An optical vortex-based asymmetric cryptosystem using QZ modulation for the double image encryption in the gyrator transform

The current study proposes a new nonlinear cryptosystem that employs complex phase masks named as optical vortices (OVs) which are obtained from the phases of the radial Hilbert mask (RHM) and Toroidal Fresnel lens (TFL). The security keys used in classical optical cryptosystems are often random phase masks (RPMs), which are vulnerable to various forms of attacks including brute force attacks. To create complex images in the frequency domain with the current encryption, the amplitude images are first phase encoded, after which the OV phase mask is used to modulate them. The complex images are further processed with QZ decomposition resulting in two upper quasi-triangular matrices, two unitary matrices, and decryption keys in the gyrator transform (GT). The use of OVs, QZ decomposition and gyrator transform settings provides additional keys to expand the key space. The proposed method is asymmetric, and the decryption process requires the private keys generated during encryption. The performance of the scheme has been evaluated in terms of mean-squared-error (MSE), peak signal-to-noise ratio (PSNR), SSIM, histograms, entropy analysis, correlation analysis, noise and occlusion attacks. Numerical simulations are presented to validate the proposed encryption method in terms of its strength and reliability.