A method for decrypting a binary phase-only mask using an amplitude-only spatial light modulator key is proposed. The system is based on a double passage through a generalized phase contrast setup. A binary amplitude key is converted to phase for decrypting a binary phase-only encrypted pattern on the first passage. The decrypted phase pattern is subsequently converted to an intensity pattern on the second passage. A compact dual-path system, applicable in reflection geometry, is suggested for this decryption operation.