Steganography - Hiding messages in the Noise of a Picture

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Introduction

This article describes how any data can be hidden in the noise pixels of an image.

Background

There is only one safe place for private data and messages: the place where nobody looks for it. A file encrypted with algorithms like PGP are not readable, but everybody knows that there is something hidden. Wouldn't it be nice, if everyone could open your encrypted files, and see noisy photos of some old friends instead of your private data? They surely wouldn't look for pieces of encrypted messages in the pixel values.

Using the code

To hide a message, open a bitmap file, then enter a password or select a key file. The key file can be any file, another bitmap for example. This password or key will be treated as a stream of bytes specifying the space between two changed pixels. I don't recommend text files, because they may result in a quite regular noise pattern. The longer your key file or password is, the less regular the noise will appear.

Next step, enter the secret message or choose a file, and click the Hide button. The application writes the length of the message in bytes into the first pixel. After that it reads a byte from the message, reads another byte from the key, and calculates the coordinates of the pixel to use for the message-byte. It increments or resets the color component index, to switch between the R, G and B component. Then it replaces the R, G or B component of the pixel (according to the color component index) with the message-byte, and repeats the procedure with the next byte of the message. At last, the new bitmap is displayed. Save the bitmap by clicking the Save button. If the grayscale flag is set, all components of the color are changed. Grayscale noise is less visible in most images.

To extract a hidden message from a bitmap, open the bitmap file and specify the password or key you used when hiding the message. Then choose a file to store the extracted message in (or leave the field blank, if you only want to view hidden Unicode text), and click the Extract button. The application steps through the pattern specified by the key and extracts the bytes from the pixels. At last, it stores the extracted stream in the file and tries to display the message. Don't bother about the character chaos, if your message is not a Unicode text. The data in the file will be all right. This works with every kind of data, you can even hide a small bitmap inside a larger bitmap. If you are really paranoid, you can encrypt your files with PGP or GnuPG before hiding them in bitmaps.

How it works

To see how the application works, you should view the source. I've commented every block, because otherwise I'd have to post nearly all the code here to explain it.

Here is a summary about hiding:

And about extracting:

The process starts with writing the length of the entire message into the first pixel. We'll need this value before extracting the message later on.

messageLength = (Int32)messageStream.Length;


//do some length conflict checking here

//...

String colorValue = messageLength.ToString("x");
colorValue = UnTrimColorString(colorValue, 6);
int red = Int16.Parse(colorValue.Substring(0,2), NumberStyles.HexNumber);
int green = Int16.Parse(colorValue.Substring(2,2), NumberStyles.HexNumber);
int blue = Int16.Parse(colorValue.Substring(4,2), NumberStyles.HexNumber);
pixelColor = Color.FromArgb(red, green, blue);
bitmap.SetPixel(0,0, pixelColor);

Then it reads a byte from the key stream to calculate the next position:

//start with second pixel

Point pixelPosition = new Point(1,0);
//Loop over the message

for(int messageIndex=0; messageIndex<messageLength; messageIndex++){
 //repeat the key, if it is shorter than the message

 if(keyStream.Position == keyStream.Length){
  keyStream.Seek(0, SeekOrigin.Begin);
 }

 //Get the next pixel-count from the key, use "1" if it's 0

 currentKeyByte = (byte)keyStream.ReadByte();
 currentStepWidth = (currentKeyByte==0) ? (byte)1 : currentKeyByte;

 //jump to reverse-read position and read from the end of the stream

 keyPosition = keyStream.Position;
 keyStream.Seek(keyPosition, SeekOrigin.End);
 currentReverseKeyByte = (byte)keyStream.ReadByte();
 //jump back to normal read position

 keyStream.Seek(keyPosition, SeekOrigin.Begin);

 //Perform line breaks, if current step is wider than the image

 while(currentStepWidth > bitmapWidth){
  currentStepWidth -= bitmapWidth;
  pixelPosition.Y++;
 }

 //Move X-position

 if((bitmapWidth - pixelPosition.X) < currentStepWidth){
  pixelPosition.X = currentStepWidth - (bitmapWidth - pixelPosition.X); 
  pixelPosition.Y++;
 }else{

  pixelPosition.X += currentStepWidth;
 }

Now, get the pixel and put the message-byte into one color component (or all components, if the grayscale flag is set):

 //Get color of the "clean" pixel

 pixelColor = bitmap.GetPixel(pixelPosition.X, pixelPosition.Y);

 //To add a bit of confusion, xor the byte with 

 //a byte read from the keyStream

 int currentByte = messageStream.ReadByte() ^ currentReverseKeyByte;

 if(useGrayscale){
  pixelColor = Color.FromArgb(currentByte, currentByte, currentByte);
 }else{
  //Change one component of the color to the message-byte

  SetColorComponent(ref pixelColor, currentColorComponent, currentByte);
  //Rotate color components

  currentColorComponent = 
   (currentColorComponent==2) ? 0 : (currentColorComponent+1);
 }
} //end of for - proceed to next byte

If the method runs in extraction mode, it reads the message's length and the color components instead of setting it. Here is how to get the length from the first pixel:

pixelColor = bitmap.GetPixel(0,0);
//UnTrimColorString fill the String with '0'-chars 

//to match the specified length

String colorString = UnTrimColorString(pixelColor.R.ToString("x"), 2)
 + UnTrimColorString(pixelColor.G.ToString("x"), 2)
 + UnTrimColorString(pixelColor.B.ToString("x"), 2);

 messageLength = Int32.Parse(colorString, NumberStyles.HexNumber);
 messageStream = new MemoryStream(messageLength);

The pixel coordinates are calculated the same way as described above. Then the hidden byte is extracted from the color value:

 //Get color of the modified pixel

 pixelColor = bitmap.GetPixel(pixelPosition.X, pixelPosition.Y);
 //Extract the hidden message-byte from the color

 byte foundByte = (byte)(currentReverseKeyByte ^ 
   GetColorComponent(pixelColor, currentColorComponent));
 messageStream.WriteByte(foundByte);
 //Rotate color components

 currentColorComponent = 
   (currentColorComponent==2) ? 0 : (currentColorComponent+1);
} //end of for - proceed to next byte