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Miloslav Ciz 2025-02-22 20:35:24 +01:00
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steganography.md
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@ -223,4 +223,4 @@ int main(int argc, char **argv)
The usage is following: make a file with a grayscale [ASCII art](ascii_art.md) picture, then pass it to the standard input of this program along with text you want to encode (maximum length of the text you can encode is given by the count of usable characters in the input image) passed as the first argument to the program, for example: `cat picture.txt | ./program "hello"`. The program will print out the image with the text embedded in. To read the text from the image similarly pass the picture to the program's input, without passing any arguments, for example: `cat picture2.txt | ./program`. The text will be written to terminal.
The method used is this: firstly for the encoded message we use our own 6 bit alphabet -- this only allows us to represent 63 symbols (which we have chosen to be uppercase and lowercase letters, space and period) but will allow us to store more of them. Each 6 bit symbol of our alphabet will be encoded by three bit pairs (3 * 2 = 6). One bit pair will be encoded in one ASCII art character by altering that character slightly -- we define groups of ASCII characters that have similar brightness. Each of these groups consists of 4 characters (e.g. `@WM0` is the group of darkest characters), so a character can be used to encode 2 bits (one bit pair of the encoded symbol). The first character in the group encodes `00`, the second one `01` etc. However not all ASCII art characters can be used for encoding, for example space (` `) has no similar brightness characters, so these are just skipped.
The method used is this: firstly for the encoded message we use our own 6 bit alphabet -- this only allows us to represent 63 symbols (which we have chosen to be uppercase and lowercase letters, space and period) but will allow us to store more of them. Each 6 bit symbol of our alphabet will be encoded by three bit pairs (3 * 2 = 6). One bit pair will be encoded in one ASCII art character by altering that character slightly -- we define groups of ASCII characters that have similar brightness. Each of these groups consists of 4 characters (e.g. `@WM0` is the group of darkest characters), so a character can be used to encode 2 bits (one bit pair of the encoded symbol)[.](sub_rosa.md) The first character in the group encodes `00`, the second one `01` etc. However not all ASCII art characters can be used for encoding, for example space (` `) has no similar brightness characters, so these are just skipped.