@misc{cryptoeprint:2011:608,
author = {Patrick Longa and Francesco Sica},
title = {Four-Dimensional Gallant-Lambert-Vanstone Scalar Multiplication},
howpublished = {Cryptology ePrint Archive, Report 2011/608},
year = {2011},
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.