Alfredo De Santis Anna Lisa Ferrara Barbara Masucci Efficient Provably-Secure Hierarchical Key Assignment Schemes. 371-382 2007 conf/mfcs/2007 MFCS http://dx.doi.org/10.1007/978-3-540-74456-6_34 db/conf/mfcs/mfcs2007.html#SantisFM07