Known-plaintext attack-free double random phase-amplitude optical encryption: Vulnerability to impulse function attack

A security enhanced double random phase-amplitude encryption (DRPAE) scheme is immune against the powerful known-plaintext attack, due to the insertion of an undercover amplitude random mask at the Fourier plane in the conventional double random phase encryption (DRPE) scheme. However, DRPAE, which nullifies the high level known-plaintext attack, is found to be susceptible to a simple impulse function attack. Information about the exact distribution of the amplitude mask as well as that of the Fourier plane random phase mask is divulged from an impulse function attack. A variant form of the impulse function attack is also able to show that the DRPAE scheme is as linear as the conventional DRPE scheme. Numerical simulation results validate the effectiveness of the simple impulse attack on the DRPAE scheme.