Steganography (in Greek, "secret" + "writing") is the practice of hiding secret information inside non-secret, or less secret, information. Various methods of steganography predate electronic/computer cryptography by centuries: invisible inks, conventions for altering public texts, code words, etc. What distinguishes steganography from plain old encryption is that an attacker does not know with certainty that there is any secret message inside another message. In some circumstances this can be important for plausible deniability; in others as a diversion for an attacker; in still others as a way of subverting a channel that an attacker might choose to leave open.

It is worth giving a couple of hypothetical examples of steganography here. In order to pass a secret message, a typed letter might include a number of deliberate "typos," the position of the words with "typos" would encode a subset of the numbers between one and the number of words in the letter. An attacker would not know whether an intercepted letter contains a subtext or subchannel -- or whether it simply has typos (as do many letters with no hidden message). Obviously, the recipient must be aware of the protocol used to encode the subchannel. Similarly, a sound recording (for example, one played on the radio) might have a number of clicks and pops added to it that are indistinguishable from scratches on a vinyl record (in fact, they could be produced by making scratches in such a vinyl record before playing it). The exact timing of the pops would encode a message (for example, the millisecond gaps between successive pops encodes a series of numbers). Because regular phonograph recordings also contain pops, an attacker would not (immediately) know whether a particular song played on the radio actually contains a subtextual message.