A dual-mode thumbnail-preserving encryption scheme based on chaotic system and reversible data hiding

The widespread adoption of cloud storage has exposed user image data to privacy leakage risks. Traditional image encryption methods convert images into entirely unreadable formats, ensuring security yet severely restricting image usability (e.g., previewing, retrieval). Thumbnail Preserving Encryption (TPE) offers a novel approach to balancing privacy and usability by maintaining visual recognisability in encrypted images. However, existing TPE methods retain only coarse visual information, remaining inadequate for non-visual usability. To address this, we propose a dual-mode TPE scheme based on chaotic systems and reversible data hiding (RDH), aiming to simultaneously achieve image privacy protection and usability enhancement. A novel two-dimensional chaotic mapping is designed to drive a dual encryption mechanism of permutation and substitution. Combined with an improved histogram shifting algorithm, this approach enables lossless embedding of non-visual information within the encrypted domain. Experimental results demonstrate that the total time for encryption and information embedding is merely 0.5 seconds, with an NPCR value exceeding 0.99, indicating robust resistance against differential attacks. Concurrently, the scheme supports lossless recovery of decrypted images and complete extraction of embedded information.