| dc.description.abstract | To prevent the confidential information from being disclosed, one needs to apply some techniques to protect it. Visual cryptography scheme is a secret sharing technique, which splits a binary image into n shares, and gathering more than k shares can recover the secret. The remarkable feature of a visual cryptography scheme is that the decoding process is done by human eyes. A conventional visual cryptography scheme encodes a pixel on the secret image into m subpixels on the share hence makes the size of the decoded image larger than that of the secret image. A few of studies pay attention on solving the problem of pixel expansion. Some studies, incorporated with halftoning or probability, can construct a visual cryptography scheme for gray-level images without pixel expansion. However, such integrated schemes seem not suitable because the visual effect of the decoded image is poor. Except for the problem of pixel expansion, the loss of contrast is also a problem for the visual cryptography. Since the secret is decoded visually, making a good visual effect and offsetting the loss of contrast are as important as keeping the image size invariant.
In this study, we propose two Multi-pixel Encoding Methods based on the visual cryptography scheme. The main purpose of the proposed method is to solve the problem of pixel expansion and generate smooth-looking decoded images. For each time, we simultaneously encode m pixels, called an encryption sequence, on the secret image into m pixels on the share. Hence the size of the decoded image is the same as that of the secret image. There are two different ways to take an encryption sequence: one is to take m pixels with the same color; the other is to take m adjacent pixels. For the former one, there are two kinds of encryption sequences on a secret image: one is black encryption sequence; the other is white encryption sequence. These two encryption sequences are encoded by black and white basis matrices respectively. For the later one, there are (m + 1) types of encryption sequences on a secret image. We propose two different ways to handle different kinds of encryption sequences. When incorporating with halftoning, we can construct a gray-level visual cryptography scheme with a smooth-looking decoded image. Moreover, we exploit the CMY color model to decompose a chromatic image into three image layers in tones of cyan, magenta, and yellow, respectively. Then, we can use the proposed Multi-pixel Encoding Methods to construct a chromatic visual cryptography scheme without pixel expansion. The experimental results show that the size of shares is the same as that of the secret image, and visual effect of the decoded image look smooth as well. Moreover, our scheme can offset the loss of contrast caused by visual cryptography.
Since visual cryptography is not suitable for transferring large amount of texts, we can use Lin and Lee’s document protection scheme to protect the secret message. They randomly choose a meaningful document as a cheating message and then encode a secret message into an index file. The index file is encrypted by IDEA (International Data Encryption Algorithm) before they are sent out. However, the content of the cheating document is restricted to the secret message, i.e. the selection of the cheating message is not truly random; therefore, they can not use any text as cheating message. Besides, the length of the cheating document cannot be shorter than that of the secret document. In addition, the size of the encoded result – index file – is too large so it must be compressed. Although some researchers proposed slight improvements on Lin and Lee’s scheme, the above major drawbacks of Lin and Lee’s scheme are unsolved.
We proposed a new document protection scheme to remedy these drawbacks mentioned above. The first method of our scheme utilizes a subtraction operator to encode the secret message. For each code of the secret message, we randomly pick a code from the cheating message and subtract it from the secret code. The difference is the cipher code of the secret code. The second method of our scheme utilizes a comparison and a logic XOR operator to encode the secret message. For each secret code, we randomly generate a equal-length code from the cheating message via comparison operator. Then we perform logic XOR on the generated code and the secret code. The result is the cipher code of the secret code. The security of the first method mainly depends on the seed of the pseudo random number generator; hence the cipher message may need to be encrypted by another cryptosystem for advancing the security more. The second method possesses the positive features of the first method and conforms to the property of perfect secrecy; hence the cipher message is not necessary to be encrypted again by another cryptosystems. In addition, the experimental results show that the cheating message is not restricted by the secret message. Therefore, the selection of the cheating message is truly random. | en_US |