@misc{cryptoeprint:2015:858,
author = {Olivier Blazy and Saqib A. Kakvi},
title = {Group Signatures Without q-Assumptions},
howpublished = {Cryptology ePrint Archive, Report 2015/858},
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.