@misc{cryptoeprint:2015:568,
author = {Bingke Ma and Bao Li and Ronglin Hao and Xiaoqian Li},
title = {Cryptanalysis of Reduced-Round Whirlwind (Full Version)},
howpublished = {Cryptology ePrint Archive, Report 2015/568},
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.