Data hiding (DH) is a technique used to embed a sequence of bits in a cover image with small visual deterioration and the means to extract it afterwards. Authentication watermarking (AW) techniques use DH to insert particular data into an image, in order to detect later any accidental or malicious alterations in the image, as well as to certify that the image came from the right source. In recent years, some AWs for binary images have been proposed in the literature. The authentication of binary images is necessary in practice, because most scanned and computer-generated document images are binary.This publication describes techniques and theories involved in binary image AW: We describe DH techniques for binary images and analyze which of them are adequate to be used in AWs; analyze the most adequate secret- and public-key cryptographic ciphers for the AWs; describe how to spatially localize the alteration in the image (besides detecting it) without compromising the security; present AWs forJBIG2-compressed binary images; present a reversible AW for binary images; and finally present our conclusions and future research.
Top1. Introduction
This publication describes techniques and theories involved in binary image authentication watermarking. The authentication of binary images is necessary in practice because most of scanned and computer-generated document images are binary. These documents must be protected against fraudulent alterations and impersonations.
Binary images can be classified as either halftone or non-halftone. Halftone images are binary representations of grayscale images. Halftoning techniques (Ulichney, 1987; Knuth, 1987; Roetling et al., 1994) simulate shades of gray by scattering proper amounts of black and white pixels. On the other hand, non-halftone binary images may be composed of characters, drawings, schematics, diagrams, cartoons, equations, etc. In many cases, a watermarking algorithm developed for halftone images cannot be applied to non-halftone images and vice-versa.
Data hiding (DH) or steganography is a technique used to embed a sequence of bits in a cover image with small visual deterioration and the means to extract it afterwards. Most DH techniques in the literature are designed for grayscale and color images and they cannot be directly applied to binary images. Many of continuous-tone DHs modify the least significant bits (Wong, 1998), modify the quantization index (Chen et al., 2001), or modify spectral components of data in a spread-spectrum-like fashion (Cox et al., 1997; Marvel et al., 1999). Many of the continuous-tone DHs makes use of transforms like DCT and wavelet. Unfortunately, none of the above concepts (least significant bits, quantization indices and spectral components) are applicable to binary images. Binary images can be viewed as special cases of grayscale images and consequently can be transformed using DCT or wavelet, resulting in continuous-tone images in transform-domain. However, modifying a transform-domain image to insert the hidden data and inverse transforming it, usually will not yield a binary image. Hence, transforms like DCT and wavelet cannot be used to hide data in binary images. As consequence of the reasoning above, special DH techniques must be designed specifically for binary images.
A watermark is a signal added to the original cover image that can be extracted later to make an assertion about the image. Digital watermarking techniques can be roughly classified as either “robust watermarks,” or “authentication watermarks.” Robust watermarks are designed to be hard to remove and to resist common image-manipulation procedures. They are useful for copyright and ownership assertion purposes.
Authentication watermarks (AWs) use DH techniques to insert the authentication data into an image, in order to detect later any accidental or malicious alterations in the image, as well as to certify that the image came from the right source. AWs can be further classified in two categories: fragile and semi-fragile watermarks.