Mostafa Mokhtari Ardakan - Department of Computer Engineering, University of Isfahan, Isfahan, Iran.
Reza Ramezani - Department of Computer Engineering, University of Isfahan, Isfahan, Iran.
Ali Mohammad Latif - Department of Computer Engineering, Yazd University, Yazd, Iran.

Visual cryptography is a method used to secure images by converting them into several shares that are finally stacked to recover the original image without any calculations. Most existing techniques that can encrypt gray and color images often convert them to binary or support only limited colors, which results in reducing the quality of recovered images. Pixel expansion is another problem with existing methods. Thus, a new approach is required to encrypt gray and color images with real value, without converting them to binary or limited-color images, and also without imposing any pixel expansions. Besides, generated shares should have security, and the recovered images should be of high quality. In this research, fuzzy random grids and a meta-heuristic algorithm are used for the share generation in the encryption step. Next, the decryption step uses the fuzzy OR operator to recover high-quality images. The evaluation results demonstrate the ability of the proposed solution in encrypting gray and color images without converting them to binary, and also without pixel expansion. Besides, the results show that the proposed method is secure as individual shares do not show any information from the original image. The quality of the decrypted images has also been evaluated using subjective and objective evaluation metrics, which prove the high quality of recovered images.

Visual cryptography, pixel expansion, fuzzy random grid, fuzzy OR, visual quality