Tzung-Her Chen

Threshold visual secret sharing by random grids

A new visual secret sharing (VSS) approach by random grids (RG-based VSS), proposed by Kafri and Keren (1987), has drawn close attention recently. With almost all advantages of visual cryptography-based VSS, RG-based VSS benefits more from keeping the same size of secret images without the problem of pixel expansion from which VC-based VSS suffer. In this paper, a threshold RG-based VSS scheme aiming at providing the wide-use version is presented. This is the first effort to develop the technique. The experimental results and theoretically analysis in visual quality and security show that the proposed scheme performs well.

Visual secret sharing by random grids revisited

Recently, the visual secret sharing (VSS) technique by random grids (RG), proposed by Kafri and Keren in 1987, has drawn attention in academia again to remove the abovementioned drawbacks. However, Kafri and Kerens scheme is a 2-out-of-2 VSS scheme but neither n-out-of-n nor 2-out-of-n(n>2). In this paper, novel n-out-of-n (Method 1) and 2-out-of-n (Method 2) secret image sharing schemes based on RG are proposed to encrypt the secret into n cipher-grids without pixel expansion and additional codebook required. In the decryption phase, while participants superimpose all (Method 1) or at least two (Method 2) cipher-grids without any extra computation, the secret is recognizable by the human visual system. To our best knowledge, this paper is the first attempt in the literature to develop new RG-based VSS schemes by means of extending the basic 2-out-of-2 scheme to the n-out-of-n as well as 2-out-of-n ones. To demonstrate the correctness of the proposed schemes, the formal analysis is given while the experimental results show the proposed schemes do work well.

Cheating in Visual Cryptography

A secret sharing scheme allows a secret to be shared among a set of participants, P, such that only authorized subsets of P can recover the secret, but any unauthorized subset cannot recover the secret. In 1995, Naor and Shamir proposed a variant of secret sharing, called visual cryptography, where the shares given to participants are xeroxed onto transparencies. If X is an authorized subset of P, then the participants in X can visually recover the secret image by stacking their transparencies together without performing any computation. In this paper, we address the issue of cheating by dishonest participants, called cheaters, in visual cryptography. The experimental results demonstrate that cheating is possible when the cheaters form a coalition in order to deceive honest participants. We also propose two simple cheating prevention visual cryptographic schemes.

A cheating prevention scheme for binary visual cryptography with homogeneous secret images

In 1995, Naor and Shamir proposed the k-out-of-n visual cryptography scheme such that only more than or equal to k participants can visually recover the secret through superimposing their transparencies. Visual cryptography schemes have been extensively investigated since their invention and extended to numerous applications such as visual authentication and identification, steganography, and image encryption. In 2006, Horng et al. proposed that cheating is possible where some participants can deceive the remaining participants by delivering forged transparencies. Meanwhile, Horng et al. also proposed two cheating prevention schemes. One scheme, however, requires extra verification transparencies and the other needs larger transparencies. In other words, compared to visual cryptography, both schemes burden each participant with an additional problem of transparency management. In this paper, a more secure scheme is given to solve the cheating problem without extra burdens by adopting multiple distinct secret images. Moreover, for sharing these secret images simultaneously, the share construction method of visual cryptography is redesigned and extended by generic algorithms. Finally, the results of the experiment and security analysis show that not only the proposed scheme is more secure in comparison with the two previous cheating prevention schemes in the literature, but extra burdens are also eliminated.

A publicly verifiable copyright-proving scheme resistant to malicious attacks

A wavelet-based copyright-proving scheme that does not require the original image for logo verification is proposed in this paper. The scheme is strong enough to resist malicious manipulations of an image including blurring, JPEG compression, noising, sharpening, scaling, rotation, cropping, scaling-cropping, and print-photocopy-scan attacks. The proposed scheme is also resistant to StirMark and unZign attacks and it is not only a robust method but also a lossless one. Experiments are conducted to show the robustness of this method. Moreover, cryptographic tools, such as digital signature and timestamp, are introduced to make copyright proving publicly verifiable.

