Steganography

Steganography is the practice of writing hidden messages in such a way that no one apart from the intended recipient even knows that a message has been sent. Generally a steganographic message will appear to be something else, like a shopping list, an article, a picture, or some other "cover" message.

Image of a tree.
By removing all but the last 2 bits of each colour component, an almost completely black image results. Making the resulting image 85 times brighter results in the following image:
Above image extracted from original image.
With a good drawing program, you can do that too. Load the first image, apply the logical and operation with the number 3 to the image, and make the image 85 times brighter, and you get the second image.

Steganographic messages are typically first encrypted by some traditional means, and then a covertext is modified in some way to contain the encrypted message, resulting in stegotext. For example, the letter size, spacing, typeface, or other characteristics of a covertext can be manipulated to carry the hidden message; only the recipient (who must know the technique used) can recover the message and then decrypt it. Francis Bacon is known to have suggested such a technique to hide messages.

The larger the cover message is (in data content terms -- nr of bits) relative to the hidden message, the easier it is to hide the latter. For this reason, digital pictures (which contain large amounts of data) are used to hide messages on the Internet and on other communication media. It is not clear how commonly this is actually done. For example: a 24 bit bitmap will have 8 bits representing each of the three colour values (red, green, and blue) at each pixel. If we consider just the blue there will be 2⁸ different values of blue. The difference between say 11111111 and 11111110 in the value for blue intensity is likely to be undetectable by the human eye. Therefore, the least significant bit can be used (more or less undetectably) for something else other than colour information. If we do it with the green and the red as well we can get one letter of ASCII text per 3 pixels.

Stated somewhat more formally, the objective for making steganographic encoding difficult to detect is to ensure that the changes to the carrier (the original signal) due to the injection of the payload (the signal to covertly embed) appear visually in the example (and ideally, statistically as well) negligible; that is to say, the changes are indistinguishable from the noise floor of the carrier.

(From an information theoretical point of view, this means that the channel must have more capacity than the 'surface' signal requires , i.e., there is redundancy. For a digital image, this may be noise from the imaging element; for digital audio, it may be noise from recording techniques or amplification equipment. Any system with an analog amplification stage will also introduce so-called thermal or "1/f" noise, which can be exploited as a noise cover. In addition, lossy compression schemes (such as JPEG) always introduce some error into the decompressed data; it is possible to exploit this for steganographic use as well.)

Steganography can be used for digital watermarking, where a message (being simply an identifier) is hidden in an image so that its source can be tracked or verified. In fact, in Japan "... the Content ID Forum and the Digital Content Association of Japan started tests with a system of digital watermarks 'to prevent piracy' (The Japan Times Online 26-08-2001)." [1]

Steganography techniques:

• Chaffing and winnowing
• Invisible ink
• Microdots
• Null ciphers

• Hidden messages in wax tablets, : in ancient Greece, people wrote messages on the wood, then covered it with wax so that it looked like an ordinary, unused, tablet.
• Hidden messages on messenger's body : also in ancient Greece. Herodotus tells the story of a message tatooed on a slave's shaved head, covered by hair regrowth, and exposed by reshaving. The message, if the story is true, carried a warning to Greece about Persian invasion plans.
• Hidden messages on paper written in secret inks under other messages or on the blank parts of other messages.

See also

• Covert channel
• DeCSS
• Security engineering
• canary traps

• http://www.spammimic.com will take a sentence that you provide and turn it into text that looks to all the world like spam.
• Steganography Articles, Links, and Whitepapers