The far-reaching Internet and inexpensive software are combining to make the technology perhaps the easiest, most powerful way ever to distribute concealed information. Bearing no perceptible difference from the original, the image can be prepared for transmission over the World Wide Web in a few minutes. Maybe you've come across steganography in seemingly innocent material without ever knowing it. A communique to a terrorist cell might be hidden in a page of text on a Web site you visited. Unlike you, the intended recipients of the hidden content would know to look for it.
Digital steganography descends from the centuries-old practice of information hiding.
World War II intelligence operators used updated forms of steganography, inserting secret information rendered in microscopic sizes and invisible inks within mundane-looking documents. The photo carrying the secret message would bear no visible sign of the alteration. E-mailing such a picture to relatives would hardly raise suspicion. But when it would arrive at its destination, the same computer program that put the message there could be used to extract it. Steganography programs most often employ photographs.