An image encoding scheme using the phase-contrast technique and a random phase distribution is proposed to encrypt images in phase masks. The robustness of the encoding is assured by the non-linearities intrinsic to the phase-contrast technique and the bandwidth of the random phase distribution. The advantage of this method, compared to the previous methods proposed, is the direct encoding of the image without any iterative calculation to generate the phase mask. This approach permits practical applications since the final phase mask could be implemented using thermoplastic plates and spatial light modulators (Slims).© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.