Steganography is the art of hiding messages by embedding them in ordinary communications. The word is Greek for “covered, or hidden writing”. The term can be traced back to Herodotus (in 440 BC) who mentions two particularly interesting examples (summarized on Wikipedia):
Demaratus sent a warning about a forthcoming attack to Greece by writing it on a wooden panel and covering it in wax. Wax tablets were in common use then as re-usable writing surfaces, sometimes used for shorthand. Another ancient example is that of Histiaeus, who shaved the head of his most trusted slave and tattooed a message on it. After his hair had grown the message was hidden. The purpose was to instigate a revolt against the Persians.
Steganography is distinct from cryptography which obscures the meaning of a message, but it does not conceal the fact that there is a message. In today’s digital age, steganography includes concealment of one’s and zero’s within data files. An ordinary-looking image file, for example, can include embedded messages that can go unnoticed unless someone is actively looking for a code, much like invisible ink. As has been noted, the advantage of steganography over cryptography that messages do not attract attention to themselves, to messengers, or to recipients.
And now, two Polish scientists with the Institute of Telecommunications in Warsaw have just revealed that they are developing a steganographic system for VoIP networks. This may eventually be a more effective way for activists and social resistance movements to communicate and evade detection. All that we need now is the software and interface to make this communication as simple as two clicks of a mouse. Any takers? Of course, repressive regimes could also use the same tactic, and I realize this presupposes Internet connection and access, but it’s a start.
iRevolutions 
As could organized crime, terrorist networks, folks cheating on their spouses, etc. Still, it’s a fascinating concept!