@misc{cryptoeprint:2000:046,
author = {Stefan Lucks},
title = {The Saturation Attack - a Bait for Twofish},
howpublished = {Cryptology ePrint Archive, Report 2000/046},
year = {2000},
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.