@misc{cryptoeprint:2009:293,
author = {Jin Li and Qian Wang and Cong Wang and Kui Ren},
title = {Enhancing Attribute-based Encryption with Attribute Hierarchy},
howpublished = {Cryptology ePrint Archive, Report 2009/293},
year = {2009},
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.