@misc{cryptoeprint:2010:319,
author = {Jian Weng and Yunlei Zhao},
title = {On the Security of a Bidirectional Proxy Re-Encryption Scheme from PKC 2010},
howpublished = {Cryptology ePrint Archive, Report 2010/319},
year = {2010},
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.