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Comment by simondotau

4 years ago

The image would need to be vaguely similar in terms of gross shapes and arrangement. It's exceedingly unlikely that any CSAM would ever be remotely similar to an ID card or a sheet of paper.

If there are ever going to be any "natural" matches to any CSAM hashes, it's probably going to be a photograph of people who are coincidentally in a similar pose at a nearly identical angle and strikingly shading.

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simondotau

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waz0wski  4 years ago

In the myriad of articles about this systems many issues there have been comments from people who have worked with the NCMEC upstream database and note that it's filled with mundane photos, empty rooms, etc - I think it was in one of the hackerfactor article discussions

This entire system is ripe for false positives AND adversarial attacks.

  • simondotau  4 years ago

    I've no doubt the totality of the database contains a lot of photos, but only photos tagged as A1, A2, B1, or B2 would be considered illegal to possess. And then only the absolute worst of the worst (images categorised as "A1") are being included in the hash set on iOS. The category definitions are:

      A = prepubescent minor
  B = pubescent minor
  1 = sex act
  2 = "lascivious exhibition"

    The categories are described in further detail (ugh) in this PDF, page 22: https://www.prosecutingattorneys.org/wp-content/uploads/Pres...

  • shuckles  4 years ago

    The NCMEC database is large and graded to distinguish types of photos. There’s evidence in the false positive calculations that Apple is only using a subset, presumably the one where photos are graded as depicting active abuse.

    It’s not reasonable to dispute the 1 in 1e12 false positive claim on mere speculation.