Efficient Attribute-Based Proxy Re-Encryption with Constant Size Ciphertexts

Abstract

Attribute-based proxy re-encryption (ABPRE) allows a semi-trusted proxy to transform an encryption under an access-policy into an encryption under a new access policy, without revealing any information about the underlying message. Such a primitive facilitates fine-grained secure sharing of encrypted data in the cloud. In its key-policy flavor, the re-encryption key is associated with an access structure that specifies which type of ciphertexts can be re-encrypted. Only two attempts have been made towards realising key-policy ABPRE (KP-ABPRE), one satisfying replayable chosen ciphertext security (RCCA security) and the other claiming to be chosen ciphertext secure (CCA secure). We show that both the systems are vulnerable to RCCA and CCA attacks respectively. We further propose a selective CCA secure KP-ABPRE scheme in this work. Since we demonstrate attacks on the only two existing schemes in the literature, our scheme becomes the first KP-ABPRE scheme satisfying selective CCA security. Moreover, our scheme has an additional attractive property, namely collusion resistance. A proxy re-encryption scheme typically consists of three parties: a delegator who delegates his decryption rights, a proxy who performs re-encryption, and a delegatee to whom the decryption power is delegated to. When a delegator wishes to share his data with a delegatee satisfying an access-policy, the proxy can collude with the malicious delegatee to attempt to obtain the private keys of the delegator during delegation period. If the private keys are exposed, security of the delegator’s data is completely compromised. Hence, achieving collusion resistance is indispensable to real-world applications. In this paper, we show that our construction satisfies collusion resistance. Our scheme is proven collusion resistant and selective CCA secure in the random oracle model, based on Bilinear Diffie-Hellman exponent assumption.