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Vehicles share information via Vehicular Ad-hoc Network (VANET) that is considered the foundation of the intelligent transportation system. Access control from functional encryption (FE) allows secure sharing over encrypted data with both utilization and privacy. However, low-powerful vehicles and the untrusted server incurs that FE-based access control mechanisms are less practical with client-side inefficiency and access unreliability. In this work, we put forth ORAC, an Optimized encrypted Access Control system with partial privacy and Reliability. ORAC is built on top of functional encryption that guarantees data partial privacy and data utilization in VANET, and separates the data access into the offline phase and the online phase, such that most of decryption computation could be done in the offline phase and vehicles could execute the real decryption efficiently. By embedding blockchain transactions into identity records and data storage, ORAC protects data accessed by vehicles from tampering and thereby manages data transfers with high reliability. It can be instantiated directly from pairing-based assumptions and features only the decryption complexity of two exponentiation and two multiplication operations. We give a formal security proof of the instantiation in the semantic security model. We also prototype our scheme by implementing ORAC over Ethereum with two types of smart contracts and the evaluation can demonstrate its efficiency optimization. [ABSTRACT FROM AUTHOR]

Currently, decentralized redactable blockchains have been widely applied in IoT systems for secure and controllable data management. Unfortunately, existing works ignore policy privacy (i.e., the content of users' redaction policies), causing severe privacy leakage threats to users since users' policies usually contain large amounts of private information (e.g., health conditions and geographical locations) and limiting the applications in IoT systems. To bridge this research gap, we propose PFRB, a policy-hidden fine-grained redactable blockchain in decentralized blockchain-based IoT systems. PFRB follows the decentralized settings and fine-grained chameleon hash-based redaction in existing redactable blockchains. In addition, PFRB hides users' policies during policy matching such that apart from successful policy matching, users' policy contents cannot be inferred and valid redactions cannot be executed. Some main technical challenges include determining how to hide policy contents and support policy matching. Inspired by Newton's interpolation formula-based secret sharing, PFRB converts policy contents into polynomial parameters and utilizes multi-authority attribute-based encryption to further hide these parameters. Theoretical analysis proves the correctness and security against the chosen-plaintext attack. Extensive experiments on the FISCO blockchain platform and IoT devices show that PFRB achieves competitive efficiency over current redactable blockchains. [ABSTRACT FROM AUTHOR]