Paper 2019/1325

Attribute-based Proxy Re-Encryption with Constant Size Ciphertexts

Arinjita Paul and S. Sharmila Deva Selvi and C. Pandu Rangan

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. This paper proposes the first CCA secure key-policy attribute-based proxy re-encryption~(KP-ABPRE) scheme allowing monotonic access structures with constant ciphertext size for both the original and re-encrypted ciphertexts. Prior to our work, only two attempts were made towards the construction of an RCCA secure and a CCA secure KP-ABPRE scheme in the literature. We show that both the systems are vulnerable to replayable chosen-ciphertext and chosen-ciphertext attack respectively. When a user shares his data by delegating decryption towards an access-policy, the proxy can collude with a malicious delegatee to attempt to obtain the private keys of the delegator during the delegation period. If the private keys are exposed, the security of the delegator's data is completely compromised. The proxy or the delegatee can obtain all confidential data of the delegator at will at any time, even after the delegation period is over. 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 CCA secure in the random oracle model, based on Bilinear Diffie-Hellman exponent assumptions.