@misc{cryptoeprint:2013:417,
author = {Yongjuan Wang and Liren Ding and Wenbao Han and Xiangyu Wang},
title = {The Improved Cube Attack on Grain-v1},
howpublished = {Cryptology ePrint Archive, Report 2013/417},
year = {2013},
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.