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> There is still a local CSAM database and still a method somewhere that returns a probability that an image is in the database, isn't there?

Yes, but that method is on the server. The client doesn't know which images match, so it can’t scan all photos and decide to upload the ones that match. From Apple:

> On-Device PSI Protocol. Given a user image, the general idea in PSI is to apply the same set of transformations on the image NeuralHash as in the database setup above and do a simple lookup against the blinded known CSAM database. However, the blinding step using the server-side secret is not possible on device because it is unknown to the device. The goal is to run the final step on the server and finish the process on server. This ensures the device doesn’t know the result of the match, but it can encode the result of the on-device match process before uploading to the server.

– CSAM Detection Technical Summary: https://www.apple.com/child-safety/pdf/CSAM_Detection_Techni...




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