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The word digital describes a sound, image, or data file that can be stored electronically, in contrast to such a file in another medium, such as a photograph printed on photo paper. The term steganography comes from the Greek words meaning “to cover tightly” and “writing.” It refers to the practice or art of concealing a message in any medium. Digital steganography refers to concealing a message in a digital format.
Steganography dates from at least the time of Herodotus, who mentions the use of tattooed messages on slaves’ shaved scalps — messages that were hidden when their hair regrew, as well as messaged carved in wood and covered with wax. Steganography, when it was first used in English in the sixteenth century, referred to cryptography — writing in codes and ciphers. Currently, the word refers primarily to messages that are unseen. Although such messages may also be encrypted, this is not necessary, and that goes for digital steganography, where the main purpose is often to create a message that defies detection.
Digital steganography makes use of the fact that in a number of file formats, data is reduplicated or some data is of little importance, and the hidden message does not cause noticeable changes to the file. It is used in graphics files, HTML, sound files, video, and text files, for example, but image files are favored and referred to as stego-images. Digital steganography is said to have played a role in the planning by al-Qaeda leading up to the September 11, 2001 attacks on the United States. Digital steganography is also used by people who are being censored, by governments and government agencies, by criminals, and for other reasons.
Digital steganography is related to hidden digital watermarking, but watermarking often uses smaller messages and has different purposes — often copyright, although to a certain extent, they are interchangeable. There is special software to enable the practice of digital steganography, and it may enable hiding a message within a digital watermark, although other methods may be more common. Some digital steganography software is better than others at effectively concealing messages.
The counterpart of digital steganography software is digital steganalysis technology and techniques. Digital steganalysis is the art of steganography detection and extraction. It is used by governments, intelligence agencies, law enforcement, and the military. Training for people who want to be certified in steganography detection and extraction is also available.