@misc{cryptoeprint:2008:100,
author = {Patrick Longa},
title = {Accelerating the Scalar Multiplication on Elliptic Curve Cryptosystems over Prime Fields},
howpublished = {Cryptology ePrint Archive, Report 2008/100},
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.