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Comment by ars | Hacker News Reader

Can you use this for encryption? Make the decryption key the prime number generated by this, and you unlock it by pointing the camera at the right thing?

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I suppose if you have a very good camera, and are super-humanly precise at aiming the camera, and you have perfectly consistent lighting, then maybe.

No, not really: if you take a picture of something, it won't have the exact same pixel values every time you take it. A QR code would be way easier.

It's scaled to 32x32 black and white. The pixel values don't have to be that similar, only reasonable close.

But then I'm guessing it fudges pixels/bits until it ends up with a prime number. Is that fudging deterministic, or just random?

It tests primality of numbers greater than the number corresponding to the input image by incrementing. So it's deterministic.

Incrementing… So pixels at the bottom right of the image are the most likely to be flipped from the source image. Interesting. (Edit: Assuming big-endianness and left-to-right, top-to-bottom printing of the pixels/bits)

Nonetheless, yes, using this to create security keys would probably be a good way to lose your bitcoin. :D

Clearly the current implementation is not enough, but I wonder if there is a way to do this properly with actual security, and yet be reproducible for a normal human (we can assume the same camera and lighting).

Maybe some kind of marker in the image that is used for cropping and scaling?

Being able to see a particular image, and at a particular time of day (or indoors I guess), would definitely be an interesting security key.

(Hollywood: feel free to take this idea and do it badly :)

Actually it's decrementing: https://github.com/geonnave/primg/blob/master/index.html#L82

I guess it could change the whole image if the last pixels are "000...000", which happens when the top-left corner has an opposite color to the bottom-right one.

This image can't be "primed" correctly, for example: https://i.imgur.com/ms3sXtF.png

But that's only 1024 bits, assuming values of each pixel is independant from its neighbors. Which it isn't, so you are well under 1024 bits.

Huh? Why do you want the decryption key to be prime?

That's how RSA encryption works.

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