Capacity of digital watermarks subjected to an optimal collusion attack

One envisioned application of digital watermarking is fingerprinting, in which different information is embedded into several copies of the same original signal. Several attackers may collude by combining their copies to produce an attacked signal. In the case of independent watermarks, a collusion-attack model is presented and shown to be analogous to the Gaussian multiple-access channel. The attack parameters are optimized to minimize the information rate under a constraint on the distortion of the attacked signal. Another fingerprinting method, collusion-secure codes, is then related to the attack. Finally, independent and collusion-secure watermarking are compared for the same attacked-signal distortion and probability of false identification.