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Steganography III - Change only one Bit per Pixel

, 3 Apr 2004
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An article about hiding each byte of a message bit-by-bit in eight pixels.

Sample Image - steganodotnet3.png

Introduction

The application described in the last article Steganography II always added visible noise to the carrier bitmaps. This article is about splitting the bytes and hiding each bit in a different pixel. The hidden message is really invisible, because only the lowest bit of one color component is changed.

Background

Before reading this, you should have read at least part one, Steganography - Hiding messages in the Noise of a Picture. This article uses the application described in part one and two, but you don't need the features added in part two to understand this one.

End of the rainbow

Replacing a whole byte of a RGB color produces a rainbow pattern. This is how a white image (all pixels just white) looks like after hiding a text message:

typical colour noise

On a colorful photo, these rainbow pixels may be alright. In grayscale mode, the same message in the same picture looks like this:

typical grayscale noise

As you can see, too many bits of the pixels have been changed. Now, we’re going to spread each byte over eight pixels and make the rainbow disappear.

Get and set bits

spreading a byte over eight pixels

For each byte of the message, we have to:

  1. Grab a pixel.
  2. Get the first bit of the message byte.
  3. Get one color component of the pixel.
  4. Get the first bit from the color component.
  5. If the color-bit is different from the message-bit, set/reset it.
  6. Do the same for the other seven bits.

The C#-functions for getting and setting single bits are simple:

private static bool GetBit(byte b, byte position){
	return ((b & (byte)(1 << position)) != 0);
}

private static byte SetBit(byte b, byte position, bool newBitValue){
	byte mask = (byte)(1 << position);
	if(newBitValue){
		return (byte)(b | mask);
	}else{
		return (byte)(b & ~mask);
	}
}

The rest of the code has not changed much. Hiding works like that now...

hide a message

...and extraction works like that:

extract a message

That's enough text for today. If you want to know the details, you should download the source.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Corinna John
Software Developer
Germany Germany
Corinna lives in Hannover/Germany (CeBIT City) and works as a Delphi developer, though her favorite language is C#.

Comments and Discussions

 
GeneralProblem with PNG Image Pinmemberlonelywind198221-Jun-04 22:09 
GeneralRe: Problem with PNG Image PinmemberCorinna John22-Jun-04 20:31 
GeneralSetting pixel in Bitmap Object PinmemberAlexsander Antunes4-Mar-04 6:23 
GeneralRe: Setting pixel in Bitmap Object PinmemberCorinna John4-Mar-04 20:16 
GeneralRe: Setting pixel in Bitmap Object Pinmemberchaitumad24-May-09 19:50 
GeneralGetBit/SetBit Pinmemberjmueller4-Nov-03 21:24 
GeneralRe: GetBit/SetBit PinmemberCorinna John5-Nov-03 9:20 
GeneralRe: GetBit/SetBit Pinmemberjmueller5-Nov-03 9:30 
I hope I didn't come across as complaining - I think this is an interesting series of articles. It just seemed like GetBit/SetBit could be called thousands or even millions of times per image, so they seemed to be good candidates for optimization. To take an extreme example (calling SetBit on 50 million bytes) the difference was 13 seconds vs. 900 milliseconds on my machine.
GeneralRe: GetBit/SetBit PinmemberCorinna John5-Nov-03 9:35 
GeneralRe: GetBit/SetBit PinmemberCorinna John4-Apr-04 21:56 
Generalavi file Pinmemberunitecsoft6-Oct-03 8:25 
GeneralRe: avi file PinmemberCorinna John6-Oct-03 19:38 
GeneralRe: avi file PinmemberCorinna John25-Oct-03 5:52 

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