The next code of interest in the AddMessage class is the StoreBits method. This method stores a string into the bitmap data array. It takes two arguments: the string containing the data and the number of bits that can be stored in each byte.

// This method stores an entire string
//   into the bitmap data array. It takes
//   two arguments: the string containing
//   the data and the number of bits that
//   can be stored in each byte. Once again,
//   note that the number of bits can actually
//   be different than the number of bits
//   that were stripped off.
void StoreBits( string strData, int nBits)
{
     // Loop through the string.
     for (int i = 0; i < strData.Length; i++)
     {
          // Get the data byte for this character.
          byte Data = (byte)strData[i];
          // Start our bit test variable at 1;
          byte DataTest = 1;
          // Loop through the 8 bits of the data byte.
          for (int j = 0; j < 8; j++)
          {
               // Start by assuming the bit is 0.
               bool Bit = false;
               if ((Data & DataTest) != 0)
               {
                    // If the bit is a 1, set the
                    //   boolean variable to true.
                    Bit = true;
               }
               // Call the StoreBit method
               //   which will do the actual
               //   storage into the bitmap
               //   data array.
               StoreBit(Bit, nBits);
               // Adjust our bit test variable.
               DataTest <<= 1;
          }
     }
}

The last method that is used by the AddMessage class to perform the message hiding is the btnSave_Click method which simply calls the InitBitStoring method, the StoreBits method, and then saves the bitmap data to disk as a BMP file. The btnSave_Click method can be seen below.

// This method stores the message data into the
//   bitmap array (by calling the appriate methods)
//   and then saves the bitmap data to a BMP file.
private void btnSave_Click(object sender, EventArgs e)
{
	// We need to format the five digit string
	//   that contains the length of the messagwe
	//   string.
	string strSize = String.Format("{0:00000}",
           txtMessage.Text.Length);
	// Call the method to initialize
	//   the bit saving variables.
	InitBitStoring();
	// Store the number of bits. Note that
	//   this is saved as a single bit so
	//   that when the file is read back,
	//   it's always assumed that the initial
	//   number of bits is in a single bit.
	StoreBits(Convert.ToString(m_nBits), 1);
	// Store the size string.
	StoreBits(strSize, m_nBits);
	// Store the message data.
	StoreBits(txtMessage.Text, m_nBits);
	try
	{
		// Create a bitmap object from
		//   the bitmap data.
		Bitmap objBitmap = 
			BitmapHelper.BitmapFromBitmapData(m_BitmapData);
		// Save the bitmap to disk.
		objBitmap.Save(txtFilename.Text, ImageFormat.Bmp);
		// Show the user that the process is complete.
		MessageBox.Show("Done!");
	}
	catch (Exception ex)
	{
		// Alert user to the error.
		MessageBox.Show(ex.Message.ToString());
	}
}

To read a bitmap from a BMP file and extract a message, the program simply reverses the process. This code can be seen in the ReadMessage class. The Figure below shows this Windows Form in use.

Conclusion

Steganography is a really intriguing subject. It's not that hard to get started, and I encourage you to try out the demo program.