Optical Cryptosystems. Naveen K. Nishchal

NPCR Number of pixel change rate OAC Optical asymmetric cryptosystem OD Optical device PCE Peak-to-correlation energy PCF Phase contrast filter PCI Photon counting imaging PD Plastic diffuser PI Peak intensity PK Private key POCSA Projection-onto constraints sets algorithm POF Phase-only function POM Phase-only mask PPM Plasmonic phase mask PRA Phase retrieval algorithm PRX Photorefractive crystal PSDOE Polarization selective diffractive optical element PSI Phase-shifting interferometry PSNR Peak signal-to-noise ratio PSR Peak-to-sidelobe ratio PT Phase-truncated PTFT Phase-truncated Fourier transform QPS Quadratic phase system QR Quick response code QWP Quarter wave plate RAM Random amplitude mask RE Relative error RGB Red, green, blue RP Retardation plate RPM Random phase mask RSAA Rivest, Shamir, Adleman algorithm SAA Simulated annealing algorithm SC Symmetric cryptosystems SHA Secure hash algorithm SLM Spatial light modulator SNR Signal-to-noise ratio SPM Structured phase mask SSE Sum squared error SWG Subwavelength grating UACI Unified average change in intensity VAR Variance VLC VanderLugt correlator WT Wavelet transform XOR Exclusive OR

      IOP Publishing

      Optical Cryptosystems

      Naveen K Nishchal

      Chapter 1

      Digital techniques of data and image encryption

      1.1 Introduction

      Information security is of paramount importance in today’s digitally connected world. This is also called the digital era, in which the encryption is being considered as a fast-moving trend. Though advanced modern information security tools, storage, and retrieval mechanisms have been developed there are still enormous challenges posed by hacking tools, unsecure transmission channels, and ubiquity of the Internet. Therefore, there has been a rise in cyber security challenges globally, hence the users must be cyber prepared. Cyber security is impacting the industry. With the advent of advanced technologies such as Internet-of-Things, Cloud Computing, and Artificial Intelligence, it is envisaged that billions of devices would be connected. While such technologies provide several opportunities, they also pose threats to information security. Today most of the global web traffic is encrypted and it is expected that in future almost all the global web traffic will be fully encrypted. While this has enabled much greater privacy and helped prevent data breaches, cyber criminals are using these encrypted channels to propagate malware and exfiltrate data knowing that they can bypass traditional security inspection solutions that do not decrypt traffic [1–4].

      The art and science of concealing information/data is called cryptography. The information/data/message to be concealed is called a plaintext (clear text) and the concealed form of message is called a ciphertext (encrypted text). In other words, cryptography is a process of converting plaintext into ciphertext and vice versa. The process of conversion from plaintext to ciphertext is called encryption and the reverse process that retrieves plaintext from ciphertext is called decryption. The ciphertext is a message that cannot be understood by anyone or is a meaningless message. A cipher is an algorithm used for encryption and decryption. The ciphertext is stored and transmitted to the intended user. The cryptography is not only used for protecting the information from theft or alteration but it is also used for user authentication [5–7].

      A cryptosystem, also referred to as a cipher system, is an implementation of cryptographic techniques and their accompanying infrastructure to provide information security services. 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. Therefore, cryptology is considered as a young science. Julius Caesar (around 100 BC) was known to use a form of encryption to convey secret messages to his Army Generals. The substitution cipher, known as the Caesar cipher is probably the most mentioned historic cipher in academic literature [3]. In this method, each character of a plaintext is substituted by another character to form the ciphertext. The variant used by Caesar was a shift by three ciphers. Each character was shifted by three places, so the character ‘A’ was replaced by ