Abstract: This paper proposes a new progressive visual cryptography (PVC) scheme for color images without any pixel expansion based on the halftoning technique. PVC is a special encryption technique which can be utilized to recover the secret image gradually by superimposing more and more shares. If we only have a few pieces of shares, we could get an outline of the secret image; by increasing the number of shares being stacked, the details of the hidden information can be revealed progressively. Firstly, a chromatic image is decomposed into three monochromatic images in tones of red, blue and green. These three images are transformed into binary images by halftone technique. The secret image shares from binary images are obtained by the unexpanded VC algorithm. To prevent attack from hackers, the secret image shares are watermarked with different cover images and are transmitted. At the receiving side the cover images are extracted from the shares and stacked one by one which reveals the secret image progressively. This scheme provides a more efficient way to hide color images in different meaningful shares without any pixel expansion, providing high security and recovered images with high contrast.

Keywords: Visual Cryptography (VC), halftoning, color image, PVC, watermarking, shares, Visual Secret Sharing (VSS).