Vulnerability to chosen-cyphertext attacks of optical encryption schemes based on double random phase keys

We show how optical encryption methods based on double random phase keys are vulnerable to an organized attack of the chosen-ciphertext type. The decryption key can be easily obtained by an opponent who has repeated access to either the encryption or decryption machines. However, we have also devised a solution that prevents the attack. Our results cast doubts on the present security of these techniques. (c) 2005 Optical Society of America.