Visual cryptography (VC) is a powerful technique that combines the notions of perfect ciphers and secret sharing in cryptography with that of raster graphics. VC takes a binary image (the secret) and divides it into two or more pieces known as shares. When the shares are printed on transparencies and then superimposed, the secret can be recovered. No computer participation is required, thus demonstrating one of the distinguishing features of VC. VC is a unique technique in the sense that the encrypted message can be decrypted directly by the human visual system (HVS). In this survey, we will summarize the latest developments of visual cryptography since its inception in 1994, introduce the main research topics in this area and outline the current problems and possible solutions. Directions and trends for future VC work shall also be examined along with possible VC applications.

A comprehensive study of visual cryptography

Resolution variant visual cryptography takes the idea of using a single share of visual cryptography (VC) to recover a secret from an image at multiple resolutions. That means, viewing the image on a one-to-one basis and superimposing the share will recover the secret. However, if the image is zoomed, using that same share we can recover other secrets at different levels. The same share is used at these varying resolutions in order to recover a large amount of hidden secrets. This process is quite similar to watermarking an image, whereby nothing can be seen while fully zoomed out, but as the zoom level is increased the watermark becomes visible. This would also be associated with a recursive style of secret sharing. This type of secret sharing scheme would be appropriate for recovering specific types of censored information, such as vehicle registration numbers within certain types of images. This adds an additional dimension to our scheme: content based visual cryptography.

Resolution variant visual cryptography for street view of Google Maps

Based on the theoretic research we conducted, in this chapter, we introduce the various applications of visual cryptography. The typical applications of visual cryptography include watermarking, resolution variant visual cryptography, multiple resolution visual cryptography scheme, etc. We hope the simple and effective encryption and decryption techniques of visual cryptography could be applied to our ordinary life.

Various Applications of Visual Cryptography

Cryptography is an emerging technology, which is important for network security. Research on cryptography is still in its developing stages and considerable research effort is required. This paper includes the complete step by step implementation of advance encryption technique, i.e. encrypting and decrypting 128 bit data using the AES and its modification for enhanced reliability and security. The encryption process consists of the combination of various classical techniques such as substitution, rearrangement and transformation encoding technique. The encryption and decryption module include the key expansion module which generates the key for all iterations. The modification includes the addition of an arithmetic operation and a route transposition cipher in the attacks iterative rounds. The key expansion module is extended to double the number of iterative rounds in order to increase its immunity against unauthorized attacks.

Performance Evaluation of Ciphers Using CRYPTOOL 2.0

Visual Cryptography is a cryptographic technique which can hide the visual data (images, pictures, text etc.) and which can be decrypted without a computer but in a tedious way. The best-known techniques that visual cryptography has are encrypting the images by breaking those images into n shares. Each n-1 share will not reveal the actual image but decrypting the n shares will give the actual information of the image. We can also make the image zoom and find out the information in it but we can hide this information through these techniques of visual cryptography. Even Watermarking is one of the beautiful techniques which protect the copyright ownership of a digital image. According to the proposed method, the watermark pattern does not have to be embedded into the original image directly, which makes it harder to detect or recover from the marked image in an illegal way. It can be retrieved from the marked image without making comparison with the original image. However, their concepts are different but their main purpose is of hiding the original information. For visual cryptography, a set of share binary images is used to protect the content of the hidden image. The hidden image can only be revealed when enough share images are obtained. For watermarking, the hidden image is usually embedded in a single halftone image while preserving the quality of the watermarked halftone image. Hence, visual cryptography requires no understanding and, because security is effectively in the hands of the end user, he or she has a feeling of control, which promotes trust.

Resultant Variant Cryptography by using Morphological Transformation Domain for Google Maps

Hiding image in a color image is a very interesting research topic, especially using very popular software such as Microsoft Windows Paint, Internet Explorer browser (IE) Mozilla Firefox, etc can achieve very amazing result. In this paper, we start from the select operation of IE, and introduce the mechanism how to hide a color image in a real natural image. Based on this, we generate an animation using this scheme. Using the select function of IE, we can toggle two completely different animations. We improve the hiding scheme and introduce how to embed the shares of visual cryptography into a natural color image, another share of visual cryptography works as a public key like the select operation of IE. We also introduce how to hide a visual cryptography share into a halftone image. The results are very robust and can resist print and scan tempering.

J. Weir

Papers

Secure mask for color image hidding