Yuliang Lu

Progressive Visual Secret Sharing for General Access Structure with Multiple Decryptions

Visual secret sharing (VSS) for general access structure (GAS) owns wider applications than (k,n) threshold VSS. VSS with multiple decryptions realizes the functionalities of both OR-based VSS (OVSS) and XOR-based VSS (XVSS), which can broaden the applications compared to one recovery method-based VSS. In this paper, we propose a progressive VSS (PVSS) scheme for GAS with the features of both OR and XOR decryptions based on random grid (RG). The different regions of the secret image and corresponding genearted random bits are employed to gain progressive property as well as GAS with OR and XOR decryptions. For the qualified sets, we can reconstruct the secret by stacking. On the other hand, if a device with XOR operation is available, we can improve the visual quality of the recovered secret image as well as reconstruct the secret image losslessly when we collect all the n shares. In addition, our scheme has neither pixel expansion nor codebook design due to RG. The effectiveness of the proposed scheme is shown in terms of experimental results and analyses.

Exploiting the Homomorphic Property of Visual Cryptography

In this paper, homomorphic visual cryptographic scheme HVCS is proposed. The proposed HVCS inherits the good features of traditional VCS, such as, loss-tolerant e.g., k, n threshold and simply reconstructed method, where simply reconstructed method means that the decryption of the secret image is based on human visual system HVS without any cryptographic computation. In addition, the proposed HVCS can support signal processing in the encrypted domain SPED, e.g., homomorphic operations and authentication, which can protect the users privacy as well as improve the security in some applications, such as, cloud computing and so on. Both the theoretical analysis and simulation results demonstrate the effectiveness and security of the proposed HVCS.

Clarity Corresponding to Contrast in Visual Cryptography

The visual quality of the recovered secret image is usually evaluated by contrast in visual cryptography (VC). Precisely, the reconstructed secret image can be recognized as the original secret image when contrast is greater than zero. It is important for the user to know the clarity of the revealed secret image corresponding to different contrast values. In this paper, the clarity corresponding to contrast is investigated through conducting subjective evaluation scores and objective evaluation indexes, which can be extended to general applications.

Greyscale-images-oriented progressive secret sharing based on the linear congruence equation

Secret image sharing (SIS) can be applied to protect a secret image when the secret is transmitted in public channels. However, classic SIS schemes, e.g., visual secret sharing (VSS) and Shamir’s polynomial-based scheme, are not suitable for progressive encryption of greyscale images, because they will lead to many problems, such as “All-or-Nothing”, lossy recovery, complex computations and so on. Based on the linear congruence equation, three novel progressive secret sharing (PSS) schemes are proposed to overcome these problems: (k, k) threshold LCSS and (k, n) threshold LCPSS aim to achieve general threshold progressive secret sharing with simple computations. Furthermore, extended LCPSS (ELCPSS) is developed to generate meaningful shadow images, which enable simple management and misleading the enemy. Both theoretical proofs and experimental results are given to demonstrate the validity of the proposed scheme.

Chinese Remainder Theorem-Based Secret Image Sharing for (k, n) Threshold

In comparison with Shamir’s original polynomial-based secret image sharing (SIS), Chinese remainder theorem-based SIS (CRTSIS) overall has the advantages of lossless recovery, low recovery computation complexity and no auxiliary encryption. Traditional CRTSIS methods generally suffer from no (k, n) threshold, lossy recovery, ignoring the image characteristics and auxiliary encryption. Based on the analysis of image characteristics and SIS, in this paper we propose a CRTSIS method for (k, n) threshold, through dividing the gray image pixel values into two intervals corresponding to two available mapping intervals. Our method realizes (k, n) threshold and lossless recovery for gray image without auxiliary encryption. Analysis and experiments are provided to indicate the effectiveness of the proposed method.

