[chrisseaton]

> You can't reverse it if the data is gone.

That's the problem - the data you think is gone isn't gone. High frequencies are gone.... but you left all the low frequencies, didn't you? You can read a face from the low frequencies.

[pbhjpbhj]

If you blur then mosaic, or vice-versa, then presumably you get rid of the low and high frequencies? Depending on the detail shown in the original image either, or both, might remove enough information to render the image anonymised.

How about replace each face with a "this is not a person" AI generated face, then blur+mosaic. Or just a non-person face using a deepfake system that matches the facial expression?

[chrisseaton]

Why do all these complicated things?

Just draw a black box over faces.

[tgsovlerkhgsel]

Because the result is a lot more ugly.

Don't tell people what not to do. Figure out why they're doing it, and provide what they actually want while still achieving the goals (here: security).

Very coarse mosaic, add noise, then blur seems reasonably safe, and doesn't have to look like crap.

[Hello71]

"seems reasonably safe" seems like a terrible cryptographic analysis. in fact, given that we already know that both blurring and mosaicing are individually reversible, and noise is easily removable from a sufficiently wide mosaic, this seems like a particularly terrible algorithm. that's not the point though: any man can create an encryption algorithm that he himself cannot break. maybe you can come up with an obfuscation algorithm that cannot be trivially broken, but that doesn't mean it's even remotely a good idea.

[tgsovlerkhgsel]

I was writing a HN comment, not a scientific paper, which is why I wrote "seems safe" instead of making stronger claims.

I'd also like to know how mosaicing is reversible, since it demonstrably reduces the total available amount of information from e.g. 20x20 = 400 RGB values to a single RGB value. This is not sufficient for text where you can start brute-forcing individual options because the search space is small and inputs can be reconstructed precisely, but I'd like to see an explanation why you think this is reversible for photos (even without noise added). I'd also like to know how you want to remove random noise applied to each mosaic block.

The mosaicing is supposed to be the security step here. The blur is optional eye candy not expected to remove further information.

In particular, if you claim that a face mosaiced with a large "pixel" size (e.g. so that the typical face is 5x5 "mosaic blocks" big), you're effectively claiming that you can perform facial recognition based on noisy 5x5 pixel images.

[Hello71]

according to https://www.bitsoffreedom.nl/2019/12/12/amazons-rekognition-..., current facial recognition software is able to distinguish faces in very blurry mosaics using statistics.

it doesn't matter though. as I've explained, it's far easier to come up with flawed schemes than prove them insecure. just because I can't explain why your specific scheme is insecure doesn't mean it stands a chance against real cryptographers.

[wool_gather]

I would be worried that a generated fake face would be similar enough to the face of a real someone, somewhere, to get that person in trouble. This isn't a crisp portrait photo; a blurry cell phone video with a lot of activity and noise already kind of leaves an opening for mis-identification.

[steerablesafe]

Nit: when you blur high frequencies are not gone. They are damped.