The rapid growth of digital multimedia technologies brings tremendous attention to the field of digital authentication. Digital watermarking has become widely recognized as an effective measure for copyright protection of multimedia data. The owner or the distributor of the digital images can insert a unique watermark into copies for different customers or receivers, which will be helpful to identify the source of illegal copies. In this chapter the authors present two original spatial authentication techniques for digital images. These new algorithms yield an invisible watermark that is robust to various kinds of attacks. The main principle is the utilization of a virtual (2D or 3D) graph embedded into the digital images. Then, the colors of some vertices of the virtual graph are slightly modified for obtaining the watermark. The proposed techniques modify pixels or voxels of the object by a spatial watermark insertion scheme. These techniques can be used for all kinds of digital images, color or black and white, and the new algorithms produce an invisible robust watermark. The techniques lower the computational complexity that normally rises with the traditional watermarking algorithms. This approach reduces computation and implementation complexity of the algorithms. These techniques seem to replace advantages of the transform domain techniques with those of the spatial domain techniques.
