Vulnerability to ciphertext-only attack of optical encryption scheme based on double random phase encoding. 
We demonstrate in this paper that the traditional double random phase encoding (DRPE) technique is vulnerable to ciphertext-only attack (COA).  In this method, an unauthorized user (or say attacker) is assumed to be able to retrieve the corresponding plaintext from the only ciphertext under some certain condition.  The proposed scheme mainly relies on a hybrid iterative phase retrieval (HIPR) algorithm, which combines various phase retrieval algorithms.  With an estimation of the number of nonzero pixels (NNP) in the original plaintext, an attacker could recover the plaintext in a large extent.  The simulation results show that this method is feasible and validate.