Revocable and Flexible Privacy-Preserving Data Computing with Bilateral Access Control for Cloud-Fog Based EHR Systems

Cloud-fog-assisted electronic health record (EHR) systems offer promising solutions for large-scale medical data storage and processing. However, they also raise critical privacy concerns, particularly regarding secure computation over sensitive data, fine-grained bilateral access control, dynamic revocation, and decryption key exposure. Existing cryptographic primitives, such as functional encryption and matchmaking encryption, address some of these challenges individually but fail to offer a unified solution. In this work, we design a revocable and privacy-preserving data computing system with bilateral access control (RPDC-BAC) for cloud-fog-assisted EHR sharing, built upon our newly proposed cryptographic primitive, called server-aided revocable attribute-based matchmaking functional encryption (SR-AB-MFE). Specifically, our scheme supports expressive bilateral access control and enables computation over encrypted data. In addition, we integrate a time-evolving decryption key mechanism to resist decryption key exposure and a server-aided revocation mechanism to efficiently exclude revoked users. To further reduce receiver-side overhead, fog nodes are delegated to verify the authenticity of the ciphertext and perform partial decryption. We formally define the syntax and construction of the SR-AB-MFE scheme, prove its security under static assumptions, and analyze the security of the proposed RPDC-BAC system against practical threat scenarios. Finally, we demonstrate the efficiency and practicality of our design through comprehensive experimental evaluation.