Title: A COLLUSION ATTACK ON STABLE SIGNATURE AND A DEFENSE USING DOMAIN-BASED SIGNATURE ASSIGNMENT
Primary area: applications to computer vision, audio, language, and other modalities
Keywords: Image watermarking, Stable Signature, Collusion Attack, domain-based signature assginment

Abstract: Stable Signature is a recent watermarking framework based on latent diffusion models, which generates images with embedded signatures by fine-tuning the decoder. While prior work has shown that watermarks can be removed while maintaining visual quality by retraining the watermarked decoder with clean images, we demonstrate that collusion among multiple users poses a practical and severe threat. Our attack begins by averaging watermarked decoders, which already provides a strong initialization for watermark removal. With encoder access, this initialization can be further fine-tuned to significantly suppress the watermark signal. Even when the encoder is not available, colluders can expand their group size to achieve comparable effectiveness, highlighting the scalability of the attack. To defend against this threat, we propose a domain-based signature assignment mechanism. In this strategy, the watermarking service provider (e.g., one using Stable Signature) partitions the signature space into domains, requiring all users in the same domain to share a fixed set of domain-index bits in their signatures. Experiments show that the domain-index bits remain robust under the collusion attack when the encoder is not available. Our studies suggest that adopting the domain-based signature assignment and keeping the encoder confidential will be good practices when Stable Signature is used as a watermarking solution.