Image Processing:

An image file is merely a binary file containing a binary representation of the color or light intensity of each picture element (pixel) comprising the image.The simplest approach to hiding data within an image file is called Least Significant Bit (LSB) insertion.Least significant bit (LSB) insertion is a common, simple approach to embedding information in a cover image. The least significant bit (in other words, the 8th bit) of some or all of the bytes inside an image is changed to a bit of the secret message. When using a 24-bit image, a bit of each of the red, green and blue colour components can be used, since they are each represented by a byte. In other words, one can store 3 bits in each pixel. An 800 × 600 pixel image, can thus store a total amount of 1,440,000 bits or 180,000 bytes of embedded data.In this method, we can take the binary representation of the hidden data and overwritethe LSB of each byte within the cover image. If we are using 24-bit color, the amount of changewill be minimal and indiscernible to the human eye. As an example, suppose that we have threeadjacent pixels (nine bytes) with the following RGB encoding:

10010101 00001101 1100100110010110 00001111 1100101010011111 00010000 11001011

Now suppose we want to "hide" the following 9 bits of data (the hidden data is usually compressed prior to being hidden): 101101101. If we overlay these 9 bits over the LSB of the 9 bytes above, we get the following (where bits in bold have been changed):

10010101 00001100 1100100110010111 00001110 1100101110011111 00010000 11001011

Note that we have hidden 9 bits successfully, but at a cost of only changing 4, or roughly 50%, of the LSBs according to the embedded message. This description is meant only as a high-level overview. Since there are 256 possible intensities of each primary colour, changing the LSB of a pixel results in small changes in the intensity of the colours. These changes cannot be perceived by the human eye - thus the message is successfully hidden. With a well-chosen image, one can even hide the message in the least as well as second to least significant bit and still not see the difference. In the above example, consecutive bytes of the image data – from the first byte to the end of the message – are used to embed the information. This approach is very easy to detect. A slightly more secure system is for the sender and receiver to share a secret key that specifies only certain pixels to be changed. Should an adversary suspect that LSB steganography has been used, he has no way of knowing which pixels to target without the secret key. In its simplest form, LSB makes use of BMP images, since they use lossless compression. Unfortunately

to be able to hide a secret message inside a BMP file, one would require a very large cover image. Nowadays, BMP images of 800 × 600 pixels are not often used on the Internet and might arouse suspicion. For this reason, LSB steganography has also been developed for use with other image file formats.

Similar methods can be applied to 8-bit color but the changes, as the reader might imagine, are more dramatic. Gray-scale images, too, are very useful for steganographic purposes. One potential problem with any of these methods is that an adversary who is looking can find them. In addition, there are other methods besides LSB insertion with which to insert hidden information. 

STEGANOGRAPHY ATTACKS:

There are several kind of attacks can be made by steganalyst on data. Some of them are mentioned below: Stego-only attack: in this attack only the stego-object is available for stegoanalysis Known cover attack: in this kind of attack the original cover-object and stego-object are both available for steganalyst.  Known message attack: Sometimes the hidden message may become known to the stegoanalyser. Analyzing the stego-object for patterns that correspond to the hidden message may be beneficial for future attacks against that system. (Even with the message, this may be very difficult and may even be considered equivalent to the stego-analysis.)  Chosen stego attack: in this kind of attack the stegoanalysis generates a stego-object from some steganography tool or algorithm from a chosen message. The goal in this attack is to determine corresponding patterns in the stego-object that may point to the use of specific steganography tools or algorithms.  Known stego attack: in this attack the steganography algorithm is known and both the original and stego-objects are available.