How Stego Online Works
The RGB Color Cube
The RGB Color Model
In the RGB color model used by GIF images, colors are described by a set of three values that range from 0 to 255. The first value is for red, the second is for green, and the third is for blue. The higher the number, the stronger the color.
Some Primary RGB Colors
| R | G | B | |
|---|---|---|---|
| red | 255 | 0 | 0 |
| green | 0 | 255 | 0 |
| blue | 0 | 0 | 255 |
| yellow | 255 | 255 | 0 |
| white | 255 | 255 | 255 |
| black | 0 | 0 | 0 |
In the RGB color model there are a total of 256 x 256 x 256 = 16,777,216 possible colors.
A Sample Image
The same image up close
Painting by Numbers
A palette is a list of the RGB colors used in an image. Most GIF files on the web today use an 8-bit palette. This means that of all the 16,777,216 possible colors, there are only 256 RGB colors in the image, because 8-bit binary number can only take on 256 distinct values.
The image can be thought of as a grid with an index into the palette in each grid cell.The index is an 8 bit binary number, a string of 8 bits that can range from 0 to 255.The grid cells are called pixels
Some Examples of 8 bit binary numbers
| Binary Number | Decimal equivalent |
|---|---|
| 00000000 | 0 |
| 00001101 | 13 |
| 01010100 | 84 |
| 11111111 | 255 |
The bit that is furthest to the right in a binary number is called the least significant bit because changing it has the least effect on the number's value.
First, Sort The Palette
EzStego copies the palette from the image. It rearranges the copy of the palette so that colors that are near to each other in the color model are near to each other in in the palette.This is what the first nine colors in the palette for the sample GIF image look like before and after sorting.
EXAMPLE
17 231 31 is color 00100101 in the sorted palette.
The index value of 00100101 is changed to 00100100.
Color 00100100 in the sorted palette is 179 233 36.
179 233 36 is color 11101110 in the original palette.
The pixel value is changed to 11101110.
One bit at a time
EzStego puts the binary data of the input file into the least significant bit of pixels in the image. Here are its instructions:
Find the index of the pixel's RGB color in the sorted palette.
Get one bit from the input file. Replace the least significant bit of the index.
Find the new RGB color that the index now points to in the sorted palette.
Find the index of the new RGB color in the original palette.
Change the pixel to the index of the new RGB color.
EXAMPLE
179 233 36 is color 00100100 in the sorted palette.
The least significant bit is 0.
Write 0 to the output file.
Recovering the input file
Find the index of the pixel's RGB color in the sorted palette.The least significant bit of the index came from the input file. Write it to the ouput file.
Stego and EzStego are (C) Romana Machado 1994,1995,1996,1997. ALL RIGHTS RESERVED.
RGB Cube appears by courtesy of Conrad Hughes.