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High-rate data transmission has become ubiquitous due to the abundant availability of internet facilities. This also makes information security vital to protecting against data theft. Traditionally, encryption has been used to successfully address such threats. This Spotlight introduces some classical information-security approaches, e.g., steganography, watermarking, and digital cryptographic methods. In this context, optical signal processing (OSP)-based encryption systems have recently received a great deal of attention because of their inherent advantages, such as natural 2-D imaging capabilities, multiple degrees of freedom, and high-speed parallelism. This book provides an overview of optical encryption from the perspective of image (both 2-D and 3-D) and optical-information processing. Section 3 introduces the classical Fourier-transform-based optical encryption method, i.e., double random phase encryption (DRPE), and several of its extensions. Section 4 discusses the conventional implementations (holographic and non-holographic) of some of the encryption systems introduced in Section 3. Section 5 briefly covers low-light-level (photon counting)-based encryption system implementations. Finally, Section 6 briefly talks about the cryptanalysis approaches used in various attempts to crack or break some of the classical encryption systems presented. Both experimental and synthesized results (images) are provided in the text for illustration. Wherever possible (space permitting), background information is provided, although some knowledge of advanced mathematics and image processing, and familiarity with standard scientific computing languages, is needed for complete comprehension.
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