@misc{cryptoeprint:2011:543,
author = {Tatsuaki Okamoto and Katsuyuki Takashima},
title = {Adaptively Attribute-Hiding (Hierarchical) Inner Product Encryption},
howpublished = {Cryptology ePrint Archive, Report 2011/543},
year = {2011},
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.