Digital watermarks have been proposed as a method for discouraging illicit copying and distribution of copyright material. This paper presents a new approach for the secure and robust copyright protection of digital images. The digital watermarks described in this paper are designed to be, as far as possible, invariant against image transformations such as rotation, translation, scaling and cropping. We concentrate especially on the desirable properties of the Fourier Transform and propose a novel technique based on an invisible template which allows us to reverse many of the effects of image processing on the digital watermark. Robustness of the watermark to operations such as lossy compression is achieved by using a perceptually adaptive spread spectrum communications approach, in which a spread spectrum signal is embedded in selected components of the magnitude spectrum of the image. The keys used to embed the spread spectrum signal are generated, certified, authenticated and securely distributed using a public key infrastructure containing an electronic copyright office and a certification authority. The security architecture used for this purpose is also outlined.