Steganography

Taken from Dr. Mark Boady's lesson on Spycraft,

Steganography is a type of spycraft. It is a method of hiding data inside of images [James Stanger, 2020]. This practice has been used for centuries. People want to communicate secretly with others, but need to do it in a public place. They hide the message they want to send inside an image. The image can be posted in a public place. Anyone who sees it will just enjoy it as an image. Only those people that know how the message is encoded can reveal the secret payload within.

Obviously if you really wanted to keep the message secret you would encrypt it, and you would use a unique image rather than one pulled off of wikipedia so there'd be nothing to compare it to!

How it works

The image is parsed to seperate the various parts of an image. As described below, bmp is used to keep things simple (no compression for example), but can be modified to accommodate other formats. Bitmap has a standard format, so the script parses out the vairous components (header, pixel array, etc.). The pixels are turned into a list of list of tuples, representing the RGBA color for each pixel where:

• R is the red value
• G is the green value
• B is the blue value
• A is the alpha transparency level

So, we'd end up with pixels like below:

[
  [
    {117, 152, 54, 255},
    {117, 150, 56, 255},
    ...
  ],
  [
    {116, 148, 58, 255},
    {114, 147, 53, 255},
    ...
  ],
  ...
]

Without the alpha channel, we're left with:

[
  [
    {117, 152, 54},
    {117, 150, 56},
    ...
  ],
  [
    {116, 148, 58},
    {114, 147, 53},
    ...
  ],
  ...
]

Because of the fact that for most humans looks like , we can change just the least significant bit by can an encoding either a 0 or 1. Also note that in that example, all three pixels are changed but very often we may only change 1 pixel, or none in the case where they all happen to end in the intended 0 or 1 already.

Since we can encode 0's and 1's, we can encode ASCII. Per Boady:

If we want to represent the letter “a” it has character code 97. We can then represent 97 as a binary value. In binary 97 is 0110 0001. Each character takes exactly 8 bits. We can hide our bits in side a pixel. We store 1 bit in each color code. We place our bit in the last digit of the color code. That means we can fit 3 bits in each pixel. We need 3 pixels to fit a whole letter, with one extra bit.

Example

hamlet_encoded.bmp has the entire text of Hamlet encoded in it (from Project Gutenberg). Can you tell any difference between this and the original.jpg?

"We save the output image as a BMP or TIFF file. These image formats have no compression, so we can be sure every pixel will stay exactly how we want it. This method can be modified for compressed images like JPG, but this requires making changes the compression won’t effect. A BMP/TIFF file is easier to work with but much larger."- Boady

Original (taken from wikipedia here)

Hamlet Encoded

Challenge

Write a decoder to parse the text out from the hamlet_encoded.bmp image and write it to a file (decoded.txt).

Dr. Boady wrote an encoder and decoder in Python, can you write an encoder as well as the decoder?

Interesting note on representing information

On the topic of how meaning can be encoded, from Dr. Boady:

"The Library of Babel takes this concept and generates all possible books with a certain number of characters."