Thank you very much for typing that out for me! And this seems to be the first sound solution I read about. I think I would happily deploy something like that on my own (potential) server. I have to think about it and try and dig up more information.

Lately, I've been following the news about EU data retention and all they come up with are solutions that are proper surveillance of every citizen, a slippery slope and come with many downsides. The justification is always "won't somebody please think of the children" and the proposed solution is to break end to end encryption for everyone. They could have just implemented this. Okay I do actually know why they don't... There is a lobby pushing for general surveillance and protecting children is just their superficial argument to gain acceptance for it. So they're not interested in effective solutions to deal with the specific problem at all. They want something that actually is a slippery slope and can also be used for other purposes, later on.

Such a hash-table would at least detect known illegal content. And it doesn't even trigger on legal content. For example if someone underage sends nudes to their partner consentually. And having it only detect multiple images makes it less likely someone can get attacked by sending them one illegal picture / planting evidence and they'll instantly be flagged and be raided by police. All the proper cases I read about they always find hundreds of images on a criminal's harddisk. And the police already said they can't handle loads of false positives. They're understaffed and overworked and implementing a solution that'd generate many false positives would lead to them having to deal with it and have even less time to deal with the actual criminals.

So this sounds like a solution that Apple has put some thought into. It tackles lots of issues that previously were arguments for me to advocate against CSAM filters.