@misc{cryptoeprint:2003:187,
author = {Tatsuaki Okamoto and Ryo Kashima},
title = {Resource Bounded Unprovability of Computational Lower Bounds },
howpublished = {Cryptology ePrint Archive, Report 2003/187},
year = {2003},
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.