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Stegnography is defined as the art of hiding information, data or messages in an image. Even the different file formats can be used for the purpose of hiding the information like for example the video or audio etc. The purpose is to pass on the information with out any regard or knowledge of others safely to the destination. The advantage of stegnography is that those who are outside the party even do not realize that some sort of communication is being done.

This is an alternative method used instead of encryption. In case of the images the bits are used and the contrast size of the image matters a lot. If the computer criminal has used some sort of stegnography then it is required for the computer forensics professional to decode that information to acquire valid evidence that can be produced in the court of law. The advantage of unused bits within the structure of a file or those bits those are mostly not detectable if altered are used. If not identified as a steganographic message; there is no chance of any leakage of information.

The message rides safely and secretly to its destination. The message can be decrypted only if it is identified. So it has an advantage over the encryption process. In case of the encrypted text; however complicated may be the cipher applied but at least the encrypted message is detectable as an encrypted message.

In general a stegnographic message can appear in many different formats. Only the person or the destination will be able to understand the hidden meaning. It appears as something else like for example an article, shopping list, a picture, or some other cover message.

If the message is much larger then data content terms are also even more. The content terms here relate to the number of bits used. They are relative to the hidden messages. More number of bits will result in much ease to hide. Hence the digital pictures and images are used for the purpose of stegnography. Usually the images with large amount of data are used for the purpose.

They can be easily stored in the computer or any other storage media. A computer forensics even if encounters the data is also not even able to realize the evidence. These stegnographic images can be easily used over the internet to pass the messages for communication. The principle of stegnography is basically from olden days.

Let us consider an example that can be used to describe how the task of stegnography is actually achieved. For example a 24 bit bitmap image will have 8 bits representing the color values. Each of the three color values that is the red, green, and blue have a relative value at each pixel.

The difference between say 1111111111 and 1111111110 in the value for blue intensity is unlikely to be detected by a human eye. Hence, it is possible to represent the least significant bit for something else rather than color information bit. It is possible to get one letter in an ASCII character set if we do it with the other colors such as green and the red as well. So for each of the three pixels it is possible to have one letter of ASCII text. Winnowing and Chaffing are the recent techniques in stegnography. Also Null ciphers can be used.

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