On channel capacity and modulation of watermarks in digital still images

Abstract

An adversary who knows a watermarking scheme can extract the watermarked coefficients and attack them directly. This situation can be understood in a similar way to jamming as known from military communications and system performance can be described in terms of channel capacity and distortion. Using a gradient method, the attack is optimized from the adversary's viewpoint by minimizing channel capacity. It turns out that then for the same level of distortion and equiprobable modulation symbols binary modulation can achieve a higher channel capacity than modulation alphabets of larger size.