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This letter presents a method for implementing visual cryptography (VC) in which an additional tag is attached to each generated share. The proposed (t, n ), 2 \leq t \leq n tagged visual cryptography (TVC) scheme works like a traditional VC scheme does, where the original image is encoded in n shares in such a way that the secret can be revealed by superimposing any t or more shares, but knowledge of less than t shares gets no secret information. A notable characteristic of TVC is that an extra tag can be revealed by folding up each share, which provides users with supplementary information such as augmented message or distinguishable patterns to identify the shares. The tagging property can easily be applied to any reported VC scheme to endow the generated shares with more capabilities. Construction methods and simulation results of the proposed ( t , n ) TVC based on conventional matrix-based VC and probabilistic-VC are illustrated in the letter. [ABSTRACT FROM PUBLISHER]

Abstract: This paper presents an image authentication scheme for digital images. The proposed scheme protects a group of n images mutually. It designs a block matching procedure to generate the recovery data for each image, and applies a (t, n −1), 2≤ t < n, threshold mechanism to encode each recovery data in n −1 shares. The recovery shares are cross-embedded in the n images using a modified reversible contrast mapping watermarking scheme. A signature-based authentication code is finally generated and stamped to provide evidence for integrity of each image. The scheme not only can detect the tampering activities, but also can locate and recover the invalid regions of the tampered image if t or more watermarked images in the same group were intact. A nice characteristic of the proposed scheme is that the original images can be reconstructed lossless if no watermarked image was tampered, making the technique feasible in the application of protecting very sensitive images such as military or medical images. Experimental results show that the proposed scheme successfully detects various kinds of image alterations such as filtering, cropping, and replacement, and the corrupted images are properly recovered using the cross-recovery scheme. [Copyright &y& Elsevier]

Abstract: Visual cryptography (VC) is an image-based secret protection mechanism in which the decoding process is conducted by inspecting the stacked shares with the unaided eye. In traditional VC schemes, the content of an image is usually processed as a single secret, which limits the whole image to unique secret property. Recently, Wang ( IEEE SPL, 16(2009) 659–663) proposed an n-level region incrementing VC scheme which has the ability of incremental revealing of the secrets on an image; however, the pixel expansion of the generated shares increases rapidly in the proposed scheme as n grows. In this paper, we propose an incrementing VC scheme using random grids. The method has the same incremental revealing effect to the secrets on an image, and the size of each share is the same as that of the original image without any expansion. The smaller size of shares makes their further processing such as storage and/or printing out more efficient. [ABSTRACT FROM AUTHOR]

Abstract: Image sharing addresses a fault-tolerant protection mechanism for important images. In a typical image sharing scheme, the generated shadow images usually have a noise-like appearance that conceals the secret image completely, but this makes them difficult to identify and manage. This paper proposes a scalable user-friendly image sharing scheme in which each generated shadow image looks like a shrunken replica of the original image. The scheme provides an easy-to-identify interface for managing the shadow images. Compared with previous user-friendly image sharing schemes, the proposed method can reconstruct the original image without any loss and still has the small-size shadow images. A notable feature is that the qualities of generated shadow images are scalable in the proposed scheme, which allows the quality of the shadow images to be adjusted according to the requirements of diverse applications. [Copyright &y& Elsevier]

This paper proposes a method for hiding an important image in a cover image whose size is limited. In this method, in order to save space, a modified search-order coding (MSOC) technique first transforms the important image, then, a randomization procedure permutes the transformed image to further increase the security. Finally, a modulus function embeds the permuted code in a cover image; notably, in the modules function, the modulus base used for a pixel is determined according to the variance of its neighboring pixels. Experimental results show that the images are of high quality. Comparisons with reported methods are provided. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a novel image-hiding method that exhibits a high hiding capacity that allows the embedded important image to be larger than the cover image, a facility that is seldom described in the literature. In the proposed method, the entire important image is divided into many nonoverlapping blocks. For each block of the important image, a block-matching procedure is used to search for the best similar block from a series of numbered candidate blocks. The obtained indices of the best-matching blocks are encoded using Huffman coding scheme, and then recorded in the least-significant-bit planes of the cover image through a monoalphabetic transposition cipher. The proposed method exhibits the following advantages over existing methods: (1) a high hiding capacity such that the embedded important image can be larger than the cover image; (2) a stego-image with a high quality, which improves the secrecy of the hidden image; and (3) a small error between the extracted important image and the original important image indicating that the extracted important image is of acceptable quality. [Copyright &y& Elsevier]

Abstract: Secret image sharing is a technique for protecting images that involves the dispersion of the secret image into many shadow images. This endows the method with a higher tolerance against data corruption or loss than other image-protection mechanisms, such as encryption or steganography. In the method proposed in this study, the difference image of the secret image is encoded using Huffman coding scheme, and the arithmetic calculations of the sharing functions are evaluated in a power-of-two Galois Field GF(2 t ). Experiment results show that each generated shadow image in the proposed method is about 40% smaller than that of the method in [Thien, C.C., Lin, J.C., 2002. Secret image sharing. Comput. Graphics 26 (1), 765–770], which improves its efficiency in storage, transmission, and data hiding. [Copyright &y& Elsevier]

Abstract: The secret sharing schemes in conventional visual cryptography are characterized by encoding one shared secret into a set of random transparencies which reveal the secret to the human visual system when they are superimposed. In this paper, we propose a visual secret sharing scheme that encodes a set of 2 secrets into two circle shares such that none of any single share leaks the secrets and the x secrets can be obtained one by one by stacking the first share and the rotated second shares with x different rotation angles. This is the first true result that discusses the sharing ability in visual cryptography up to any general number of multiple secrets in two circle shares. [Copyright &y& Elsevier]