Multi-Client Searchable Symmetric Encryption in Redactable Blockchain for Conjunctive Queries

Sharing and searching encrypted data securely in outsourced environments poses a challenge due to possible cooperation between compromised users and untrusted servers. This paper studies the problem of multi-client dynamic searchable symmetric encryption, where a data owner stores encrypted documents on an untrusted remote server and selectively allows multiple users to access them through keyword search queries. The paper proposes a practical multi-client conjunctive searchable symmetric encryption scheme in a redactable blockchain to address this challenge. This scheme achieves multi-client sublinear conjunctive keyword search, and the data owner can authorize clients to access the documents. The scheme combines encryption primitives with novel access control techniques and constructs a redactable blockchain$\zeta$-oblivious group cross tags for sublinear search. The system's security is proven in a simulation-based security model. A prototype implementation using a blockchain-based approach is developed and evaluated on a real-world database containing millions of documents to demonstrate its practicality.