Steganography

Steganography is the art or science of writing hidden messages.

Generally a steganographic work will appear to be something else, like a shopping list, an article, picture or any form of eltronic media. It differs from regular cryptography as often encoded works are sent to someone in a form that is obviously encoded with the strength of the encryption providing the protection.

A very old type of steganology is the secret method of communication used by the Roman empire. They would send stips of paper that would make so sense unless they were bound around a rod of a specific diameter, which Roman commanders carried, and read. These messages were created by the stoganographer writing on the paper strips wound around such standard rods.

Digital watermarking generally uses this type of approach so people either don't know that a watermark exists or can not work out how to remove it.