@misc{cryptoeprint:2011:649,
author = {Fanbao Liu and Changxiang Shen and Tao Xie and Dengguo Feng},
title = {On the Security of NMAC and Its Variants},
howpublished = {Cryptology ePrint Archive, Report 2011/649},
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.