Digital Watermarking : A Tutorial Review

The growth of high speed computer networks and that of Internet, in particular, has explored means of new business, scientific, entertainment, and social opportunities. Ironically, the cause for the growth is also of the apprehension use of digital formatted data. Digital media offer several distinct advantages over analog media, such as high quality, easy editing, high fidelity copying. The ease by which a digital information can be duplicated and distributed has led to the need for effective copyright protection tools. Various software products have been recently introduced in attempt to address these growing concerns. It is done by hidding data (information) within digital audio, images and video files. One way such data hidding is digital signature, copyright label or digital watermark, that completely characterizes the person who applies it and, therefore, marks it as being his intellectual property. Digital Watermarking is the process that embeds data called a watermark into a multimedia object such that watermark can be detected or extracted later to make an asertion about the object. Watermarking is either ”visible” or ”invisible”. Although visible and invisible are visual terms watermarking is not limited to images, it can also be used to protect other types of multimedia object. This work is a tutorial review of the digital watermarking techniques appreared in the literature. 1 Information Hiding Techniques In this section, we briefly discuss the historical development of steganography / watermarking. We also introduce various data hiding terminologies used in current literature and attempt have clear distinction of them. This report was written in 1999 when the author was at the Indian Institute of Science, Bangalore 1.1 History of Information Hiding The idea of communicating secretely is as old as communication itself. The earliest allusion to secret writing in the West appears in Homer’s Ilaid [9]. Steganographic methods made their record debut a few centuries later in several tales by Herodotus, the father of history [10]. Some of them can also be found in [7, 19, 23]. Kautilya’s Arthasa’stra and LalitaVista’ra, and Vatsa’yana’s Ka’masu’tra are few famous examples of the Indian literature in which secret writting / steganography have been used. Few other examples of steganography can be found in [7, 19, 23]. An important technique was the use of sympathetic inks. Ovid in his ”Art of Love” suggests using milk to write invisibly. Later, chemically affected sympathetic inks were developed. This was used in World Wars 1 and 2. The origin of steganography is biological and physiological. The term ”steganography” came into use in 1500’s after the appearance of Trithemius’ book on the subject ”Steganographia”. A whole other branch of steganography, ”linguistic steganography”, consists of linguistic or language forms of hidden writing. These are the ”semagrams” and the ”open code” [16, 19, 23]. A semagram is a secret message that is not in a written form. For example, a system can use long blades of grass in a picture as dashes in Morse code, with short blades for dots. People have also used musical notes for letters -but it doesn’t look anything at all like music and it doesn’t sound like music. Open codes use illuions or code words. In World War 1, for example, German spies used fake orders for cigars to represent various types of British warships-cruisers and destroyers. Thus 5000 cigars needed in Portsmouth meant that five cruisers were in Portsmouth. Watermarking technique has eveloved from steganography. The use of watermarks is almost as old as paper manufacturing [32]. Our ancients poured their half-stuff slurry of fiber and water on to mesh molds to collect the fiber, then dispersed the slurry within deckle frames to add shape and uniformity, and finally applied great pressure to expel