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The other thread demonstrates a similar attack with pictures of dogs. The same questions still apply. Why would you save tons of pictures of dogs to your photo

The other thread demonstrates a similar attack with pictures of dogs. 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?

nullc 5y ago
I'm really surprised to see a fellow HN poster making such a stark failure to generalize. 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.
- hannasanarion 5y ago
  What kind of non-child-porn imagery is so similar to child porn that it fools the NCMEC investigator who is looking at it side-by-side with its maliciously-collisioned hash match, but is at the same time so so dissimilar from real child porn that, as you say, the target won't report its sudden appearance on their device and the the sender won't be at legal risk? Your scenario requires the image (not the hash, but the regular human-perceptible image) to be both unremarkably innocent and incontrovertibly damning at the same time.
  - nullc 5y ago
    You're assuming that there is a side-by-side comparison with a hash matching image. I think that is an extremely big assumption which assumes facts not in evidence at all. 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/ https://www.crimeandfederalism.com/page/61/ )
    - hannasanarion 5y ago
      You still haven't explained how an image can be at the same time so benign that the user doesn't delete it from their phone and cloud service and the sender is in no legal danger, and yet so obviously pornographic that a jury will be convinced beyond any reasonable doubt that it's child porn. 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.

nullc 5y ago

I'm really surprised to see a fellow HN poster making such a stark failure to generalize. 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.

hannasanarion 5y ago
What kind of non-child-porn imagery is so similar to child porn that it fools the NCMEC investigator who is looking at it side-by-side with its maliciously-collisioned hash match, but is at the same time so so dissimilar from real child porn that, as you say, the target won't report its sudden appearance on their device and the the sender won't be at legal risk? Your scenario requires the image (not the hash, but the regular human-perceptible image) to be both unremarkably innocent and incontrovertibly damning at the same time.
- nullc 5y ago
  You're assuming that there is a side-by-side comparison with a hash matching image. I think that is an extremely big assumption which assumes facts not in evidence at all. 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/ https://www.crimeandfederalism.com/page/61/ )
  - hannasanarion 5y ago
    You still haven't explained how an image can be at the same time so benign that the user doesn't delete it from their phone and cloud service and the sender is in no legal danger, and yet so obviously pornographic that a jury will be convinced beyond any reasonable doubt that it's child porn. 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.