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by Juan C. Dürsteler [message nº 101]

In the last decade, steganography and particularly the techniques incorporating copyright messages (Watermarking) into documents, images and soundtracks have experienced an important boom.

Steganography is the art of conveying information in such a way that the very existence of it goes unnoticed, typically hidden within a text or an image. It comes from the Greek words steganos (covered) and graptos (writing), literally covered writing, with covered understood in the sense of hidden.

Unlike cryptography, where the message is visible and leads to suspicion, here information is subtly embedded in a vehicle, be it text, image or soundtrack, of a completely normal appearance. The information it contains can only be revealed by applying an appropriate algorithm or procedure.

For many people, Steganopgraphy has left its hideout since September 11 and with the possibility that the terrorist network of Al Qaeda is using these techniques in order to convey messages through images or soundtracks freely available in the World Wide Web .

Some newspapers, like USA Today and New York Times, among others, published articles about the topic in fall 2001. The University of Michigan took them seriously and started a project to analyse several million images accessible through the Internet, without any success.

Steganography is by no means new. Herodotus, in his Histories already described how the wax covered tablets used to write were used to warn about the danger of Xerxes invading Sparta. The trick was to remove the wax, write on the wooden frame that served as a support of the wax and cover it again with wax.

The oldest known book about the topic is the “Gasparis Shotti Schola Steganographica” dated 1665. During World War II steganography was profusely used in many of its guises, with the aid of invisible inks or as innocent messages which, applying a punch card that selected certain words or letters, provided secret messages. 

The advent of the computer has given it a new projection and also applications more honourable than espionage. In particular, the techniques known as Watermarking for hiding copyright messages and Fingerprinting for identifying information like serial numbers or telling the difference between specific objects that are otherwise similar have become increasingly popular.

Its importance resides in the fact that the protection of intellectual property rights of images, soundtracks and written documents has become more and more difficult in a world where downloading an image or an MP3 song is just one click away. Remember the problems of the record industry with Napster or the software piracy.

Watermarking systems subtly modify the constituting bits of the document, image or soundtrack in such a way that the resulting product is almost indistinguishable from the original. Through an appropriate decoding algorithm you can reconstruct the “digital signature” embedded in it.

An excellent web site on the topic is the one of Fabien A.P. Petitcolas from Microsoft Research. It contains, among others, a page with an interesting list of steganographic software and another about audio watermarking.

Since 1996, the year of the first conference on the speciality, the interest in it has been growing steadily. Currently there are a large variety of techniques and algorithms, some of which do not need to use the computer in order to reveal its contents.

One of them, devised by Ren-jun Hwang and Chin-Chen Chang (“Hiding a picture in two pictures”, Optical Engineering, Vol 40(3) pp 342-351) modifies two images with whatever contents by computer in such a way that photocopying them onto slides and superimposing them the hidden message appears.

As we can see, information isn’t always visualised by means that help understanding and clarity. Sometimes the information is in front of our eyes, deliberately hidden. 

The dark side of InfoVis?…

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