Constant Ciphertext Size Multi-Authority Attribute-Based Scheme Without Key Escrow

With the development of cloud computing application, attribute based encryption (ABE) with flexibly fine-grained data access control is adopted widely. However, the honest but curious authorities often peep at the user data. How to eliminate the key escrow is also an important and challenging problem in ABE schemes. In this paper, a constant ciphertext size multi-authority ciphertext-policy ABE scheme (RKE-MA-ABE) which resists key escrow is presented. In the proposed scheme, a user credential issuer (UCI) is introduced to generate a credential for each user credibly. UCI decentralizes the managing privilege of attribute authorities (AAs) and helps to embed the user's secret value into the decryption key issued by the corresponding AAs. Additionally, AAs in this scheme work independently without interacting with each other to generate the master public key of the system during the system initialization phase. Our scheme avoids the collusion attacks by vicious users or authorities and has constant ciphertext length. It is proven CPA-secure under the decisional q-Bilinear Diffie-Hellman Exponent (q-BDHE) assumption in random oracle model.