@misc{cryptoeprint:2015:249,
author = {Keita Xagawa},
title = {Improved (Hierarchical) Inner-Product Encryption from Lattices},
howpublished = {Cryptology ePrint Archive, Report 2015/249},
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.