WeiQi Yan

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.

Efficient Image Copy Detection Using Multiscale Fingerprints

A multiscale scale-invariant feature transform (SIFT) descriptor can help improve our ability to discriminate between images when using copy detection to identify illegal image copies.

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.

Progressive Audio Scrambling in Compressed Domain

Audio scrambling can be employed to ensure confidentiality in audio distribution. We first describe scrambling for raw audio using the discrete wavelet transform (DWT) first and then focus on MP3 audio scrambling. We perform scrambling based on a set of keys which allows for a set of audio outputs having different qualities. During descrambling, the number of keys provided and the number of rounds of descrambling performed will decide the audio output quality. We also perform scrambling by using multiple keys on the MP3 audio format. With a subset of keys, we can descramble to obtain a low quality audio. However, we can obtain the original quality audio by using all of the keys. Our experiments show that the proposed algorithms are effective, fast, simple to implement while providing flexible control over the progressive quality of the audio output. The security level provided by the scheme is sufficient for protecting MP3 music content.

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.

Fine-search for image copy detection based on local affine-invariant descriptor and spatial dependent matching

Copies are somehow a subset of near-duplicates, but the approaches extensively employed in near-duplicate retrieval only obtain rough and imprecise query results. Therefore a fine-search scheme is proposed to refine these rough results and attempt to completely detect the real copies accurately. This scheme first employs a local affine-invariant descriptor based on polar-mapping and discrete Fourier transform. Then a spatial dependent matching method is proposed combining nearest neighbor distance ratio with the spatial relationships among the local features. Experimental results demonstrate that the employed descriptor is more robust, distinctive and suitable for copy detection in comparison with the SIFT descriptor. And the spatial dependent matching is able to improve the recall and precision, and lower the false positives and ambiguities.

A novel collusion attack strategy for digital fingerprinting

Digital fingerprinting is a technology which aims to embed unique marks with traceability in order to identify users who use their multimedia content for unintended purpose. A cost-efficient attack against digital fingerprinting, known as collusion attack, involves a group of users who combine their fingerprinted content for the purpose of attenuating or removing the fingerprints. In this paper, we analyze and simulate the effect of Gaussian noise with different energies added in the noise-free forgery on both the detection performance of correlation-based detector and the perceptual quality of the attacked content. Based upon the analysis and the principal of informed watermark embedding, we propose a novel collusion attack strategy, self-adaptive noise optimization (SANO) collusion attack. The experimental results, under the assumption that orthogonal fingerprints are used, show that the proposed collusion attack performs more effectively than the most of existed collusion attacks. Less than three pieces of fingerprinted content can sufficiently interrupt orthogonal fingerprints which accommodate many thousands of users. Meanwhile, high fidelity of the attacked content is retained after the proposed collusion attack.