Ghostor: Toward a Secure Data-Sharing System from Decentralized Trust

Data-sharing systems are often used to store sensitive data. Both academia and industry have proposed numerous solutions to protect the user privacy and data integrity from a compromised server. Practical state-of-the-art solutions, however, use weak threat models based on centralized trust-they assume that part of the server will remain uncompromised, or that the adversary will not perform active attacks. We propose Ghostor, a data-sharing system that, using only decentralized trust, (1) hides user identities from the server, and (2) allows users to detect server-side integrity violations. To achieve (1), Ghostor avoids keeping any per-user state at the server, requiring us to redesign the system to avoid common paradigms like per-user authentication and user-specific mailboxes. To achieve (2), Ghostor develops a technique called verifiable anonymous history. Ghostor leverages a blockchain rarely, publishing only a single hash to the blockchain for the entire system once every epoch. We measured that Ghostor incurs a 4-5x throughput overhead compared to an insecure baseline. Although significant, Ghostor's overhead may be worth it for security-and privacy-sensitive applications.