[GistNoesis]

How do you defend against semi transparent overlaying ?

Take an image you want to censor, overlay with a very transparent offensive image. Also publish the original image (or a second image with a different transparency value), and an in-browser extension can reconstruct the offensive image.

Your hash database will be flooded with wrong values.

[lonelappde]

That seems like symmetric key encryption with extra steps.

[GistNoesis]

This is a general attack against perceptual hashing. It tries to achieve multiple objectives. Using stenography you could add any data in the image, like you would with encryption as you suggest. But here it is deeper.

By superposing a very transparent bad image with the good one, the perceptual hash won't be affected but the image would be still labelled as offensive by the poor guy labeling the data because the transparent offensive image is still visible. This mean that the good data will be marked as potentially offensive.

This allows anyone to target any individual user, or website. You take some of their published content, overlay offensive image and republish. The image will be reported as offensive, yet have a similar hash (i.e. a collision) to the good image which automated systems will pick-up as the original image being offensive and blacklisting its user or tanking down the website on search engine results.

Because this is sensitive data, by law it should be deleted as soon as it is detected which means the proofs get deleted automatically.