For instance, a message may be hidden by using invisible ink between the visible lines of innocuous documents.

This apparent message is the 'covertext'. Generally, a steganographic message will appear to be something else: a picture, an article, a shopping list, or some other message. Later, Johannes Trithemius published the book ' Steganographia', a treatise on cryptography and steganography disguised as a grimoire. The purpose was to instigate a revolt against the Persians. After his hair had grown the message was hidden. Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. Wax tablets were in common use then as re-usable writing surfaces, sometimes used for shorthand. Demaratus sent a warning about a forthcoming attack to Greece by writing it on a wooden panel and covering it in wax. Herodotus mentions two examples of steganography in ' The Histories of Herodotus'. Its ancient origins can be traced back to 440 BC. The word 'steganography' is of Greek origin and means 'covered, or hidden writing'.

For example, the sender might start with an ordinary-looking image file, then adjust the color of every 100th pixel to correspond to a letter in the alphabet-a change so subtle that someone who isn't actively looking for it is unlikely to notice it. Today, the term steganography includes the concealment of digital information within computer files. By contrast, cryptography obscures the meaning of a message, but it does not conceal the fact that there 'is' a message. Steganography is the art and science of writing hidden messages in such a way that no one apart from the sender and intended recipient even realizes there 'is' a hidden message.

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