SanIdea: Exploiting Secure Blockchain-Based Access Control via Sanitizable Encryption

Cryptographic access control guarantees that authorized users can access data while unauthorized get nothing. Such an all-or-nothing access mode achieves secrecy but does not fit strong-privacy scenarios. FE-based access control breaks it and reaches a balance between data privacy and data utilization. To resist malicious senders, Damgard et al. introduced sanitizable functional encryption that enables a bi-directional control to both senders and receivers. However, its centralized structure means that the compromise of the authority incurs massive secret leakage and undermines the system's reliability. In this work, we present SanIdea, a sanitizable, decentralized and privacy-preserving access control framework which embraces a sanitizer in the distributed-authority-domain access control setting. We instantiate it by proposing a cryptographic primitive named sMABE, which adds a Sanitize algorithm over multi-authority attribute-based encryption. We formally prove its security in the IND-CPA model and the Sanitization Security model under the DBDH assumption. We demonstrate its reasonable efficiency through algorithm simulation, where the sanitization time is less than 0.1s with the configuration of 5 attribute authorities and 25 user attributes. We design an SABC system by integrating SanIdea with the blockchain, where SABC uses a smart contract to ensure the correctness of the distributed secret key parts. We implement SABC in an Ethereum testbed and the experiment results show that the upload algorithm costs about 163000 user gas and the download algorithm costs about 84000 user gas, which is cost-reasonable.