Secure Authentication Watermarking for Binary Images

Authentication watermarking is a hidden data inserted into an image, in order to detect any alterations. It seems to be almost impossible to design a really secure authentication watermarking without making use of the solid cryptography theory and techniques. In a cryptography-based authentication watermarking, a message authentication code (or digital signature) of the whole image is computed and the resulting code is inserted into the image itself. However, inserting the code alters the image and consequently its authentication code, invalidating the watermark. To avoid this problem, for gray-scale or color image, usually the least significant bits (LSBs) are cleared, the authentication code of the LSB-cleared image is computed and then the code is inserted into LSBs. Surely, one cannot perform the same procedure for binary images. This paper proposes a quite simple solution for inserting a secure authentication watermarking in dispersed-dot halftone images. This technique can also be applied to any kind of binary images (including clustered-dot halftones), though the visual quality is not as good as when applied to dispersed-dot halftones. The proposed technique can be used with both secret-key or public-key ciphers.