Secure digital imaging is an important research area combining methods and techniques coming from cryptography and image processing. Visual cryptography and in general secret image sharing techniques enable distributing sensitive visual materials to involved participants through public communication channels, as the generated secure images do not reveal any information if they are not combined in the prescribed way. In visual cryptography, the decoding process is performed directly by the human eyes, while in general, the shared images need some processing to reconstruct the secret image. The increasing number of possibilities to create, publish, and distribute images calls for novel protection methods, offering new sharing and access control mechanisms for the information contained in the published images. Secure image sharing techniques overcome the traditional cryptographic approach, providing new solutions for the development of new and secure imaging applications.

Since both digital imaging and privacy protection are two research themes that involve a continuously growing number of people and applications, the book focuses on secure image sharing techniques, offering an interesting reference point for studying and developing solutions needed in such areas. This book aims to fill the existing literature gap, providing a valid guide for professional people and/or researchers interested in such fields of research. It provides a scientifically and scholarly sound treatment of state-of-the-art techniques to students, researchers, academics, and practitioners who are interested or involved in the study, research, use, design, and development of image sharing techniques. It can serve as a guide for making use of visual cryptographic algorithms and as a basis to develop applications based on image secret sharing solutions.

The book has the form of a contributed volume, where well-known experts address an extensive range of topics related to visual cryptography techniques and solutions for secure image sharing. Besides introducing the relevant issues of such research fields, and providing a coverage of theoretical results, the book illustrates some aspects and interesting results concerning recent research directions. It covers the most prominent topics in the area, such as the possibility of sharing multiple secrets, visual cryptography schemes based on the probabilistic reconstruction of the secret image, possibility to include pictures in the distributed shares, contrast enhancement techniques, visual cryptography schemes based on different logical operations for combining the shared images, color images visual cryptography, cheating prevention, alignment problem for image shares, and the description of some practical applications of visual cryptography. In addition to the above, steganography and authentication are discussed, and different methods of image sharing are presented, including mathematical and probabilistic techniques for generating the image shares.

Each chapter provides the fundamentals for the topics under consideration and the detailed description of the relevant methods, presenting examples and practical applications to demonstrate the effectiveness of the surveyed techniques. Chapters 1–2 introduce visual cryptography and its application to halftone and color images, respectively. The techniques presented in these chapters focus on the problem of providing high quality shares using halftoning, and providing an adaptation of visual cryptography to color images, resolving the problem of darkening caused from the superposition of the shares in the reconstruction phase. Chapter 3 presents an extension of the general technique useful for sharing multiple images using the same set of shares. Chapter 4 shows how visual cryptography schemes can be applied to photograph images, obtaining interesting results for practical applications.

Chapters 5–7 focus on different extensions of basic visual cryptography technique. Probabilistic visual cryptography schemes trade the guarantee of a correct reconstruction of the secret image with a reduced pixel expansion of the generated shares. In Chapter 6 the basic superposition mechanism used for the reconstruction of the secret image is abandoned for a XOR- based reconstruction phase, improving on the contrast of the reconstructed image. Chapter 7 presents random grid techniques, which are derived from visual cryptography, but provide a reduced pixel expansion in the generated shares and do not need a basis matrix in the generation phase of the shares.

Chapters 8–9 focus on the contrast of the reconstructed images. In Chapter 8 the bounds on the contrast of the most known schemes are discussed. In Chapter 9 techniques based on the reversing of the images are presented, where black pixels are reversed to white ones and vice versa. Using such an operation, schemes with perfect reconstruction of both white and black pixels can be derived, avoiding any loss of resolution for the shared image.

Chapters 10–12 are more focused on the problems to be faced in the practical applications of visual cryptography schemes. One is cheating prevention, as discussed in Chapter 10, where the problem of malicious participants to the schemes is discussed. The other problem relates to the alignment of the shares in the reconstruction phase. In general, techniques to prevent the alignment of the shares and introduce some degree of tolerance in the reconstruction phase are discussed. Finally, Chapter 12 presents some applications of visual cryptography in the field of securing online transactions and e-voting.

Chapters 13–17 are more focused on general image sharing techniques. Image sharing in halftone images is discussed in Chapter 13, where two general techniques of data hiding are presented and analyzed. An algorithm for color image sharing is presented in Chapter 14, where interpolation is used for hiding secret images. Image sharing techniques based on polynomial secret sharing are discussed in Chapter 15. Chapter 16 discusses steganography and authentication in the context of image sharing, presenting some novel schemes. Finally, Chapter 17 presents a novel technique for image sharing, where the reconstruction phase can be performed both via the superposition of the shares, or via some computation.

In all the chapters, figures, tables, and examples are used to improve the presentation and the analysis of the discussed methods and techniques. Bibliographic references are included in each chapter and provide a good starting point for deeper research and further exploration of the topics covered in this volume.