This is the art or technique of creating and transmitting hidden messages.
The dictionaries — those few that include it, for it has been in the past an uncommon word — say this is connected with cryptography. But those in the spy business make a careful distinction between the techniques of cryptography and steganography, although both are aspects of the art of secret communication.
Cryptography is the technique of scrambling a message in a systematic way so that (hopefully) it can be read only by its intended recipient. Steganography, on the other hand, keeps the message secret by hiding the fact that it exists, a technique that cryptographers sometimes sniffily refer to as “security through obscurity”. So the microdot of the Cold War spy novels — in which a document is photographically reduced to the size of a pinhead and stuck to an otherwise innocuous typescript — is an example of steganography. Invisible ink is another example. You can, of course, combine the two techniques if you believe in the braces-and-belt approach to life.
The word has become more common in the digital age, since it is now relatively easy to hide messages within otherwise innocuous-looking images, audio recordings or other data. This has proved of benefit to spies and terrorist organisations.
In 2006, law enforcement officials found that by entering a 27-character code into the computer of the Cambridge spies, they were able to access a special steganography program that makes the messages appear on digital images downloaded from specific websites that they used to communicate secret messages.
Boston Globe, 2 Nov. 2011.
Steganography derives from the Greek steganos, hidden or covered, plus graphein, to write. Someone who uses this technique is a steganographer.