@misc{cryptoeprint:2008:227,
author = {Ruilin Li, Bing Sun, Peng Zhang and Chao Li},
title = {New Impossible Differential Cryptanalysis of ARIA},
howpublished = {Cryptology ePrint Archive, Report 2008/227},
year = {2008},
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.