ABSTRACT
This chapter describes the fundamentals of watermark-based image authentication and presents the main motivations and the rationale for research, development, and use of this technology and state-of-the-art techniques for image authentication. It considers theoretical and practical aspects related to feasibility, effectiveness, and security. The chapter discusses a technique for fragile image authentication derived from the extreme sensitiveness of linear ill-posed operators to small distortions in the input data. The watermarking process is achieved by linking the watermarked image with a very ill-conditioned matrix derived from a secret key and the original image. The solution of the underlying ill-posed operator is extremely sensitive to changes in the watermarked image. Semifragile watermarks are designed to discriminate between expected image changes in most cases due to application constraints and intentional image tampering. During the last few years, different attacks and countermeasures have been proposed to either break or increase the security of watermarking-based image authentication algorithms.
