A Fully Homomorphic Encryption Scheme Suitable for Ciphertext Retrieval

Ciphertext data retrieval in cloud databases suffers from some critical limitations, such as inadequate security measures, disorganized key management practices, and insufficient retrieval access control capabilities. To address these problems, this paper proposes an enhanced Fully Homomorphic Encryption (FHE) algorithm based on an improved DGHV algorithm, coupled with an optimized ciphertext retrieval scheme. Our specific contributions are outlined as follows: First, we employ an authorization code to verify the user’s retrieval authority and perform hierarchical access control on cloud storage data. Second, a triple-key encryption mechanism, which separates the data encryption key, retrieval authorization key, and retrieval key, is designed. Different keys are provided to different entities to run corresponding system functions. The key separation architecture proves particularly advantageous in multi-verifier coexistence scenarios, environments involving untrusted third-party retrieval services. Finally, the enhanced DGHV-based retrieval mechanism extends conventional functionality by enabling multi-keyword queries with similarity-ranked results, thereby significantly improving both the functionality and usability of the FHE system.