“Appa Deepo Bhavah”

      Be a Light unto Yourself.

      Gautama Buddha

Preface

      In the digital era of contemporary society, information in any form, such as a message, text, data, image, audio, or video, can be treated as wealth. Therefore, securing information is as important as protecting property. In the history of the human race, the significance of security in one form or the other can easily be traced. Though cryptographic techniques have been in use for protecting information for thousands of years, the systematic study of cryptology as a science started around one hundred years ago. Julius Caesar (around 100 BC) was known to use a form of encryption to convey secret messages to his Army Generals. In modern times, digital techniques of information security are already in use wherein there exists scope for further improvements in terms of security level and computation cost.

      Owing to the unique features of light, such as parallel processing, high speed, and several degrees of freedom, it is envisaged that information can be highly secured and communicated to the intended recipients or authentic users employing optical technologies. It can be foreseen that with the multifaceted uses of advanced technologies, such as Artificial Intelligence, Big Data, Cloud Computing, and Internet-of-Things, security will always remain an important challenge. Technologies provide several opportunities, but, at the same time, they also pose threats to information theft or misuse. Searching for a cyber expert or the attackers who attacked the digital algorithm would be very hard, because they can exist in large numbers anywhere in the world. On the other hand, finding out an attacker in the optics domain would be relatively easier. The security can be in terms of storage, in dissemination of the message, communication/transmission over conventional channels, protection of copyright/ownership, and steganography. Therefore, developments of newer alternative technologies are required to meet the challenges in the domains of scientific investigation.

      This book intends to provide a collection of optical technologies for secure storage, secure communication, and the protection of copyright in terms of watermarking. Most of the optical techniques reported in literature can be traced around a double random phase encoding algorithm. Furthermore, many variants of this scheme have been proposed and demonstrated with improvements and different levels of complexity. This book aims to provide help to researchers in the field to get first-hand information of its progress.

      This book starts with a general discussion on digital algorithms already in use in chapter 1 with more emphasis on the principles of optical techniques for image/data security in chapter 2. The growth of literature on optical technologies has been exponential with the publication of the first report in 1995. A bar chart has been provided that shows the growth of the literature. Use of fully-phased data provides additional security and robustness against noise, therefore such techniques have been dealt with in chapter 3. There is another aspect associated with security that is called authentication, in which the retrieval of original information is not intended. This can be solved with the use of an optical correlator, called a joint transform correlator, which is discussed in chapter 4. Optical techniques of watermarking and hiding are discussed in chapter 5. Polarization is one of the important properties of light, which is suited to developing a practical system because in this case the parameter that is dealt with is intensity, not the phase. Therefore, storage and transmission of intensity data is easier than phase-only information. This has been detailed in chapter 6.

      Digital holography helps record 3D data and recording with digital sensors offers advantages in image/data security. The digital holograms can be stored in a personal computer and transmitted anywhere in the world and can be numerically reconstructed at any point of time. This has been discussed in chapter 7. Processing and security of multispectral data is very important in many applications, particularly in defence, remote sensing, and surveillance. This has been discussed in chapter 8. Chaos has always been very attractive in cryptographic studies in key design. Chapter 9 has been devoted to this topic, which has the ability to combine with other optical technologies. Phase retrieval techniques are important in regenerating object-dependent phase keys used for securing data. There are different algorithms reported in literature, which find use in image security. This has been dealt with in chapter 10.

      No cryptographic technique can be considered very strong and useful unless cryptanalysis is carried out. There are several types of attacks reported in literature, which have been stated in terms of optical technologies in chapter 11. The optical technologies differ with digital counterparts, whereby in optical schemes either physical keys are used or keys are designed considering physical parameters as compared to digital keys used in electronic systems. There are various types of keys implemented in optical methods, which are discussed in chapter 12.

      In all the chapters, the basic principles have been explained with examples. In some of the chapters, numerical simulation results have been provided for better understanding of the subject. Considering the requirement on some of the relevant topics, MATLAB codes have been provided. At the end of each chapter, a list of relevant literature has been provided.

      The book is open to comments, criticisms, and suggestions from the readers in improving the quality of the book for future editions.

Acknowledgement

      First of all, I would like to thank my mentors at the Indian Institute of Technology (IIT) Delhi during my doctoral studies. I especially thank my PhD supervisors, Professor Kehar Singh and Professor Joby Joseph, for introducing me to the exciting world of optical information processing research. Over the years they have been much more than just a thesis advisor to me. I am sure that they will be happy to see this book out in print. I must thank Dr G Unnikrishnan from IRDE Dehradun with whom I have had numerous discussions. I wish to extend my sincere thanks to Dr A K Gupta, Ex-Director IRDE Dehradun and Professor R S Sirohi, Ex-Director, IIT Delhi, for their encouragement.

      My colleagues at IIT Patna deserve due acknowledgments for their encouragement. I acknowledge my gratitude to IIT Patna for providing facilities and a congenial environment. I must thank my colleagues from abroad, Professor Bahram Javidi, University of Connecticut; Professor John T Sheridan, University College Dublin; Professor Thomas J Naughton, Maynooth University; Professor Ayman AlFalou, ISEN/Yncrea; Professor Christian Brosseau, Universite Britagne Occidentale; Professor Cornelia Denz, Universitat Muenster; Professor Yan Zhang, Capital Normal University; Professor Takanori Nomura, Wakayama University; Professor Osamu Matoba, Kobe University; Professor Guohai Situ, Shanghai Institute of Optics and Fine Mechanics; and Professor