@misc{cryptoeprint:2015:020,
author = {Phong Q. Nguyen and Jiang Zhang and Zhenfeng Zhang},
title = {Simpler Efficient Group Signatures from Lattices},
howpublished = {Cryptology ePrint Archive, Report 2015/020},
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.