Key-Policy Multi-Authority Attribute-Based Encryption
  Bilinear groups are often used to create Attribute-Based Encryption (ABE) algorithms. In particular, they have been used to create an ABE system with multi authorities, but limited to the ciphertext-policy instance. Here, for the first time, we propose a multi-authority key-policy ABE system. In our proposal, the authorities may be set up in any moment and without any coordination. A party can simply act as an ABE authority by creating its own public parameters and issuing private keys to the users. A user can thus encrypt data choosing both a set of attributes and a set of trusted authorities, maintaining full control unless all his chosen authorities collude against him. We prove our system secure under the bilinear Diffie-Hellman assumption. 
Riccardo Longo, Chiara Marcolla, Massimiliano Sala
cs.CR