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Watermark defences that hide their detection boundary can be reconstructed by anyone who can query the detector in batches

AspireAI community · arXiv
Latent diffusion watermarks hide their detection boundary behind secret long-term transformations, on the theory that a boundary nobody knows is a boundary nobody can attack. This paper shows that fails under a stronger and more realistic threat model: an adversary with batched access to a publicly reachable detector can reconstruct the transformations and permanently re-expose the decision boundary. The proposed scheme mixes long-term and one-time secret transformations so structure cannot be reused across samples and the long-term secret stays hidden. The authors prove security in the sense that no probabilistic polynomial-time adversary gains a non-negligible advantage over one restricted to adding isotropic white noise with a sqrt(2)-larger perturbation budget, and show that factor is tight. Evaluations on Stable Diffusion 2.1 and XL demonstrate the attack against the existing defence and show the new scheme keeps generation quality while withstanding common perturbations and targeted removal.
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