Secure and fair outsourced decryption service of attribute-based ciphertext using blockchain

Attribute-based encryption (ABE) is a promising primitive to protect data privacy and achieve flexible data sharing. Considering the increasing number of resource-limited devices in the Internet of Things (IoT) era, outsourcing the expensive decryption operations of ABE to a powerful proxy is helpful and necessary for users. However, the innate distrust between the user and the proxy makes security and fairness challenging. That is, the user would worry about whether the proxy executes the outsourced decryption honestly without breaking data confidentiality. Conversely, the proxy may be concerned that if she can get the deserved reward as promised. In this paper, we propose a blockchain-based scheme to provide secure and fair outsourced decryption service of attribute-based ciphertext. Specifically, we propose an efficient attribute-based encryption scheme with verifiable outsourced decryption (ABE-VOD), which enables the user to outsource most of the decryption operations securely and verify the returned result with little cost. We then integrate blockchain into ABE-VOD to manage the outsourced decryption tasks and deal with the reward and punishment issues as a trusted mediator, which could effectively guarantee fairness. Finally, we implement a proof-of-concept prototype on a local Ethereum testnet to evaluate the feasibility and performance of the proposed scheme.