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copied!<p>I'm sure there are many ways. Here's one:</p> <p>In a photograph, minor variations in color would often be unnoticable to the naked eye, or even if noticed, might easily be mistaken for flaws in the quality of the picture.</p> <p>So to take a simple example, suppose you had a gray-scale GIF image where the pallette is arranged in order from white to black with a smooth range of grays in between. I'm not sure how much you know about graphic file formats, but in GIF you have one byte per pixel, with each possible byte value mapping to some specific color. So in this case we could say pallette #0=RGB(0,0,0), pallette #1=RGB(1,1,1), ... palette #255=RGB(255,255,255).</p> <p>Then you take an ordinary, real photograph. Break your secret message into individual bits. Set the last bit of each pallette index number to successive bits of your message.</p> <p>For example, suppose the first eight pixels of the original photo are, say, 01 00 C9 FF FF C8 42 43. Your message begins with the letter "C", ascii code 0110 0111. So you change the last bit of the first byte to 0, changing the byte from 01 to 00. You change the last bit of the second byte to 1, changing the byte from 00 to 01. You change the last bit of the third byte to 1. It's already 1, so that makes no difference. Etc. You end up with the coded 8 bytes being 00 01 C9 FE FF C9 43 43.</p> <p>The changes to the colors would be so subtle that it's unlikely that anyone looking at the picture would notice. Even if they did notice, unless they had a reason to be suspicious, they would likely just conclude that the picture was of less-than-perfect quality.</p> <p>Of course nothing says you have to use 1 bit per byte for the secret message. Depending on how much degradation in quality you think you can get away with, you could use 2 bits per byte, or just change 1 bit in every other byte, etc.</p> <p>Of course the same technique can be used with color photos: change the last bit in each of the RGB components to encode 3 bits per pixel, etc. </p>

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