this post was submitted on 28 Aug 2023
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Traditional hash like MD5 and SHA256 are not locality-sensitive. Can't be used to detect match with certain degree. Otherwise, yes you are correct. Perceptual hashes can create false positive. Very unlikely, but yes it is possible. This is not a problem with perfect solution. Extraordinary edge cases must be resolved on a case by case basis.
And yes, simplest solution must be implemented first always. Tracking post reputation, captcha before post, wait for account to mature before can post, etc. The problem is that right now the only defense we have access to are mods. Mods are people, usually with eyeballs. Eyeballs which will be poisoned by CSAM so we can post memes and funnies without issues. This is not fair to them. We must do all we can, and if all we can includes perceptual hashing, we have moral obligation to do so.
Something I thought about that might be helpful is if mods had the ability to add a post delay on a community basis. Basically, the delay would be moderator adjustable, but only moderators and admins would be able to see the post for X number of minutes after being posted. It'd help for situations like ongoing attacks where you don't necessarily want to have to manually approve posts, but you want a chance to catch any garbage before the post goes public.
Edit: and yeah, one of the reasons I'm aware that perceptual hashes can have collisions is because a number of image viewers/cataloging tools like xnview mp or hydrus network use hash collisions to help identify duplicate images. However, I've seen collisions between unrelated images when lowering the sensitivity which is why I was wondering if there was a way to use multiple hashing algorithms to help reduce false positives without sacrificing the usefulness of it.
Or just making posts approval only with a mod queue