Attribute specified identity-based encryption

Fine-grained access control of encrypted data without trusted third party is a challenging task. Using the simple attribute-based encryption has the problem of key escrow, since there exists a trusted authority who is able to generate the secret keys of all users. Delegating this ability to multiple authorities can only minimize the risk, but not eliminating the possibility that all authorities may collude. We develop a new cryptosystem called Attribute Specified Identity-Based Encryption (AS-IBE) to solve this problem. We employ the idea of certificateless encryption and extend it to the attribute-based setting. Each user chooses his own public and secret key pairs to use, in addition to his attribute-based secret key. Therefore, the authority cannot decrypt without the knowledge the user's own secret key. Yet, the resulting AS-IBE system has some fundamental differences with the original attribute-based encryption. In this paper, we give the security model for the new AS-IBE cryptosystems, and propose two variants for the construction, namely the key policy AS-IBE and ciphertext policy AS-IBE.