  We revisit Tree-Ring Watermarking, a recent diffusion model watermarking
method that demonstrates great robustness to various attacks. We conduct an
in-depth study on it and reveal that the distribution shift unintentionally
introduced by the watermarking process, apart from watermark pattern matching,
contributes to its exceptional robustness. Our investigation further exposes
inherent flaws in its original design, particularly in its ability to identify
multiple distinct keys, where distribution shift offers no assistance. Based on
these findings and analysis, we present RingID for enhanced multi-key
identification. It consists of a novel multi-channel heterogeneous watermarking
approach designed to seamlessly amalgamate distinctive advantages from diverse
watermarks. Coupled with a series of suggested enhancements, RingID exhibits
substantial advancements in multi-key identification. Github Page:
https://github.com/showlab/RingID
