@misc{cryptoeprint:2015:565,
author = {Craig Costello and Patrick Longa},
title = {FourQ: four-dimensional decompositions on a Q-curve over the Mersenne prime},
howpublished = {Cryptology ePrint Archive, Report 2015/565},
year = {2015},
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.