This chapter describes image watermarking, the most common and widespread category of media files are images. The evolution of the Internet and the ease by which images can be duplicated and distributed has led to the need for effective copyright protection tools and techniques in order to provide a secure way to the producers and the owners of these media files. These techniques are described below with an introduction to information hiding. Various software products have been introduced with an aim to address these growing concerns; some categories are presented here. The fundamental technique which allows an individual to add hidden copyright notices or other verification messages to digital images is called digital image watermarking and constitutes the main part of the chapter. Finally authors provide future trends and directions of image watermarking.
History Of Information Hiding
In this section we are going to give some important landmarks about information hiding without intending to cover the whole history of it. Information hiding techniques belong to a wide category of techniques that try to embed a signal, called digital signature or copyright label or watermark in the initial data. These techniques combine many different scientific areas like cryptography, digital signal processing, communication theory, etc. Generally there are two basic methods of information hiding, cryptography and steganography which overlap in some cases because they share some techniques.
Cryptography is about protecting the content of a message with the use of disguise. The initial message is called the plain text and the disguised message is called the cipher text. The process of converting this plain text to a cipher text is called enciphering or in other words encryption. The opposite process is called deciphering or decryption. Encryption protects content during the transmission of the data from the sender to receiver. However, after receipt and subsequent decryption, the data is no longer protected and is clear. While cryptography is about protecting the content of messages, steganography is about concealing their very existence.
Steganography lies in devising astute and undetectable methods of concealing the message themselves. It comes from Greek roots (στεγανός, γραφείν), literally meaning «covered writing» (Clarendon Press, 1933), and is usually interpreted to mean hiding information in other information. Examples include sending a message to a spy by marking certain letters in a newspaper using invisible ink. Steganography hides messages in plain sight rather than encrypting the message. It is embedded in the data and does not require secret transmission. A whole other branch of steganography is linguistic steganography which consists of linguistic or language forms of hidden writing (David Kahn, 1996).
There has been a growing interest, by different research communities, in the fields of steganography, digital watermarking and fingerprinting. This led to some confusion in the terminology. For this reason in Figure 1 we present a classification of various information hiding techniques.
A classification of information hiding techniques (Based on Birgit Pfitzmann, 1996)