Agreed entirely.
I want to add some technical details, since this is a peeve I've also had for many years now:
The standard for this is Microsoft's PhotoDNA, a paid and gatekept software-as-a-service which maintains a database of "perceptual hashes." (Unlike cryptographic hashes, these are robust against common modifications).
It'd be very simple for Microsoft to release a small library which just wraps (1) the perceptual hash algorithm and provides (2) a bloom filter (or newer, similar structures, like an XOR filter) to allow developers to check set membership against it.
There are some concerns that an individual perceptual hash can be reversed to a create legible image, so I wouldn't expect or want that hash database to be widely available. But you almost certainly can't do the same with something like a bloom filter.
If Microsoft wanted to keep both the hash algorithm and even an XOR filter of the hash database proprietary, that's understandable. But then that's ok too, because we also have mature implementations of zero-knowledge set membership proofs.
The only reason I could see is that security-by-obscurity might be a strategy that makes it infeasible for people to find adversarial ways to defeat the proprietary secret-sauce in their perceptual hash algorithm. But I that means giving up opportunities to improve the algorithm, while excluding so many ways it could be useful to combat CSAM.
I want to add some technical details, since this is a peeve I've also had for many years now:
The standard for this is Microsoft's PhotoDNA, a paid and gatekept software-as-a-service which maintains a database of "perceptual hashes." (Unlike cryptographic hashes, these are robust against common modifications).
It'd be very simple for Microsoft to release a small library which just wraps (1) the perceptual hash algorithm and provides (2) a bloom filter (or newer, similar structures, like an XOR filter) to allow developers to check set membership against it.
There are some concerns that an individual perceptual hash can be reversed to a create legible image, so I wouldn't expect or want that hash database to be widely available. But you almost certainly can't do the same with something like a bloom filter.
If Microsoft wanted to keep both the hash algorithm and even an XOR filter of the hash database proprietary, that's understandable. But then that's ok too, because we also have mature implementations of zero-knowledge set membership proofs.
The only reason I could see is that security-by-obscurity might be a strategy that makes it infeasible for people to find adversarial ways to defeat the proprietary secret-sauce in their perceptual hash algorithm. But I that means giving up opportunities to improve the algorithm, while excluding so many ways it could be useful to combat CSAM.