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Perceptual hashes are not exact hashes, otherwise they would be useless for this task; you would just mirror the image or change 1 pixel.

They are instead fuzzy classifiers, and thus have non-zero error rates.

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But this trigger requires many of these false flags to exceed the threshold (most likely why it exists). I imagine the "one in a trillion" numbers they claim for false flagging rate are probably cemented in reality, and make it trivial for human review.

I've explained the problem here: https://news.ycombinator.com/item?id=28099927

How does this relate to the allegation I was responding to? There is a difference between false positives and false negatives you know. False negative is where the criminal evades the system through measures you describe. False positive — what I was responding to — is where Apple incorrectly accuses an innocent person and thus violates their privacy during the subsequent review.

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