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Comment by the8472 | Hacker News Reader

they can verify that after they have found a match.

if there is no match you gave them no data of interest. if there is a match then they already have that image anyway and you didn't make your privacy situation worse, except maybe telling them that you claim that is you, now allowing them to associate more images was you, but that's probably an acceptable tradeoff if there is actual revenge porn of you out there.

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It's not a simple hash. From Alex Stamos: “There are algorithms that can be used to create a fingerprint of a photo/video that is resilient to simple transforms like resizing.” That doesn't make it clear that they are making use of the powerful classifiers that Facebook has, but it's clearly not md5, either.

Regardless, I think your point stands.

They could also combine locally computed perceptual hashes of the unwanted images with face recognition of already uploaded regular images on the profile. That combination makes it even less necessary to send nudes to facebook.

My guess would be that they trained an autoencoder network for facial recognition and they use the encoder part for fingerprinting and check images by the decoder part.

https://en.wikipedia.org/wiki/Autoencoder

If they already have the image, then you don't need this system.

If they don't have it, but are doing verify-after, then the image won't actually be blocked right away. The proposed system would be of pretty marginal value. Some value, yes, but low enough that facebook decided it wasn't worth building that system.

You can't avoid having a tradeoff somewhere.

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