@misc{cryptoeprint:2003:245,
author = {Willi Geiselmann and Rainer Steinwandt},
title = {A Key Substitution Attack on SFLASH^{v3}},
howpublished = {Cryptology ePrint Archive, Report 2003/245},
year = {2003},
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.