Such a disguised message is considered to be an example of steganography, which is a sub-branch of general cryptography.
Invisible ink is one form of steganography, and it has been used in espionage.
However, acrostics may also be used as a form of steganography, where the author seeks to conceal the message rather than proclaim it.
But whereas steganography aims for imperceptibility to human senses, digital watermarking tries to control the robustness as top priority.
In addition, steganography may be used to hide information embedded within graphic files on websites.
Another type of concealment is the hiding of messages in the text or on a book's pages by printing in code -- a form of steganography.
Although it bears similarities to both traditional encryption and steganography, it can not be classified under either category.
Typical network steganography methods involve modification of the properties of a single network protocol.
