@misc{cryptoeprint:2008:165,
author = {Pinhui Ke},
title = {Improved lower bound on the number of balanced symmetric functions over GF(p)},
howpublished = {Cryptology ePrint Archive, Report 2008/165},
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.