Block-Distortion-Free Reversible Data Hiding in Encryption-Then-Compression Images with Fully Flexible Access Privileges

In this paper, we propose a block-distortion-free reversible data hiding method for encryption-then-compression (EtC) images that supports fully flexible access privileges without constraints on the restoration order. The proposed approach redesigns the pre-processing strategy of previous work to ensure a clear separation of processing roles between the image owner and the data hider. It also introduces a pixel-value modification process that divides the target range into two regions to mitigate the influence of negative–positive inversion during restoration. As a result, block distortion in marked images is eliminated while preserving role separation between the image owner and the data hider. The proposed method offers four key advantages: flexible access privileges, elimination of block distortion, explicit role separation, and competitive hiding capacity comparable to existing methods with flexible restoration capabilities. Experimental results demonstrate that the proposed method achieves a high marked-image quality and competitive hiding capacity while maintaining the compression performance of marked EtC images. Furthermore, security analysis confirms the robustness of the generated EtC images against a representative ciphertext-only attack.