@misc{cryptoeprint:2011:619,
author = {Bo Zhu and Guang Gong},
title = {Multidimensional Meet-in-the-Middle Attack and Its Applications to KATAN32/48/64},
howpublished = {Cryptology ePrint Archive, Report 2011/619},
year = {2011},
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.