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.

I'm surprised this isn't linked, there are services that does this for you.

And they are free.

https://blog.cloudflare.com/the-csam-scanning-tool/

I beleive there are several readily available databases of hashes of csam material for exactly this kind of scanning. Looks like there are some open source ones.

Some top results: https://github.com/topics/csam

This looks to be the top project: https://prostasia.org/project/csam-scanning-plugins/

Not true.

A simple CAPTCHA got rid of a huge set of idiotic script-kiddies. CSAM being what it is, could (and should) result in an immediate IP ban. So if you're "dumb" enough to try to upload a well-known CSAM hash, then you absolutely deserve the harshest immediate ban automatically.

You're pretty much like the story of the economist who refuses to believe that $20 exists on a sidewalk. "Oh, but if that $20 really existed on the sidewalk there, then it would have been arbitraged away already". Well guess what? Human nature ain't economic theory. Human nature ain't cybersecurity.

Idiots will do dumb, easy attacks because they're dumb and easy. We need to defend against the dumb-and-easy attacks, before spending more time working on the harder, rarer attacks.

Good question. Yes. Also artefacts from compression can fuck it up. However hash comparison returns percentage of match. If match is good enough, it is CSAM. Davai ban. There is bigger issue however for developers of Lemmy, I assume. It is a philosophical pizdec. It is that if we elect to use PhotoDNA and CSAI Match, Lemmy is now at the whims of Microsoft and Google respectively.

If they hash the file binary data, like CRC32 or SHA, yes. But there are other hash types out there, which are more like "fingerprints" of an image. Think of how Shazam or Sound Hound can recognize a song playing, despite the extra wind, static, etc that's present. There are similar algorithms for images/videos.

No idea how difficult those are to implement, though.

One bit, in fact. Luckily there are other ways of comparing images without actually showing them to human eyes that allow you to calculate a percentage of similarity.

There are several other solutions including ones from Microsoft and Facebook.

The sad thing is that all we can usually do is make it harder for attackers. Which is absolutely still worth doing, to be clear. But if an attacker wants to cause trouble badly enough, there's always ways around everything. Eg, image detection can be foiled with enough transformation, account age limits can be gotten past by a patient attacker. Minimum karma can be botted (even easier than ever with AI) and Lemmy is especially easy to bot karma because you can just spin up an instance with all the bots your heart desires. If posts have to be approved, attackers can even just hotlink to innocent images and then change the image after it's approved.

Law enforcement can do a lot more than we can, by subpoenaing ISPs or VPNs. But law enforcement is slow and unreliable, so that's also imperfect.