Security enhanced optical image cryptosystem based on phase encoding by generating a sparse volumetric ciphertext

Abstract

Previously proposed optical image cryptosystems based on phase encoding mainly utilized 2D phase retrieval to retrieve ciphertexts, and the relationship between the plaintext and the ciphertext is very simple, which makes those cryptosystems vulnerable to several attacks. In this paper, we propose a security enhanced optical image cryptosystem based on phase encoding by generating a sparse volumetric ciphertext. Several complementary binary amplitude masks (BAMs) without overlapping are used as constraints to build a sparse volume field, and a sparse volumetric ciphertext can be retrieved by a novel volumetric phase retrieval algorithm. Afterwards, all pixels of the volumetric ciphertext are multiplexed into one 2D ciphertext for convenient storage and transmission. For decryption, the pixels should first be redistributed into the volume field. Once a beam with correct wavelength crosses the volumetric field sequentially, the decrypted image can be obtained by recording an intensity pattern. In this scheme, the parameters of the volume field serve as additional keys, which enlarges the key space and confuses the relationship between the plaintext and the ciphertext to realize much higher security. The effectiveness and security of this scheme is verified by some numerical simulations.