Silhouette-free interference-based multiple-image encryption using cascaded fractional Fourier transforms

An optical approach of silhouette-free multiple-image encryption based on interference is proposed, with two layers to enhance the level of security. In the first layer, a group of plain images are encoded into an amplitude distribution, called interim, using the cascaded fractional Fourier transforms. In the second layer, the interim is encrypted into two noisy ciphertext images. The two layers are compactly combined into a single whole to assure that the inherent silhouette problem is removed through the involvement of a random phase-only mask as the built-in function of cryptosystem. In addition to the phase mask keys generated in the process of encryption, the security is enhanced further by considering the fractional order as an additional key. Numerical simulations are given to demonstrate the feasibility and validity of this approach.