Preview Helps Selection: Previewable Image Watermarking With Client-Side Embedding

The increasing sharing of images on social networks is prompting the involvement of digital watermarking to protect copyright and combat illegal redistribution. Owner-side embedding and client-side embedding are two modes of digital watermarking, among which the latter enables better system scalability than the former due to its higher owner-side efficiency. However, the existing client-side watermarking schemes do not take into account the preview needs of users, in which users are prevented from acquiring any visual information about the original image before decryption because it is encrypted to be fully blurred. As a result, users cannot select the desired one by previewing when a batch of encrypted images is given. To solve this problem, we overcome the incompatibility between techniques and innovatively combine client-side watermarking with thumbnail-preserving encryption to render the degraded visual perception of the original image onto the encrypted one. Specifically, the image is first fully encrypted as usual client-side watermarking, and then pixel adjustments are performed to approximate the sum of the original pixels in each block for rendering the degraded visual perception. In this way, two schemes with different performance emphasis are proposed, which implement watermark embedding based on spread spectrum and quantization index modulation separately. In terms of performance evaluation, the security of both schemes is thoroughly demonstrated, and experiments are conducted to assess their feasibility, robustness, and efficiency.