Why would you save tons of gray blobs to your photo library? Why would gray blobs look like child porn? Why would Apple reviewers think gray blobs are child porn? Why would the NCMEC think gray blobs are child porn? Why would law enforcement spend time arresting someone for gray blobs?
Since the grey blob exists, I believe it is fully possible to construct natural(-ish) images that have a selected hash.
So, you should perhaps instead imagine attackers that modify lawful nude images to have matching hashes with child porn images.
With that in mind, most of your questions are answered, except perhaps for "Why would the NCMEC think"-- and the answer would be "because its porn and the computer says its child porn".
Of course, an attacker could just as well use REAL child porn. But there are logistic advantages in having to do less handling of unlawful material, and it's less likely that the target will notice. Imagine: if the target already has nude images on their host the attacker might use those as the templates. I doubt most people would notice if their nude image collection was replaced with noisier versions of the same stuff. And even if they did notice, "someone is trying to frame me for child porn" wouldn't be their first guess. :)
The same questions still apply. Why would you save tons of pictures of dogs to your photo library? Why would pictures of dogs look like child porn? Why would Apple reviewers think pictures of dogs are child porn? Why would the NCMEC think pictures of dogs are child porn? Why would law enforcement spend time arresting someone for pictures of dogs?
What is the scenario where someone can be harmed reputationally or criminally because of a hash collision attack, where the same attack could not be performed more easily and causing more damage by actually using real CSAM images?
You see that they can do this with pictures of dogs. What makes you think they can't do exactly the same with pictures of crotches?
Presumably you don't believe there is some kind inherent dog-nature that makes dog images more likely to undermine the hash. :) People are using pictures of dogs because they are a tasteful safe-for-work example, not because anyone actually imagines using images of dogs.
An actual attack would use ordinary nude images, probably ones selected so that if you were primed to expect child porn you'd believe it if you didn't spend a while studying the image.
> where the same attack could not be performed more easily and causing more damage by actually using real CSAM images?
Actual child porn images are more likely to get deleted by the target and/or reported by the target. The attacker also takes on some additional risk that their possession of the images is a strict liability crime. This means that if the planting party gets found with the real child porn they'll be in trouble vs with the hash-matching-legal-images they'll only be in trouble if they get caught planting it (and potentially only exposed to a civil lawsuit, rather than a felony, depending on how they were planting it).
Personally, I agree that the second-preimage-images are not the most interesting attack! But they are a weakness that makes the system even more dangerous. We could debate how much more dangerous they make it.
As far as what kind of image would be believed to be child porn without a side-by-side with the supposed match: ordinary porn. Without context plenty would be hard to distinguish, especially with the popularity of waxed smooth bodies and explicit close up shots.
Prosecution over "child porn" that isn't is already a proved thing, with instances where the government was happily trotting out 'experts' to claim that the images were clearly a child based on physical characteristics only to be rebuffed by the actual actress taking the stand. ( https://www.crimeandfederalism.com/page/61/ )
The entire point of this process is to catch only images from a known catalogue of existing images, of course there will be opportunity for side-by-side comparison. And if the side-by-side comparison can convince a jury that the images are the same, then the result is literally identical to if you had just sent the original image, the hash checking service hasn't impacted the attack whatsoever.
The idea that new images of nudity could be caught in it is because of a mixup by the press after it was announced, because people confused it with a different parental control feature that detects nudity in images taken by your kids' phones. This is not the same thing.
There are plenty of examples of US persons being charged for lawful pornography which a prosecutor accused of being child porn, I linked to one such example-- where the accused was rescued only by the testimony of the actress, and where the prosecution had expert witnessess testifying that the images of an adult were images of a child.
The attacker in this case would know the origin of the images and could point to them. The victim would have no idea where they came from.
I have searched the case law, and can find no case where the NCMEC provided an image for comparison. Are you aware of any?
You also seem to have continued to move the goalpost. Being merely accused of possessing child porn would be extremely damaging to a person. The fact that they might escape conviction by a jury after years of legal ordeal, incarceration, and having their reputation ruined, is not that much consolation.
> The idea that new images of nudity could be caught in it is because
This is an absurd claim. I have demonstrated (https://github.com/AsuharietYgvar/AppleNeuralHash2ONNX/issue...) that it is possible to alter existing images to match an arbitrary neuralhash resulting in a perfectly normal looking image. This point is not a matter for dispute, anymore.
I could easily do so with a nude or pornographic image, I only chose to use SFW pictures out of good taste.
My interest in apple's scheme arouse out of their malicious use of cryptography to shield their actions from accountability. I didn't even hear about the imessage nudie scanner until some time after. You can be certain no such confusion applies here.
At the very least, there are large classes of images that, while not realistic photographs, people will willingly download, that should be very easy to craft collisions for:
Memes. In particular, given the distortions of surreal and deep-fried memes, it should be pretty easy to craft collisions there.
So someone need only find a collection of likely included images and compute their hashes and publish it.
The attack works just as well even if the attacker isn't sure that every hash is in the database, they just need to know enough likely-matches such that they get enough hits.
Consider it this way, some attacker spends a couple hours searching for child porn and finds some stuff. ... what kind of failure would the NCMEC database be if it didn't include that material?