Steganography
Steganography is more ancient than codes and ciphers, and is the art of hidden writing. For example, a message might be written on paper, coated with wax, and swallowed to conceal it, only to be regurgitated later. Another way is to tattoo the message on the shaved head of a messenger and wait for the hair to regrow to cover up the ink. The best stenography uses innocent everyday objects to carry messages. A once-popular technique in England was to use a newspaper with tiny dots under letters on the front page indicating which ones should be read to spell out the message. Some people would spell out a message using the first letter of every word, or use invisible ink. Rival countries have shrunk writing down so that an entire page of text becomes the size of a pixel which is easily missed by prying eyes. Steganography is best used in conjunction with a code or cipher, as a hidden message always carries the risk of being found.