@misc{cryptoeprint:2008:442,
author = {Anton Kargl and Stefan Pyka and Hermann Seuschek},
title = {Fast Arithmetic on ATmega128 for Elliptic Curve Cryptography},
howpublished = {Cryptology ePrint Archive, Report 2008/442},
year = {2008},
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.