Attribute-based encryption with outsourced decryption in blockchain

Attribute-based encryption (ABE) is a powerful cryptographic primitive for access control and fine-grained sharing on encrypted data. Due to this functionality, ABE is usually adopted in encrypted cloud storage for flexible data sharing. However, the main drawback of ABE is that the computational cost grows linearly with the complexity of the access policy. One of the promising solutions for the problem is to outsource computation securely. For example, the user can outsource most of the decryption cost to a proxy, while the underlying plaintext remains confidential. Nonetheless, all the existing ABE schemes with outsourced decryption ignore the fairness between the user and the proxy, i.e., the user may refuse to pay even if he/she obtain the valid result. To address this problem, in this paper we propose a new ABE scheme with fair outsourced decryption by using blockchain and sampling technique. In particular, we make use of the smart contract in blockchain to guarantee that the proxy can always get the reward with the valid outsourced decryption result. Furthermore, we apply the sampling technique to enable the miners in blockchain to check the validity of the outsourced decryption result. The detailed analysis conducts that our proposal is secure and fair under some reasonable assumptions, and the experimental results demonstrate that our proposal is efficient. At last, it may be of independent interest that our proposal is a generic construction for pairing-based ABE schemes.