Microsoft Paint and Photos embed a server-issued GUID as an invisible watermark into AI-generated images created locally on Windows devices. These GUIDs, returned by Microsoft's remote prompt moderation service, are embedded pixel-wise via a specialized watermarking DLL and are also included in the image's C2PA metadata for provenance, linking prompt moderation to the final image content. This system ensures AI-generated images carry a unique, machine-readable identifier even when generated locally, combining remote moderation with local watermarking and signed metadata.
https://xusheng.dev/posts/reversing/mspaint_invisible_watermark/main/