Jonathan Weir

A comprehensive study of visual cryptography

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.

Sharing multiple secrets using visual cryptography

Visual cryptography provides a very powerful technique by which one secret can be distributed into two or more pieces known as shares. When the shares on transparencies are superimposed exactly together the original secret can be discovered without computer participation. In this paper, we take multiple secrets into consideration, and generate a master key for all the secrets; correspondingly, we share each secret using the master key and obtain multiple shares. We merge these shares into a combined share, we adjust the master key and generate a new key. The secrets are revealed when the key is superimposed on the combined share in different locations using the proposed scheme. We provide the corresponding results in this paper.

Resolution variant visual cryptography for street view of Google Maps

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.

Authenticating visual cryptography shares using 2d barcodes

One of the problems pertinent with many visual cryptography (VC) schemes is that of authentication. VC provides a way of sharing secrets between a number of participants. The secrets are in the form of an image that is encoded into multiple pieces known as shares. When these shares are physically superimposed, the secret can be instantly observed. A known problem is that of authentication. How is it possible to know that the secret being recovered is genuine? There has been some work devoted to this using so called cheating prevention schemes which attempt to provide a means of traceability or authentication via a set of additional shares that are used to check authenticity. This paper proposes a scheme that attempts to alleviate this suspicion by using 2D barcodes as a means of authentication which may have more practicality in terms of real world usage. Results are provided using an application that is available on mobile devices for portable barcode reading.

Dot-Size Variant Visual Cryptography

In this paper, we propose a scheme by which a secure random share can be generated using a dot-size variant form of visual cryptography (VC). We generate two extended style VC shares, when the share is viewed, it appears as a normal random visual cryptography share. However, this scheme is designed with spatial filtering in mind, this is the dot-size variant part of the scheme. Dot-size variant means that instead of having single black and white dots which make up a VC share, we use a cluster of smaller dots to represent these black and white pixels. This means that after printing, if the share is scanned or photocopied or even viewed with a mobile phone or digital camera, the smallest dots in the scheme are filtered. This loss of information during the copying process allows the original share to have additional security in that accurate copies cannot be created, as well as the fact that due to this loss, the copied share looks totally different from the original. This technique can be used to detect possible counterfeit shares and copies as they will be noticeably different from the original. One major advantage of our scheme is that it works with traditional print techniques and required no special materials. We present our results within this paper.

Image hatching for visual cryptography

Image hatching (or non-photorealistic line-art) is a technique widely applied in the printing or engraving of currency. Diverse styles of brush strokes have previously been adopted for different areas of an image to create textures and shading. Because there is no continuous tone within these types of images, we propose a multi-level scheme, which uses different textures based on a threshold level. These textures are then applied to the different levels and are then combined to build up the final image. We propose a technique by which one can hide a secret using visual cryptography (VC) within the hatched images. Visual cryptography provides a very powerful means by which one secret can be distributed into two or more pieces known as shares. When the shares are superimposed exactly together, the original secret can be discovered without computer participation.

Plane transform visual cryptography

Plane transformation visual cryptography takes a unique approach to some of the current shortcomings of current visual cryptography techniques. Typically, the direction and placement of the encrypted shares is critical when attempting to recover the secret. Many schemes are highly dependant on this stacking order. Within this paper, the scheme presented illustrates a technique whereby this restriction is loosened such that the number of acceptable alignment points is increased by performing a simple plane transform on one of the shares. This results in the same secret being recovered when the shares correctly aligned. The technique has also been extended to encompass multiple secrets, each of which can be recovered depending on the type of transformation performed on the shares.

Image Hatching for Visual Cryptography

Image hatching (or non-photorealistic line-art) is a technique widely applied in the printing or engraving of currency. Diverse styles of brush strokes have previously been adopted for different areas of an image to create textures and shading. Because there is no continuous tone within these types of images, we propose a multi-level scheme, which uses different textures based on a threshold level. These textures are then applied to the different levels and are then combined to build up the final image. We propose a technique by which one can hide a secret using visual cryptography (VC) within the hatched images. Visual cryptography provides a very powerful means by which one secret can be distributed into two or more pieces known as shares. When the shares are superimposed exactly together, the original secret can be discovered without computer participation.

Secure mask for color image hidding

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.

Secure masks for visual cryptography

Visual cryptography provides a very powerful technique by which one secret can be distributed into two or more pieces known as shares. When the shares are printed on transparencies and then superimposed exactly together, the original secret can be recovered without computer participation. In this paper, we take multiple secrets into consideration and generate a key share for all the secrets; correspondingly, we share each secret using this key share. The secrets are recovered when the key is superimposed on the combined share in different locations using the proposed scheme. Also discussed and illustrated within this paper is how to embed a share of visual cryptography into halftone and colour images. The remaining share is used as a key share in order to perform the decryption. It is also worth noting that no information regarding the secrets is leaked in any of our proposed schemes. We provide the corresponding results in this paper.