1. A system for hiding secret data in halftone images, comprising: an image input module, reading an original image data and a secret image data; a halftoning processing module coupled to the image input module, receiving the original image data and the secret image data, performing a first error diffusion process to the original image to generate a first halftone image and performing a second error diffusion process to the original image data and the secret image data to generate a second halftone image: and an image output module coupled to the halftoning processing module, outputting the first halftone image and the second halftone image, wherein the second error diffusion process comprises adding or subtracting a variable noise strength to or from each pixel of the secret image.