Mitigating Watermark Stealing Attacks in Generative Models via Multi-Key Watermarking

Toluwani Aremu, Noor Hussein, Munachiso Nwadike, Samuele Poppi, Jie Zhang, Karthik Nandakumar, Neil Gong, Nils Lukas

Abstract: Watermarking offers a promising solution for GenAI providers to establish the provenance of their generated content. A watermark is a hidden signal embedded in the generated content, whose presence can later be verified using a secret watermarking key. A security threat to GenAI providers are \emph{forgery attacks}, where malicious users insert the provider's watermark into generated content that was \emph{not} produced by the provider's models, potentially damaging their reputation and undermining trust. One potential defense to resist forgery is using multiple keys to watermark generated content. However, it has been shown that forgery attacks remain successful when adversaries can collect sufficiently many watermarked samples. We propose an improved multi-key watermarking method that resists all surveyed forgery attacks and scales independently of the number of watermarked samples collected by the adversary. Our method accepts content as genuinely watermarked only if \emph{exactly} one watermark is detected. We focus on the image and text modalities, but our detection method is modality-agnostic, since it treats the underlying watermarking method as a black-box. We derive theoretical bounds on forgery-resistance and empirically validate them using Mistral-7B. Our results show a decrease in forgery success from up to $100\%$ using single-key baselines to only $2\%$. While our method resists all surveyed attacks, we find that highly capable, adaptive attackers can still achieve success rates of up to $65\%$ if watermarked content generated using different keys is easily separable.