Information Hiding
and its Applications
Steganography and Watermarking
A detailed look at Steganography
   ◦ Text Steganography
   ◦ Hypertext Steganography
   ◦ Audio Steganography
   ◦ Image Steganography
   ◦ Steganography in Open System
Image Steganography Techniques
   ◦ Spatial Domain LSB Insertion
   ◦ Masking and Filtering
   ◦ DCT-based Steganography
   ◦ Wavelet-based Steganography
How to Detect Steganography
   ◦ Blind Detection
   ◦ Analytical Detection

Image Steganography Techniques

Spatial domain LSB insertion

Usually 24-bit or 8-bit files are used to store digital images. The former one provides more space for information hiding; however, it can be quite large. The colored representations of the pixels are derived from three primary colors: red, green and blue. 24-bit images use 3 bytes for each pixel, where each primary color is represented by 1 byte. Using 24-bit images each pixel can represent 16,777,216 color values. We can use the lower two bits of these color channels to hide data, then the maximum color change in a pixel could be of 64-color values, but this causes so little change that is undetectable for the human vision system. This simple method is known as Least Significant Bit insertion. Using this method, it is possible to embed a significant amount of information with no visible degradation of the cover image. The following Figure shows the insertion process.

Several versions of LSB insertion exist. It is possible to use a random number generator initialized with a stego-key and its output is combined with the input data, and this is embedded to a cover image. For example in the presence of an active warden it is not enough to embed a message in a known place (or in a known sequence of bits) because the warden is able to modify these bits, even if he can’t decide whether there is a secret message or not, or he can’t read it because it is encrypted. The usage of a stego-key is important, because the security of a protection system should not be based on the secrecy of the algorithm itself, instead of the choice of a secret key, the figure below shows the process.

The LSB inserting usually operates on bitmap images. The embedded data cannot be considered as a watermark, because even if a small change occurs in a picture (cropping, lossy compression, and color degradation) the embedded information will be lost although the change which is occurred during the embedding process is invisible. Finally, the LSB insertion method is a very popular method because of its simplicity. But the main drawback of this technique is that its weakness against some attacks such as lossy compression, where the hidden information will be lost.

Masking and filtering Steganography

Masking and filtering techniques are mostly used on 24 bit and grey scale images. Masking images entails changing the luminance of the masked area. The smaller the luminance change, the less of a chance that it can be detected. In other words Masking and filtering techniques hide information by marking an image in a manner similar to paper watermarks.

Because watermarking techniques are more integrated into the image, they may be applied without fear of image destruction from lossy compression. By covering, or masking a faint but perceptible signal with another to make the first non-perceptible, we exploit the fact that the human visual system cannot detect slight changes in certain temporal domains of the image.

Here it is good to remember the difference between watermarking and steganography; Digital watermarks may include information as copyright, ownership, or license. In digital watermarking, the object of communication is the cover. But in steganography, the object of communication is the hidden secret message.

To perform steganography within an image, the luminance of the masked area is increased by 15 percent. The luminance must be changed by a smaller percentage, so the mask would be undetected by the human eye. Now we can use the image to hide plaintext or encoded information. Masking is more robust than LSB insertion with respect to compression, cropping, and some image processing. Masking techniques embed information in significant areas so that the hidden message is more integral to the cover image than just hiding it in the “noise” level. This makes it more suitable than LSB with lossy JPEG images.
 

Reference:
• STEGANOGRAPHIC METHODS, by József LENTI , June 2000.