Michel Abdalla Eike Kiltz Gregory Neven Generalized Key Delegation for Hierarchical Identity-Based Encryption. 139-154 2007 conf/esorics/2007 ESORICS http://dx.doi.org/10.1007/978-3-540-74835-9_10 db/conf/esorics/esorics2007.html#AbdallaKN07