@misc{cryptoeprint:2015:047,
author = {Guilhem Castagnos and Fabien Laguillaumie},
title = {Linearly Homomorphic Encryption from DDH},
howpublished = {Cryptology ePrint Archive, Report 2015/047},
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.