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While this (client-side hashing) would be better in terms of privacy protection for this specific case it is not going to happen because Facebook is not allowed

by evgen 9y ago

While this (client-side hashing) would be better in terms of privacy protection for this specific case it is not going to happen because Facebook is not allowed to perform the specific photo hashing client-side as this would expose the hashing mechanism to analysis.

dmitrygr 9y ago
> this would expose the hashing mechanism to analysis Security through obscurity. Works every time (tm)
- kelnos 9y ago
  I mean, it worked pretty well for Google's ranking algorithm, so much so that it spawned an entire industry (SEO, which still doesn't let you get away with much in the way of black-hat type stuff). Even just the rate-limiting aspect of keeping the algo on the server (in this case behind a human, but that's even not necessary) is useful: I can't sit offline with the algo and keep beating it with slightly different images until the hash changes enough to be considered unique.
the8472 9y ago
maybe they shouldn't ask for nudes until they have designed an open source algorithm?
- 15155 9y ago
  How would an open source fingerprinting algorithm work in this case? There would be a browser extension to "refingerprint" images within 24 hours of release.
  - the8472 9y ago
    Make the algorithms public, let them be broken, refine them etc. until the margin between identification and refingerprinting becomes so small that refingerprinting involves such significant changes that we are reasonably confident that we are no longer dealing with theseus' ship. That's how we got good crypto. By not keeping it proprietary. And a side benefit that we would get some amazingly alteration-resistant perceptual image hashes out of that.
    - Freak_NL 9y ago
      It would be interesting to see what people come up with, but I doubt any algorithm can stop someone dedicated to figure out a way to defeat it. Having the algorithm open for public review will lead to better algorithms, but allow for direct feedback if you are attempting to circumvent it; whilst keeping it proprietary will prevent direct feedback, but might keep the algorithm less clever than it could be. If Facebook is using just PhotoDNA, defeating it should be trivial¹: > […] It works by converting the image to black and white, re-sizing it, breaking it into a grid, and looking at intensity gradients or edges […] That's fairly generic, but effective enough for resizing, colour adjustments, minor changes, etc., but not good enough for porn and an adversary willing to do some experimenting with the photo. If I were to guess at how Facebook uses this, I would expect that they use a preprocessing routine that strips out anything that doesn't resemble a nude or semi-nude person, to prevent alterations to the person's surroundings from changing the hash — which is what you would do if you wanted to bypass the filter whilst keeping the essence of the picture intact. Keeping that fact hidden would be integral to their strategy I suppose. 1: https://en.wikipedia.org/wiki/PhotoDNA https://en.wikipedia.org/wiki/PhotoDNA

by evgen 9y ago

dmitrygr 9y ago

> this would expose the hashing mechanism to analysis Security through obscurity. Works every time (tm)

kelnos 9y ago
I mean, it worked pretty well for Google's ranking algorithm, so much so that it spawned an entire industry (SEO, which still doesn't let you get away with much in the way of black-hat type stuff). Even just the rate-limiting aspect of keeping the algo on the server (in this case behind a human, but that's even not necessary) is useful: I can't sit offline with the algo and keep beating it with slightly different images until the hash changes enough to be considered unique.

the8472 9y ago

maybe they shouldn't ask for nudes until they have designed an open source algorithm?

15155 9y ago
How would an open source fingerprinting algorithm work in this case? There would be a browser extension to "refingerprint" images within 24 hours of release.
- the8472 9y ago
  Make the algorithms public, let them be broken, refine them etc. until the margin between identification and refingerprinting becomes so small that refingerprinting involves such significant changes that we are reasonably confident that we are no longer dealing with theseus' ship. That's how we got good crypto. By not keeping it proprietary. And a side benefit that we would get some amazingly alteration-resistant perceptual image hashes out of that.
  - Freak_NL 9y ago
    It would be interesting to see what people come up with, but I doubt any algorithm can stop someone dedicated to figure out a way to defeat it. Having the algorithm open for public review will lead to better algorithms, but allow for direct feedback if you are attempting to circumvent it; whilst keeping it proprietary will prevent direct feedback, but might keep the algorithm less clever than it could be. If Facebook is using just PhotoDNA, defeating it should be trivial¹: > […] It works by converting the image to black and white, re-sizing it, breaking it into a grid, and looking at intensity gradients or edges […] That's fairly generic, but effective enough for resizing, colour adjustments, minor changes, etc., but not good enough for porn and an adversary willing to do some experimenting with the photo. If I were to guess at how Facebook uses this, I would expect that they use a preprocessing routine that strips out anything that doesn't resemble a nude or semi-nude person, to prevent alterations to the person's surroundings from changing the hash — which is what you would do if you wanted to bypass the filter whilst keeping the essence of the picture intact. Keeping that fact hidden would be integral to their strategy I suppose. 1: https://en.wikipedia.org/wiki/PhotoDNA https://en.wikipedia.org/wiki/PhotoDNA