Taking the advantages of the traditional double random phase encoding and nonlinear optics techniques, a novel nonlinear optical image encryption technique based on self-phase modulation of photorefractive crystal is proposed, the robustness and security are analyzed. The plaintext image is encrypted into a stationary complex random white noise via two statistically independent random phase masks and two sequential nonlinear propagations. The decryption is the inverse process of the encryption, which can be implemented either optically or digitally using nonlinear digital holography. Numerical simulation results demonstrate that the proposed nonlinear optical image encryption technique is robust against additive and multiplicative noise, resists effectively against the chosen plaintext attack or known plaintext attack based on phase retrieval in comparison with the traditional linear encryption techniques.