Multi-authority ciphertext-policy attribute-based encryption with accountability

Attribute-based encryption (ABE) is a promising tool for implementing fine-grained cryptographic access control. Very recently, motivated by reducing the trust assumption on the authority, and enhancing the privacy of users, a multiple-authority key-policy ABE system, together with a semi-generic anonymous key-issuing protocol, have been proposed by Chase and Chow in CCS 2009. Since ABE allows encryption for multiple users with attributes satisfying the same policy, it may not be always possible to associate a decryption key to a particular individual. A misbehaving user could abuse the anonymity by leaking the key to someone else, without worrying of being traced. In this paper, we propose a multi-authority ciphertext-policy (AND gates with wildcard) ABE scheme with accountability, which allows tracing the identity of a misbehaving user who leaked the decryption key to others, and thus reduces the trust assumptions not only on the authorities but also the users. The tracing process is efficient and its computational overhead is only proportional to the length of the identity.