Cryptoanalysis on the optical image encryption scheme based on full phaseencoding and equal modulus decomposition

In this paper, the security of a security-enhanced optical cryptosystem based on full phase encoding and equal modulus decomposition (EMD) is evaluated. Compared to the original EMD-based image scheme in which plaintext is the amplitude information of the spectrum to generate two complex-valued masks with equal moduli, phase-encoded plaintext is regarded as the input of EMD-based structure to generate masks in the full phase encoding and EMD combined cryptosystem. It seems that the security strength has been improved by decreasing the number of constraints in the iterative attack; however, it is found that this scheme is still under security risk. Thus, we propose two iterative attacks based on normalization operator and phase-retrieval techniques with different constraints to break the security-enhanced scheme. Numerical simulations are carried out to demonstrate the feasibility and effectiveness of the proposed attacks.