@misc{cryptoeprint:2014:852,
author = {Robert Granger and Michael Scott},
title = {Faster ECC over $\mathbb{F}_{2^{521}-1}$},
howpublished = {Cryptology ePrint Archive, Report 2014/852},
year = {2014},
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.