README.md

This Project

This tool will encode any message you write into the three colour channels of any image of your choosing. It will do this by altering the remainder under modular arithmetic of that channels' colour codes, in order of red, green, blue.

Encode your message into image.png with the command python stegano.py image.png -t "this is my message." or from a file source.txt by using python stegano.py image.png -i source.txt

Decode such a message from encoded.png with the command python stegano.py encoded.png -d. If the encoded message is very long, it's recommended you pipe the result into a file with the > operator; python stegano.py encoded.png -d > target.txt.

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Encoding Options

Bit depth with -b or --bitlevel (default=1)

This option determines how many bits of each colour of an encoded pixel are used to store information rather than colour data. Default is $1$, maximum $8$. This is mostly unnoticeable until -b 3, after which the generated noise will start to be more visible, though random.

Skip pixels with -s or --skipping (default=1)

You can decide to encode the data into only every $N\text{th}$ pixel by using the flag -s N or --skipping N. This will of course cut the storage capacity of the image to $\frac{1}{N}$ of the original. Default is 1, which uses every pixel in sequence. -s 0 attempts to spread the data as evenly as possible across the image, finding the largest skipping number that can still fit all of the data in.

Offset encoding with -o or --offset (default=0)

-o K or --offset K will begin encoding the data at the $(K + 1)\text{th}$ pixel rather than the first, from the top left down. Default is 0, no maximum.

To encode textfile.txt into image.png with bitlevel=3, skipping=15, offset=200:

python stegano.py image.png -i textfile.txt -b 8 -s 15 -o 20

The program will attempt to automatically detect the message, but you can override that by using any of the flags -b, -s, -o: python stegano.py encoded.png -d -b 4 -s 3 will attempt to decode without searching for a message. Note that the automatic detection cannot find messages that use an offset, as this would increase the time-complexity of the algorithm by essentially an arbitrary factor.

Encoding multiple messages within one image

With the skipping number $N$, it is possible to encode $N$ separate messages by cycling through all offsets $0$ to $(N-1)$ (or $1$ to $N$), courtesy of modular arithmetic. The automatic decoding feature will not work for images with multiple messages and the analysis will likely find many false positives.

Example: to encode two messages, file1.txt and file2.txt, use

python stegano.py image.png -i file1.txt -s 2 -o 0
python stegano.py image.png -i file1.txt -s 2 -o 1

and decode using

python stegano.py encoded.png -d -s 2 -o 0
python stegano.py encoded.png -d -s 2 -o 1

Help

usage: Steganography Tool [-h] [-i TEXTFILE] [-t MESSAGE] [-d] [-b BITS_PER_PIXEL] [-s N] [-o K] [-a] filename

Encode and decode a message into and from the colour channels of an image.

positional arguments:
filename              Name of an image file.

options:
-h, --help            show this help message and exit
-i TEXTFILE, --input TEXTFILE
                        Contents of this file will be encoded into the image.
-t MESSAGE, --type MESSAGE
                        Type directly to encode a message into the image file.
-d, --decode          Read a message from the image file.
-b BITS_PER_PIXEL, --bitlevel BITS_PER_PIXEL
                        Store n bits per pixel. 1-8. Higher = more storage, less discreet, more colour data lost.
-s N, --skipping N    Encode to every Nth pixel. 0 to populate the image evenly.
-o K, --offset K      Start encoding at the Kth pixel, allows for multiple messages per image, assuming you use the same skipping number (>1).
-a, --analyze         Tries to automatically find an encoded message and its settings.

Method of steganography used

Each pixel in the image contains three colours, red, green, and blue. Their values range from $0$ to $255$ ($2^8$ values, $8$ bits). By adjusting each colour value to the nearest value (random direction) that corresponds with a binary representation of your text data, we're able to encode bitlevel bits of information in each colour channel of each pixel under $\text{mod } 2^{\text{bitlevel}}$ modular arithmetic.

For example: at bitlevel = $3$, we're working with $2^3 = \text{mod } 8$ arithmetic. If our character is an 'a', that is, $97$ or $1100001$, the first $3$ bits are $110$, or $6$, we adjust the first colour channel value to the nearest value whose remainder under division by $8$ is $6$, thereby encoding the bits. This changes the least significant $3$ bits to store the information and leaves $5$ bits of colour data. This naturally introduces some noise, but is completely invisible at low bitlevels (up to about $\text{3--4}$). At $8$ bits, the entire underlying image is lost, as $0$ bits of colour information are retained of each channel.

• N.B. All printable characters can be represented with 7 bits, and therefore the only functional difference between -b 8 and -b 7 is that the former destroys more of the image.

Initialisation sequence

The first $8$ pixels of any encoding are used for a sequence of alternating bits to identify the beginning of a message. The second $8$ pixels are used to encode length information about the message, so the program knows where to stop. If you encode an image with bitlevel $8$, you'll see this as $8$ gray/white pixels in the top left, followed by $8$ mostly black and then a coloured pixel or two, as below.

Space considerations

Calculating required image size

Given $N$ bytes of text data, the number of pixels this requires is exactly $$\frac{8N}{3\cdot\text{bitlevel}} + 16$$ The $+ 16$ comes from the message initialisation sequence. For a square image, then, one needs a $$\sqrt{\frac{8N}{3\cdot\text{bitlevel}} + 16}$$ wide and tall image, dimensions rounded up. For, say, $15\text{kB}$ at $\text{bitlevel}=3$, this means a $116 \times 116$ picture.

Calculating image storage capacity

Given a $W \times H$ image, the data storage capacity without the initialisation sequences is $\frac{3WH\cdot\text{bitlevel}}{\text{skipping}}$ bits. Subtracting from it the initial sequence, $3\cdot16\cdot\text{bitlevel}\cdot\text{skipping}$ bits, we're left with

$$\frac{3\cdot\text{bitlevel}}{8}\left\lbrack\frac{W\cdot H}{S} - 16\cdot S - \text{offset}\right\rbrack$$

bytes of storage. This way, for instance, the absolute maximum storage capacity of a 400 x 400 image is about 60kB, destroying all the colour information.

Note that compressing the image after encoding will likely destroy the encoded information as it relies on precise numerical values.

Included examples

I've encoded something into the ./resources/example_encoded.png that you can test the program on. Simply run python stegano.py example_encoded.png -d > result.txt to see what it is.