Image encryption using separable amplitude-based virtual image and iteratively retrieved phase information

An image cryptosystem that encrypts a plain image using an amplitude-based virtual image and a separable phase function is proposed. The proposed cryp tosystem is based on a 4f optical correlator that is a common architecture for image encryption. Unlike the noise-like amplitude distribution of the e ncrypted data in conventional phase encoding techniques, the amplitude dist ribution in the proposed cryptosystem is meaningful and represented as a vi rtual image. Therefore, the illegal users who steal the virtual image could be confused or treat it as a plain image. The plain image, in fact, is enc rypted by a separate phase function, which is iteratively retrieved by usin g the plain image, an arbitrary virtual image, and another fixed random pha se function. The phase function is determined when both the iterated image and the plain image are identical, or the error between both images is less than a threshold value. Two types of the cryptosystem are proposed, such t hat the separate phase function can be located in either the input plane or the Fourier plane in a 4f correlator. The decryption process can be perfor med in digital methods or implemented by optics at high speed. (C) 2001 Soc iety of Photo-Optical Instrumentation Engineers.