5 ms·

How will you be able to reduce the field of possible values presented by sha256 if you approximately know the source image? There is still noise in the image an

by tjoff 9y ago

How will you be able to reduce the field of possible values presented by sha256 if you approximately know the source image? There is still noise in the image and several orders of magnitude more data. You assume that brute forcing similar images and hashing the result would be easier than just brute forcing the sha256.

IgorPartola 9y ago
That's exactly what I assume. If I know 90% of the image, but need to brute force 10%, I just reduce the possible field of values by an order of magnitude. The better I know what the image was to begin with, the easier it gets. That's exactly why poor RNGs are bad: I can brute force the value by knowing your method + approximately your input. And if I can affect the image in a way that makes it even more predictable (black sheet over the camera, bright white light into the camera), it gets even easier.
- vokep 9y ago
  but unless the image is very low resolution, chances are sha256 is a smaller space no?
- silotis 9y ago
  Doing a little back-of-the-envelope math, a 1080p image has a total of 1920x1080x3 = 6,220,800 subpixels. That means each subpixel needs to have less than 0.00004 bits of entropy for there to be less than 256 bits in a single image. Surely a typical consumer CCD will introduce more noise than that. A potential catch is that the noise is likely correlated to a degree. Still, it's not clear that even a webcam pointed at a black sheet of paper is insufficient as a source of entropy.
- tjoff 9y ago
  10% of 1 megapixels divided by 24 is 4166. So, even if you only consider the least significant bit of one of the three subpixels in 10% of the image you still have 16 times more information than what you can represent in a SHA256 hash from a measly 1 megapixel image. I think it is valid to question how you could reduce the search space enough to be more beneficial than brute forcing SHA256.

by tjoff 9y ago

IgorPartola 9y ago

That's exactly what I assume. If I know 90% of the image, but need to brute force 10%, I just reduce the possible field of values by an order of magnitude. The better I know what the image was to begin with, the easier it gets. That's exactly why poor RNGs are bad: I can brute force the value by knowing your method + approximately your input. And if I can affect the image in a way that makes it even more predictable (black sheet over the camera, bright white light into the camera), it gets even easier.

vokep 9y ago
but unless the image is very low resolution, chances are sha256 is a smaller space no?
silotis 9y ago
Doing a little back-of-the-envelope math, a 1080p image has a total of 1920x1080x3 = 6,220,800 subpixels. That means each subpixel needs to have less than 0.00004 bits of entropy for there to be less than 256 bits in a single image. Surely a typical consumer CCD will introduce more noise than that. A potential catch is that the noise is likely correlated to a degree. Still, it's not clear that even a webcam pointed at a black sheet of paper is insufficient as a source of entropy.
tjoff 9y ago
10% of 1 megapixels divided by 24 is 4166. So, even if you only consider the least significant bit of one of the three subpixels in 10% of the image you still have 16 times more information than what you can represent in a SHA256 hash from a measly 1 megapixel image. I think it is valid to question how you could reduce the search space enough to be more beneficial than brute forcing SHA256.