Steganography (Listeni / ɛɡstə.ɒɡnɒɡ.rə.fi /, steg-ə-nog-rəfee) is the practice of hiding a file, message, image or video within another file, message, image or video. The word steganography combines the Greek words steganos (στεγανός), meaning "covered, hidden or protected", and graphein (γράφειν) 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, hidden messages appear to be (or be part of) something else: pictures, articles, shopping lists, or some other cover story. For example, the hidden message may be in invisible ink between the visible lines of a private letter. Some steganography implementations that lack a shared secret are forms of security through the dark, while key-dependent steganographic schemes adhere to the Kerckhoffs principle.
The advantage of steganography over cryptography alone is that the intended secret message does not attract attention to itself as an object of scrutiny. Clearly visible encrypted messages - irrespective of what is unbreakable - arouse interest and can themselves be incriminating in countries where encryption is illegal. Thus, while cryptography is the practice of protecting the content of a message alone, steganography is concerned with hiding the fact that a secret message is being sent, as well as hiding the content of the message.
Steganography includes concealment of information within computer files. In digital steganography, electronic communications may include steganographic encoding within a transport layer, such as a document file, an image file, a program or a protocol. Media files are ideal for steganography transmission because of its large size. For example, a sender can start with a harmless image file and adjust the color of every 100th pixel to match a letter of the alphabet, a change so subtle that someone who does not specifically look for it will not notice it.
The earliest recorded uses of steganography date back to 440 BC when Herodotus mentions two examples in his Histories. Histiaeus sent a message to his vassal, Aristagoras, shaving the head of his most trusted servant, "marking" the message on his scalp, and then sending him on his way once his hair had grown back, with the instruction: When you come to Miletus, have Aristagoras shave his head, and look at it. "In addition, Demaratus sent a warning about an upcoming attack on Greece by writing it directly on the wooden back of a wax tablet before applying its wax surface from bee. Then as reusable writing surfaces, sometimes used for shorthand.
In his work Polygraphiae Johannes Trithemius developed his so-called "Ave-Maria-Cipher" which can hide information in a Latin praise of God. "Auctor Sapientissimus Conseruans Angelica Deferat Nobis Charitas Potentissimi Creatoris" for example contains the hidden word VICIPEDIA.
