A secure authentication watermarking for halftone and binary images

An authentication watermark is a hidden data inserted into an image that can be used to detect any accidental or malicious alteration in the image. Many authentication-watermarking techniques for continuous-tone images are described in the literature, but only a quite small number of secure authentication watermarking techniques are available for binary/halftone images. This article proposes a simple solution for inserting a secure authentication watermark in binary/halftone images. It consists of choosing a set of pseudo-random pixels in the image, clearing them, computing the message authentication code (or the digital signature) of the now-cleared image, and inserting the resulting code into the selected random pixels. Dispersed-dot halftone images watermarked by the proposed technique present better visual quality than do watermarked generic binary images. However, in practice, the visual degradation is hardly noticeable in either case. The proposed technique seems to be the only binary/halftone watermarking scheme that can detect even a single pixel alteration in the host image. It can be used with secret-key or public-key ciphers. (C) 2004 Wiley Periodicals, Inc.