>could be defeated by making a 1 pixel change to any photos a pedo would hide on their phone (since any changes to the image would totally change the hash). Th

>could be defeated by making a 1 pixel change to any photos a pedo would hide on their phone (since any changes to the image would totally change the hash). This isn't the way those hashes work. A 1 pixel change would still hash similar enough to be matched. Maybe there are adversarial 1 pixel changes that could break the hashing, but I doubt it. Even cropping, watermarks and other manipulations like that would still match. "Perceptual hashing", very different to cryptographic hashing. It's basically checking if an image looks "similar enough". I believe this is why they needed multiple matches, because otherwise there must have been too many false positives. This may be too oversimplified, but imagine that in a series of CSAM images, there might be, for example, a wall or furniture or something, that could appear similar enough to a wall in one of your own photos. That's a match, off to the gulag with you!