As Internet usage continues to grow, people are becoming more aware of the need to protect the display and presentation of digital documents. Digital watermarking offers a means to protect such documents. It works by embedding a piece of information into these documents – either visibly or invisibly – which can be detected or extracted from the document at a later stage to prove ownership of the document. This chapter will review, in detail, the process of digital image watermarking and will evaluate the different types of watermarks which may be used and the different techniques used to embed the watermarks. An examination of the domains used to embed the watermarks and some of the techniques used for detection of the embedded watermark are provided. Finally future trends within the area of digital watermarking are outlined.
Digital watermarking aims to embed a piece of information in some cover document without affecting the overall appearance of the image. Before going into detail on digital watermarking we will first look at some similar techniques, namely Steganography and Cryptography.
Steganography is a term derived from the Greek words steganos, which means “covered”, and graphia, which means “writing”. Steganography is the art of concealed communication: the very existence of the message is a secret. An often used example of steganography is a story from the histories of Herodotus, which tells of a slave sent by his master, Histiaeus, to the Ionian city of Miletus with a secret message tattooed on his scalp. After tattooing, the slave grew his hair back in order to conceal the message. He then travelled to Miletus and, upon arriving, his hair was shaved to reveal the message to the city’s regent, Aristagoras. The message encouraged Aristagoras to start a revolt against the Persian king. In this case the message is of primary value to Histiaeus and the slave is simply the carrier of the message. In steganography, the message being hidden is of upmost importance and the document it is hidden within (the carrier) is of minor importance (de Sélincourt, 1954).
Steganography is the study of techniques for hiding the existence of a secondary message in the presence of a primary message. The primary message is referred to as the carrier signal or carrier message; the secondary message is referred to as the payload signal or payload message. Arnold (2003) states that classical steganography (i.e., steganographic techniques invented prior to the use of digital media for communication) can be divided into two areas; technical steganography and linguistic steganography. Examples of technical steganography include the above mentioned story about Histiaeus and the use of invisible ink. Examples of linguistic steganography include marking specific characters with punctuation or lowering certain characters and using spacing between letters in a word to hide a secret message.
Key Terms in this Chapter
Cryptography: Cryptography is the process or skill of communicating in or deciphering secret writings or ciphers.
Pseudorandom Watermark: A watermark generated by iterating a pseudorandom number generator. These watermarks are generally white in nature.
Watermark Attacks: An attack is any attempt (malicious or coincidental) to inhibit a watermarking system.
Watermark Embedding: A watermark embedding technique is an algorithm for inserting a watermark into a cover document.
Steganography: Steganography is the art of concealed communication: the very existence of the message is a secret. Steganography is the study of techniques for hiding the existence of a secondary message in the presence of a primary message.
Watermark Detection: A watermark detector is a hardware device or software application that detects and decodes a watermark.
Digital Watermarking: Digital watermarking is a technique that aims to embed a piece of information permanently into some digital media, which may be used at a later stage to prove owner authentication and attempt to provide protection to documents. Watermarking, unlike steganography, has the additional notion of resilience against attempts to remove the hidden data.
Chaotic Watermark: A watermark generated by iterating a chaotic function. These watermarks offer the user more control over the spectral properties of the generated watermarks.
Robustness: Robustness refers to the ability to detect the watermark after common signal processing operations and attacks.
Capacity: The capacity of a system refers to the number of bits of a watermark that may be encoded within a cover document.