In this paper, we propose a novel input wave front modulation method to enhance the security level of a Mach-Zender interferometer-based Fourier encryption system. The input data is encoded in the two wave fronts propagated in the arms of the interferometer. Both arms contain a 4f setup, and two independent Fourier keys are used to encrypt these wave fronts. During decryption the encrypted wave fronts are propagated through the interferometer. In the case when correct Fourier keys are used for decryption, the reconstructed data page is shown by the interference pattern of the output. We propose a method to synthesize two phase modulated input images for this cryptosystem. The modulation method has a user defined phase parameter. We prove that the security level of the proposed cryptosystem can be significantly improved compared with previous solutions, by using an optimally chosen phase parameter.