What is Digital Watermark

Multimedia document authentication allows the judging of the authenticity and integrity of digital documents. Today a variety of such tools exist which are constructed using different approaches, namely forensic methods, perceptual hashes and digital watermarks. Digital document forensics identifies the source of a document as well as its integrity by means of passive estimation. Perceptual hashing allows computing of short digests of documents; the hashes are insensitive against some signal processing operations and may serve as proof of integrity. Finally, authentication watermarking even allows gaining precise and reliable knowledge on the nature of modifications that a digital document underwent. In this chapter, we give an overview of the three complementary technologies, survey state-of-the-art methods and provide an analysis of their strength and weaknesses.