@misc{cryptoeprint:2014:505,
author = {David Kohel, Kristin Lauter, Christophe Petit, Jean-Pierre Tignol},
title = {On the quaternion $\ell$-isogeny path problem},
howpublished = {Cryptology ePrint Archive, Report 2014/505},
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.