@misc{cryptoeprint:2008:540,
author = {Jiqiang Lu and Orr Dunkelman and Nathan Keller and Jongsung Kim},
title = {New Impossible Differential Attacks on AES},
howpublished = {Cryptology ePrint Archive, Report 2008/540},
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.