@misc{cryptoeprint:2007:321,
author = {Selcuk Kavut and Melek Diker Yucel},
title = {Balanced Boolean Functions with Nonlinearity > 2^{n-1} - 2^{(n-1)/2}},
howpublished = {Cryptology ePrint Archive, Report 2007/321},
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.