Asymmetric optical cryptosystem using C-point polarization singularity speckles and polar decomposition in 2D-NS-LCT domain

The study introduces a novel asymmetric optical cryptosystem that utilizes bright C-point polarization singularity speckle (BCPSS) patterns as security keys while offering multiuser capabilities. The C-point singular beams, with spatially varying polarization distributions, are created by superposing optical vortex modes of different magnitudes into an orthogonal polarization basis. This complex light beam is then scattered through a rough surface to generate the BCPSS patterns. These generated speckle patterns inherit some unique properties due to the vectorial light field and the randomness of the rough surface, which make them nearly impossible to duplicate. To generate a complex image, the BCPSS phase mask is used to further modify the original image after it has been phase encoded. The final encrypted image is then obtained by processing the intermediate complex image using two-dimensional non-separable linear canonical transform (2D-NS-LCT) and polar decomposition. The 2D-NS-LCT has ten independent parameters which expends the key space, improving its resistance to various attacks. The implementation of polar decomposition in the proposed cryptosystem enables us to have two private keys, helping in multiuser functionality. The proposed method is also validated by testing it against various potential attacks, including contamination and plaintext attacks. Numerical simulations confirm the authenticity and reliability of the proposed cryptosystem.