@misc{cryptoeprint:2004:226,
author = {Boaz Barak and Yehuda Lindell and Salil Vadhan},
title = {Lower Bounds for Non-Black-Box Zero Knowledge},
howpublished = {Cryptology ePrint Archive, Report 2004/226},
year = {2004},
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.