Cryptoanalysis and enhancement of a binary image encryption system based on interference

In this paper, cryptoanalysis on a binary image encryption system based on interference is conducted. In the cryptosystem under study, the binary plaintext image modulated by a random phase mask (RPM) is separated directly into two phase-only masks (POMs) as private keys. Phase wrapping operation is applied to modulate two separated POMs further for silhouette removal. The plaintext image can be reconstructed by compositing two phase-wrapped POMs. However, since the RPM used in the encryption process is irrelative to the plaintexts, it is possible to retrieve the RPM by a known-plaintext attack (KPA). And then with the help of the retrieved RPM, the information encoded in the arbitrarily given ciphertext can be reconstructed by a ciphertext-only attack (COA). Based on our analysis, a hybrid attack including a KPA and a COA with different constraints is proposed in this study. Besides, the cryptosystem under study can only be used to encode binary plaintexts, which would limit the application of this scheme in the information security. Consequently, an improved cryptosystem in which both binary and gray-scale plaintext images can be encoded is proposed. In addition, the RPM to generate two private keys in the enhanced system is dependent on the plaintexts, which makes the proposed encryption scheme immune to the proposed hybrid attack. The feasibility and effectiveness of the security-enhanced cryptosystem have been validated by numerical simulations.