Fine explanation of steganography — the practice of hiding messages inside of other files — and the detection thereof:

What do these statistical artifacts look like? In most cases, the files get _more_ random looking as data is hidden inside them. This is because digital cameras and scanners aren’t very precise. The least significant bit is often highly correlated with the more significant bits. Think of a very bright spot on the image, perhaps caused by a glint of sunlight. These peg the pixels at the maximum value, usually 255 (11111111 in binary). There aren’t that many 254s skewing the number of 1’s and 0’s in the least significant bit plane.

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