In previous works, the attacking methods to optical encryption systems are mainly investigated from the aspect of cryptanalysis. An alternative forgery attack is proposed recently for an encrypted computational ghost imaging (CGI) system by confusing the receiver with forged keys and fake decrypted images. But the limitation is that it only works for binary single-pixel intensity values. We propose a different forgery attacking algorithm that works for grayscale single-pixel intensity values in a general CGI system. In addition, we transfer the concept of forgery attack to the well-known double random phase encoding (DRPE) system and propose a corresponding iterative attacking algorithm. Furthermore, we propose several metrics to evaluate the imperceptibility level of the forged keys. We prove that a forgery attack can be implemented not only in a particular situation but also for general optical encryption systems.