Conventional photographs may easily be used in court as evidence. The complete negative may be inspected. Subsequent numbers are a reliable proof that sequences of pictures have been generated. Modifications are usually quickly detected without major technical efforts. This is not true anymore for digital images, since they may easily be manipulated. This poses a problem, for example, for surveillance cameras of automatic teller machines in financial institutions. Digital watermarking techniques have been proposed to address this problem. In this chapter, a new public-key watermarking system will be presented. In contrast to digital signatures and other public-key watermarking techniques, it is two-dimensional and, hence, especially well-suited for applications involving digital images.