Cryptology ePrint Archive: BibTeX Citation of Report 2006/479

@misc{cryptoeprint:2006:479,
    author = {Alfredo De Santis and Anna Lisa Ferrara and Barbara Masucci},
    title = {Efficient Provably-Secure Hierarchical Key Assignment Schemes},
    howpublished = {Cryptology ePrint Archive, Report 2006/479},
    year = {2006},
    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.