An optical cryptosystem via Fourier ptychography with double random phase masks is proposed. The encryption process cannot be precisely simulated except by optical experiment due to vignetting, which is space variant and can act as a one-way function from the perspective of pure optics and improve the security of our system. In addition, the encryption for a high-resolution, large field-of-view, and complex-valued image is achievable. Optical experiments are presented to prove the validity and security of the proposed system. Our method will give more insight into separating optical cryptography from computer cryptography in nature. (C) 2019 Optical Society of America