The cryptography is most likely done at a higher level than the perception comparison and is quite likely done to protect the CSAM hashes than your privacy.
My interpretation of this is that they still use some sort of a perception based matching algorithm they just encrypt the hashes and then use some “zero knowledge proof” when comparing the locally generated hashes against the list, the result of which would be just that X hashes marched but not which X.
This way there would be no way to reverse engineer the CSAM hash list or bypass the process by altering key regions of the image.
> the result of which would be just that X hashes marched but not which X
That means you can't prove an incriminating file was not deleted even if you're the victim of a false positive. So they will suspect you and put you through the whole police investigation routine.
Not necessarily it just means that you don’t know/prove until a certain threshold is reached, in guessing above a specific one that hashes and the photo is then uploaded to Apple for verification and preservation.
My interpretation of this is that they still use some sort of a perception based matching algorithm they just encrypt the hashes and then use some “zero knowledge proof” when comparing the locally generated hashes against the list, the result of which would be just that X hashes marched but not which X.
This way there would be no way to reverse engineer the CSAM hash list or bypass the process by altering key regions of the image.
reply