Steganography is the practice of concealing a message within another message. The advantage of steganography over cryptography alone is that the intended secret message does not attract attention to itself as an object of scrutiny. Plainly visible encrypted messages, no matter how unbreakable they are, arouse interest whereas, steganography both conceals the message and hides that a message is even being sent. The disadvantage to steganography is that it is very complicated to do as both the coded message and the cover message must make sense to the reader.
The simplest versions of steganography, involve embedding the message within the text using ea pattern such as “read every third word.” Colonial spies often were able to smuggle their messages out of New York, Philadelphia, and Williamsburg in this manner by publishing the message in the newspaper.
A more complex approach to steganography is to employ a mask. Letters are written describing ordinary business, like personal letters, orders for merchandise, business contracts, or similar correspondence not likely to draw the attention of those who might intercept the message. These documents were intended to be viewed by a recipient who would place a shaped template over the full letter. The true message of the letter would then appear within the boundaries of the “mask.” The letter and the “mask” were usually delivered by separate couriers to ensure that the trick would go undetected.
One example of masked letter is the message (below) sent from General Henry Clinton to General John Burgoyne just before the battle of Saratoga.