Unsymmetric Image Encryption Using Lower-Upper Decomposition and Structured Phase Mask in the Fractional Fourier Domain

Background: An asymmetric cryptosystem using Structured Phase Mask (SPM) and Random Phase Mask (RPM) in fractional Fourier transform (FrFT) using Lower-Upper decomposition with partial pivoting is proposed in order to enhance security for an existing system. The usage of structured phase mask offers additional parameters in encryption. In the encoded process, the phase-truncation (PT) part is replaced by the Lower-Upper decomposition part. Objective: Introducing the asymmetric cryptosystem using LUDP is to prevent quick identification of encrypted image in the FrFT domain. Method: Initially input image is convoluted using SPM, FrFT and finally LUDP.Then the obtained result is multiplied using RPM, inverse FrFT and LUDP. Results: The strength and legitimacy of the proposed scheme have been verified by using numerical analysis on MATLAB R2018a (9.4.0.813654). For checking the viability of the proposed scheme, mathematical simulations have been carried out which inturn determines the performance and better quality of the image. These all simulations based on key sensitivity, occlusion attack, noise attacks and histograms. Conclusion: A novel asymmetric cryptosystem is proposed by using two phase masks; one is SPM and another is RPM. LUDP is proposed in which the encoded procedure is different from the decoded procedure. Security is enhanced by increasing the number of keys and the scheme is also robust against attacks. Statistical simulations are also carried out for inspecting the strength and viability of the algorithm.