Optical image encryption in Fresnel domain using spiral phase transform

In this study, we propose a new nonlinear optical image encryption technique using spiral phase transform (SPT). First, the primary image is phase encoded and multiplied with a random amplitude mask (RAM), and using power function, the product is then powered to m. This powered output is Fresnel propagated with distance z(1) and then modulated with a random phase mask (RPM). The modulated image is further Fresnel propagated with distance z(2). Similarly, a security image is also modulated with another RAM and then Fresnel propagated with distance z(3). Next, the two modulated images after Fresnel propagations, are interfered and further Fresnel propagated with distance z(4) to get a complex image. Finally, this complex image is SPT with particular spiral phase function (SPF), to get the final encrypted image for transmission. In the proposed technique, the security keys are Fresnel propagation distances, the security image, RPM, RAMs, power order, m, and order of SPF, q. Numerical simulation results confirm the validity and effectiveness of the proposed technique. The proposed technique is robust against noise and brutal force attacks.