Steganography is the art and science of writing hidden messages in such a way that no one, apart from the sender and intended recipient, suspects the existence of the message, a form of security through obscurity. The word steganography is of Greek origin and means "concealed writing" from the Greek words steganos (στεγανός) meaning "covered or protected", and graphei (γραφή) meaning "writing". The first recorded use of the term was in 1499 by Johannes Trithemius in his Steganographia, a treatise on cryptography and steganography disguised as a book on magic. Generally, messages will appear to be something else: images, articles, shopping lists, or some other covertext and, classically, the hidden message may be in invisible ink between the visible lines of a private letter.

The advantage of steganography, over cryptography alone, is that messages do not attract attention to themselves. Plainly visible encrypted messages—no matter how unbreakable—will arouse suspicion, and may in themselves be incriminating in countries where encryption is illegal.[1] Therefore, whereas cryptography protects the contents of a message, steganography can be said to protect both messages and communicating parties.

Steganography includes the concealment of information within computer files. In digital steganography, electronic communications may include steganographic coding inside of a transport layer, such as a document file, image file, program or protocol. Media files are ideal for steganographic transmission because of their large size. As a simple example, a sender might start with an innocuous image file and adjust the color of every 100th pixel to correspond to a letter in the alphabet, a change so subtle that someone not specifically looking for it is unlikely to notice it.
Contents
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    1 History
    2 Techniques
        2.1 Physical
        2.2 Digital
        2.3 Network
        2.4 Printed
        2.5 Audio
        2.6 Text
        2.7 Using Sudoku puzzles
    3 Data embedding security schemes
    4 Additional terminology
    5 Countermeasures and detection
    6 Applications
        6.1 Usage in modern printers
        6.2 Example from modern practice
        6.3 Alleged use by terrorists
        6.4 Alleged use by intelligence services
    7 See also
    8 Citations
    9 References
    10 External links

[edit] History

The first recorded uses of steganography can be traced back to 440 BC when Herodotus mentions two examples of steganography in his Histories.[2] Demaratus sent a warning about a forthcoming attack to Greece by writing it directly on the wooden backing of a wax tablet before applying its beeswax surface. Wax tablets were in common use then as reusable writing surfaces, sometimes used for shorthand.

In his work "Polygraphiae" Johannes Trithemius developed his so-called "Ave-Maria-Cipher" with which one can hide information in a Latin praise of God. "Auctor Sapientissimus Conseruans Angelica Deferat Nobis Charitas Potentissimi Creatoris" for example contains the concealed word VICIPEDIA.[3]
[edit] Techniques
[edit] Physical

Steganography has been widely used, including in recent historical times and the present day. Possible permutations are endless and known examples include:

    Hidden messages within wax tablets — in ancient Greece, people wrote messages on the wood, then covered it with wax upon which an innocent covering message was written.
    Hidden messages on messenger's body — also used in ancient Greece. Herodotus tells the story of a message tattooed on the shaved head of a slave of Histiaeus, hidden by the hair that afterwards grew over it, and exposed by shaving the head again. The message allegedly carried a warning to Greece about Persian invasion plans. This method has obvious drawbacks, such as delayed transmission while waiting for the slave's hair to grow, and the restrictions on the number and size of messages that can be encoded on one person's scalp.
    During World War II, the French Resistance sent some messages written on the backs of couriers using invisible ink.
    Hidden messages on paper written in secret inks, under other messages or on the blank parts of other messages.
    Messages written in Morse code on knitting yarn and then knitted into a piece of clothing worn by a courier.