@misc{cryptoeprint:2009:357,
author = {Zhongming Wu and Xuejia Lai and Bo Zhu and Yiyuan Luo},
title = {Impossible Differential Cryptanalysis of FOX},
howpublished = {Cryptology ePrint Archive, Report 2009/357},
year = {2009},
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.