Neil F. Johnson. Steganography. Technical Report. November 1995.

3. PC Software that Provide Steganographic Services

3.1 Background

Steganographic software is new and very effective. Such software enables information to be hidden in graphic, sound and apparently "blank" media. Charles Kurak and John McHugh discuss the implications of downgrading an image (security downgrading) when it may contain some other information [Kurak92]. Though not explicitly stated the author(s) of StegoDos mention embedding viruses in images [StegoDos].

In the computer, an image is an array of numbers that represent light intensities at various points (pixels1) in the image. A common image size is 640 by 480 and 256 colors (or 8 bits per pixel). Such an image could contain about 300 kilobits of data.

There are usually two type of files used when embedding data into an image. The innocent looking image which will hold the hidden information is a "container." A "message" is the information to be hidden. A message may be plain-text, ciphertext, other images or any thing that can be embedded in the least significant bits (LSB) of an image.

For example:

Suppose we have a 24-bit image 1024 x 768 (this is a common resolution for satellite images, electronic astral photographs and other high resolution graphics). This may produce a file over 2 megabytes in size (1024x768x24/8 = 2,359,296 bytes). All color variations are derived from three primary colors, Red, Green and Blue. Each primary color is represented by 1 byte (8 bits). 24-bit images use 3 bytes per pixel. If information is stored in the least significant bit (LSB) of each byte, 3 bits can be a stored in each pixel. The "container" image will look identical to the human eye, even if viewing the picture side by side with the original. Unfortunately, 24-bit images are uncommon (with exception of the formats mentioned earlier) and quite large. They would draw attention to themselves when being transmitted across a network. Compression would be beneficial if not necessary to transmit such a file. But file compression may interfere with the storage of information.

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1 A pixel is an instance of color, a point in a picture. (go back)

Next Section: 3.2 Evaluation Method