This paper introduces steganography, the art and science of hidden writing. The purpose of steganography is to hide the existence of a secret message from a third party. The most popular modern application of steganography involves concealing messages within least significant bits of image or sound files. Another application of modern steganography is a steganographic file system. Steganography is also used by some modern printers, where tiny yellow dots that contain encoded printer serial numbers, as well as other information, are added to each printed page. I chose modern steganography as a topic for my course paper primarily because I have had an interest in the topic before, and would like to use this paper as an opportunity to expand my knowledge of steganographic techniques, especially in their modern applications.
Steganography is the art and science of hidden writing. The purpose of steganography is to hide the existence of a secret communication from a third party. Usually, a steganographic message will appear to be something else: a picture, an audio file, or some other message (the covertext). The advantage of steganography over cryptography alone is that steganographic message does not attract attention to communicating parties. An apparent encrypted message, no matter how unbreakable it is, can arouse suspicion and may in itself be incriminating. If suspicion is raised, then the goal of steganography is defeated. Since there is always a possibility that secret communication will be uncovered, steganography is often used together with cryptography. A steganographic message (the plaintext) is first encrypted by some modern encryption algorithms, and then a covertext is modified in some way to contain the encrypted message (the ciphertext), resulting in stegotext. Digital Steganography
A typical digital steganographic encoder is presented on Figure 1. The message is the data that the sender wishes to remain secret. It can be text, image, audio, video, or any other data that can be represented by a stream of bits. The cover media is the carrier medium in which the message is embedded and serves to hide the presence of the message. The message embedding technique is strongly dependent on the structure of the cover media, and in this discussion it is restricted to being digital image. The image with the secretly embedded message produced by the encoder is the stegoimage. The purpose of the stegoimage is to resemble the coverimage under casual inspection and analysis. In addition, the encoder usually employs a stegokey which ensures that only recipients who know the corresponding decoding key will be able to extract the message from a stegoimage.
Figure 1. Steganographic Encoding
Recovering the message from a stegoimage requires the stegoimage itself and a corresponding decoding key if a stegokey was used during the encoding process. The original coverimage may or may not be required. In most applications it is desirable that the cover image is not needed to extract the message. A typical digital steganographic decoder is shown on Figure 2.
Figure 2. Steganographic Decoding
Most modern computer graphics standards use a combination of tree component colors - red, green, and blue to represent a picture element (pixel). The RGB color cube is shown on Figure 3. The absence of all colors yields black, shown as the intersection of the zero point of the three-color axes. The mixture of 100% red, 100% blue, and the complete absence of green forms magenta; cyan is 100% green and 100% blue without any red; 100% green and 100% red with no blue form yellow. White is 100% mixture of all three colors.
Figure 3. The RGB Color Cube
Most digital image encoding standards today support 24-bit true color, where each pixel is encoded in 24 bits, comprising the three RGB bytes as described above. The most common steganography method in digital image files employs some type of...
References:  Michael E. Whitman, Herbert J. Mattord. "Principles of Information Security". Course Technology; 2 edition. November 2004.
 Wikipedia. February 2007. "Steganography". http://en.wikipedia.org/wiki/Steganography
 Wikipedia. February 2007. "Printer Steganography". http://en.wikipedia.org/wiki/Printer_steganography
 Wikipedia. February 2007. "Steganographic file system". http://en.wikipedia.org/wiki/Steganographic_file_system
 Gary C. Kessler. July 2004. "An Overview of Steganography for the Computer Forensics". http://www.fbi.gov/hq/lab/fsc/backissu/july2004/research/2004_03_research01.htm.
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