steganography

A command-line C program that hides / reveals messages in Bitmap images (BMP) using steganography. This program supports the LSB method (least significant bit).

This program conforms to the POSIX.1 standard so it will work on any POSIX.1 compliant system.

Disclaimer: this was created for educational purposes. I do not take any responsibility for any misuse of this program.

Usage

Dependencies

This program will compile on any POSIX.1 compliant system with the following:

• gcc (gnu11)
• make

Build / Installation

Simply download / clone the repo then run:

$ cd /path/to/code/
$ make

The executable steg should be created.

To use steg:

# Encode the message below into the provided sample BMP `tree.bpm`
# This will output a file in the current directory like `fileXXXXXX`
$ ./steg -m lsb -t message -e "Hidden message" samples/tree.bmp

# Decode the message from above
$ ./steg -m lsb -t message -d `fileXXXXXX`

# Encode message using 'simple' method
# Check out the differences in output between 'lsb' and 'simple' with `hexdump`
$ ./steg -m simple -t message -e "Hidden message" samples/tree.bmp

# Decode the message from above
$ ./steg -m simple -d `fileXXXXXX`

# Encode / decode files
$ ./steg -m lsb -t file -e <SOMEFILE> samples/tree.bmp
$ ./steg -m lsb -t file -d `fileXXXXXX`

# See more usage help
$ ./steg -h

How does it work?

Bitmap images are very simple files. They contain some header information, then raw RGB pixels. The RGB pixels can be thought of as:

struct RGB {
	unsigned char blue;
	unsigned char green;
	unsigned char red;
};

As you can see, there is 1 byte for every channel. To perform steganography, we can replace one of the channels with a byte from the message.

For example, if your message was "hello", then the 'h' will replace the first RGB byte, 'e' will replace the second RGB byte, and so on.

In this program, I chose to overwrite the blue channel for no other reason than it is the first channel in Bitmap images.

For the LSB method, instead of simply overwriting the image data, each bit from the message is spread across 8 (blue) RGB bytes. This method is significantly better at disguising the message, compared to the simple method.

Note: there are 7 different types of Bitmap files. See this Wikipedia page: https://en.wikipedia.org/wiki/BMP_file_format to read more about them. This program only supports the BITMAPV5HEADER type and 24 bpp format (meaning 8 bits per channel [1 byte]) which seems to be the most popular type on the Internet. The images in samples/ are some examples of that.

TODO

• Add LSB (least significant bit) method instead of simply overwriting bytes
• Add images within images
• Add more support for different images

Contribution

Please fork and submit a pull request. If you find a bug, submit an issue. I welcome feedback as well.

License

This program is free software, distributed under the terms of the [GNU] General Public License as published by the Free Software Foundation, version 3 of the License (or any later version). For more information, see the file LICENSE.