A new method of synthesizing computer generated holograms has been developed and experimentally tested. The method has been implemented by an iterative optimization algorithm on a joint transform correlator. The proposed hologram is a combination of two separated sub-holograms, and its reconstructed image is obtained as a result of a spatial correlation between the hologram's two parts. The double-holograms are displayed on the correlator input plane illuminated by a plane wave. Consequently, a desired image is constructed on part of the correlator output plane. The above method has been extended for deciphering watermarks, and other hidden information, from printed images. A spatial correlation between two concealograms, one containing the watermark and the other containing the deciphering key, reveals the concealed watermark from the printed picture, only when the two specific holograms are matched. Both simulation and experimental results are presented.