@misc{cryptoeprint:2007:466,
author = {Wei Wang and Xiaoyun Wang},
title = {Improved Impossible Differential Cryptanalysis of CLEFIA},
howpublished = {Cryptology ePrint Archive, Report 2007/466},
year = {2007},
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.