ePrint Archive, Report 2010/260},
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.