A secure authentication watermarking for halftone and binary images

Authentication watermark is a hidden data inserted into an image that can be used to detect any accidental or malicious alteration in the image. In the literature, there are many authentication-watermarking techniques for continuous-tone images. However, quite a small number of secure authentication watermarking techniques are available for binary/halftone images. This paper proposes a simple solution for inserting a secure authentication watermark in binary/halftone images. It consists on choosing a set of pseudorandom pixels in the image, clearing them, computing the message authentication code (or the digital signature) of the random-pixels-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 watermarked generic binary images. However, in practice, the size of the host image uses to be much larger than the size of the inserted code, and consequently 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 secretkey or public-key ciphers.