@misc{cryptoeprint:2009:284,
author = {Jin Li, Kui Ren, Bo Zhu, and Zhiguo Wan},
title = {Privacy-aware Attribute-based Encryption with User Accountability},
howpublished = {Cryptology ePrint Archive, Report 2009/284},
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.