Partial secret image sharing for (k,n) threshold based on image inpainting

Shamir’s polynomial-based secret image sharing (SIS) scheme and visual secret sharing (VSS) also called visual cryptography scheme (VCS), are the primary branches in SIS. In traditional (k, n) threshold secret sharing, a secret image is fully (entirely) generated into n shadow images (shares) distributed to n associated participants. The secret image can be recovered by collecting any k or more shadow images. The previous SIS schemes dealt with the full secret image neglecting the possible situation that only part of the secret image needs protection. However, in some applications, only target part of the secret image may need to be protected while other parts may be not in a full image. In this paper, we consider the partial secret image sharing (PSIS) issue as well as propose a PSIS scheme for (n, n) threshold based on image inpainting and linear congruence (LC). First the target part is manually selected or marked in the color secret image. Second, the target part is automatically removed from the original secret image to obtain the same input cover images (unpainted shadow images). Third, the target secret part is generated into the pixels corresponding to shadow images by LC in the processing of shadow images texture synthesis (inpainting), so as to obtain the shadow images in a visually plausible way. As a result, the full secret image including the target secret part and other parts will be recovered losslessly by adding all the inpainted meaningful shadow images. Experiments are conducted to evaluate the efficiency of the proposed scheme.

Security Analysis of Secret Image Sharing

Differently from pure data encryption, secret image sharing (SIS) mainly focuses on image protection through generating a secret image into n shadow images (shares) distributed to n associated participants. The secret image can be reconstructed by collecting sufficient shadow images. In recent years, many SIS schemes are proposed, among which Shamir’s polynomial-based SIS scheme and visual secret sharing (VSS) also called visual cryptography scheme (VCS) are the primary branches. However, as the basic research issues, the security analysis and security level classification of SIS are rarely discussed. In this paper, based on the study of image feature and typical SIS schemes, four security levels are classified as well as the security of typical SIS schemes are analyzed. Furthermore, experiments are conducted to evaluate the efficiency of our analysis by employing illustrations and evaluation metrics.

Visual Secret Sharing Scheme with (k, n) Threshold Based on QR Codes

In this paper, a novel visual secret sharing (VSS) scheme with using QR codes is investigated. The proposed visual secret sharing scheme based on QR codes(VSSQR) can visually reveal secret image by stacking k or more shares (shadow images) from all the n QR codes as well as scan the QR code by a QR code reader. Our VSSQR exploits the error correction mechanism in the QR code structure, to embed the bits corresponding to shares generated by VSS from a secret bit into the same locations of QR codes in the processing of encoding QR. Each output share is a valid QR code, which may reduce the likelihood of attracting the attention of potential attackers, that can be scanned and decoded utilizing a QR code reader. The secret image can be recovered by stacking for case (k, n) based on the human visual system without any computation. In addition, it can assist alignment for VSS recovery. The experiment results show the effectiveness of our scheme.

Random Grids-Based Threshold Visual Secret Sharing with Improved Visual Quality

Visual secret sharing (VSS) by random grids (RG) has gained much attention since it avoids the pixel expansion problem as well as requires no codebook design. However, most of the previous RG-based threshold VSS still suffer from low visual quality or worse reconstructed secrets when more shares are stacked. In this paper, a new RG-based VSS with improved visual quality is proposed. The random bits are utilized to improve the visual quality as well as to decrease the darkness of the reconstructed secret image in the proposed scheme. Experimental results and analyses show the effectiveness of the proposed scheme.

Quality-adaptive threshold visual secret sharing by random grids

Visual secret sharing (VSS) by random grids (RG) has gained much attention since it can avoid pixel expansion problem without codebook design. However, the previous RG-based threshold VSS schemes still suffer from low visual quality or no threshold. Even the more shares are stacked, the worse reconstructed secrets are revealed. In this paper, a new quality-adaptive threshold RG-based VSS with improved and progressive visual quality is proposed. The proposed scheme decreases the black bits in the white area of the stacked image without white bits in the black area of the stacked image. Experimental results and security analyses show the effectiveness of the proposed scheme.