Enhancing security of optical cryptosystem against ciphertext-only attack with position-multiplexing and ultra-broadband illumination

Abstract

A position-multiplexing technique with ultra-broadband illumination is proposed to enhance the information security of an incoherent optical cryptosystem. The simplified optical encryption system only contains one diffuser acting as the random phase mask (RPM). Light coming from a plaintext passes through this RPM and generates the corresponding ciphertext on a camera. The proposed system effectively reduces problems of misalignment and coherent noise that are found in the coherent illumination. Here, the use of ultra-broadband illumination has the advantage of making a strong scattering and complex ciphertext by reducing the speckle contrast. Reduction of the ciphertext size further increases the strength of the ciphering. The unique spatial keys are utilized for the individual decryption as the plaintext locates at different spatial positions, and a complete decrypted image could be concatenated with high fidelity. Benefiting from the ultra-broadband illumination and position-multiplexing, the information of interest is scrambled together by a truly random method in a small ciphertext. Only the authorized user can decrypt this information with the correct keys. Therefore, a high performance security for an optical cryptosystem could be achieved.