Efficient multi-secret image sharing based on Boolean operations

(n,n) visual secret sharing (VSS), first proposed by Naor and Shamir (1995) [4], is used to encode (encrypt) a secret image into n meaningless share images to be superimposed later to decode (decrypt) the original secret by human visual system after collecting all n secret images. In recent years, VSS-based image sharing (encryption) and image hiding schemes, two of a variety of applications based on VSS, have drawn much attention. In this paper, an efficient (n+1,n+1) multi-secret image sharing scheme based on Boolean-based VSS is proposed to not only keep the secret images confidential but also increase the capacity of sharing multiple secrets. The Boolean-based VSS technology, used to encode the secret images, generates n random matrices; then the n secret images are subsequently encoded into the n+1 meaningless share images. It is worthwhile to note that n secret images can be hidden by means of sharing only n+1 share images in the proposed scheme instead of 2n share images. Thus, the present scheme thus benefits from (1) reducing the demand of image transmission bandwidth, (2) easing the management overhead of meaningless share images, and (3) involving neither significant extra computational cost nor distortion for reconstructed secret images. The experimental results show the performance in terms of feasibility and image sharing capacity. Applied into image hiding schemes, the proposed scheme can enhance the hiding capacity.

A public verifiable copy protection technique for still images

It is well known that a digital signature scheme and digital timestamp are two well-defined security methods for copyright protection. However, these techniques are not suitable for directly dealing with digital images. For this reason, digital watermarking has received considerable attention for copyright protection. Unfortunately, there are still some challenges in the proposed watermarking schemes. Here, we will introduce cryptographic tools into watermarking, in such a way that both the advantages of cryptographic tools and digital watermarking are available. The publicly verifiable property and tolerant distortion property are therefore possible. Based on this idea, a lossless and robust copyright-protection scheme, distinct from conventional watermarking techniques, is proposed. Our scheme is strong enough to resist common image processing, geometric distortions and some intentional attacks. Some of the attacks are still challenges for the proposed watermarking methods. A series of experiments are conducted to prove the robustness property. The StirMark and unZign attacks are adopted as benchmark verification.

A novel secret image sharing scheme for true-color images with size constraint

Rapid development of telecommunication and service has made researchers think of intelligent tools to assist users in delivering critical data securely. When it comes to share digital images, owing to high frequent use of Mega pixel digital cameras or camera phones, true-color images become one common image type. In the last few years, several researches have been devoted to study of secret image sharing. What seems lacking is a scheme for sharing true-color secret images with size constraint. This paper proposes a new secret image sharing scheme for true-color secret images. Through combination of neural networks and variant visual secret sharing, the quality of the reconstructed secret image and camouflage images are visually the same as the corresponding original images. Compared with other schemes, the proposed one alone supports true-color secret image with size constraint on shares. Experimental results and comparisons demonstrate the feasibility of this scheme.

A round- and computation-efficient three-party authenticated key exchange protocol

In three-party authenticated key exchange protocols, each client shares a secret only with a trusted server with assists in generating a session key used for securely sending messages between two communication clients. Compared with two-party authenticated key exchange protocols where each pair of parties must share a secret with each other, a three-party protocol does not cause any key management problem for the parties. In the literature, mainly there exist three issues in three-party authenticated key exchange protocols are discussed that need to be further improved: (1) to reduce latency, communication steps in the protocol should be as parallel as possible; (2) as the existence of a security-sensitive table on the server side may cause the server to become compromised, the table should be removed; (3) resources required for computation should be as few as possible to avoid the protocol to become an efficiency bottleneck. In various applications over networks, a quick response is required especially by light-weight clients in the mobile e-commerce. In this paper, a round- and computation-efficient three-party authenticated key exchange protocol is proposed which fulfils all of the above mentioned requirements.

Multi-image encryption by circular random grids

Visual secret sharing (VSS) can encode a secret image into several share images where the original secret can be reconstructed and recognized by sight by stacking all share images. There are two categories of VSS schemes: visual cryptography (VC) and random grids (RG). VC has three main drawbacks: the large size, the need for the codebook to be redesigned for specific applications, and the ability to encode only one secret image at a time. RG removes the first two drawbacks. This paper proposes a novel RG-based VSS scheme that encodes multiple secret images at a time. The scheme encrypts multiple secret images into two circular cipher-grids and decrypts the images by stacking two circular cipher-grids to obtain the first secret and gradually rotating one circular cipher-grid at a fixed degree (based on the quantity of the secret images encrypted) to disclose other secrets. Compared with conventional VC-based VSS, the proposed scheme has no pixel expansion, a higher capacity for secret sharing, and no need for a complex VC codebook to be redesigned. Theoretical analysis of visual quality and security is demonstrated.