A Lossless Watermarking Scheme for Halftone Image Authentication

Summary Nowadays, halftone images appear routinely in books, magazines, printer outputs, and fax documents. It is desirable to embed data in halftone images for copyright protection and content authentication. This paper proposes a novel watermarking scheme for halftone image authentication, exploiting image hash as a fragile watermark. After pixel blocking and ordering, a lookup table is constructed according to blocks' frequency of occurrence. Watermark embedding is to displace the original blocks with the corresponding similar blocks in the lookup table, and in the reverse process watermark is extracted. Some extra blocks are randomly selected with a secret key for the lookup table embedding, and the original data of these blocks are also inserted into the image. In image authentication, the lookup table is reconstructed first with the secret key, and then a simple table-look-up procedure is employed to extract the watermark hash, finally we only need to compare the watermark hash with the hash of recovered image: if they are equal, the original image suffers no alteration; otherwise it is changed. As a lossless technique, the original image can be perfectly recovered by performing the reverse process of the watermark embedding if the watermarked image is intact. It is necessary to keep the content of original host image unchanged in some specific applications, where content accuracy of the host image must be guaranteed, e.g. military maps, medical images, great works of art, etc. As a fragile watermarking, even one pixel toggling can be detected. Because of the small quantity of watermark, low quality distortion is introduced to the halftone image. Experiment results demonstrate the effectiveness of the scheme.