Abstract: Traditional encryption protects the privacy of images stored in the cloud while completely depriving them of visual usability. Though thumbnail-preserving encryption (TPE) can balance image privacy and visual usability, none of the existing options consider the preservation of non-visual usability. In view of this, a usability-enhanced TPE scheme is proposed. This scheme takes advantage of the reversibility and flexibility of data hiding, freeing up part of the original image space for pixel adjustment to keep the ciphertext thumbnail similar to the original one. Before performing pixel adjustment, the image is traditionally encrypted for security. After the pixel adjustment is completed, the remaining space capacity is used to embed additional information that provides non-visual usability to users. Therefore, the proposed TPE solution not only balances privacy and visual usability, but also preserves non-visual usability to a certain extent. Finally, experimental results confirm the effectiveness and advancement of the proposed scheme.

Key words: thumbnail-preserving encryption, visual privacy protection, data hiding, image encryption, cloud storage