Security analysis on optical cryptosystem based on interference and phase-retrieval technique

Abstract

In this paper, the security strength of an optical cryptosystem based on interference and phase-retrieval technique has been evaluated. Compared to the conventional interference-based scheme in which private keys are generated according to the principle of vector decomposition, a modified error-reduction phase retrieval algorithm is employed to produce a plaintext-dependent key with the smaller size than that of the plaintext in the improved cryptosystem; thus, resulting in higher resistance against iterative attacks and lower burden of data transmission. However, it is found that only one phase-only mask (POM) served as the private key is unknown to illegal users, the key space is limited; thus, it is possible to investigate an iterative process to recover the unknown key with the aid of public keys and the arbitrarily given ciphertext as additional constraints. And employing the recovered private key, encoded information may be reconstructed. Based on this cryptoanalysis, the iterative ciphertext-only attack (COA) with normalization and image smoothing techniques (i.e., the median filter, the bilateral filter) is proposed to crack the enhanced scheme. Numerical simulations are carried out to validate the feasibility and effectiveness of the proposed COA.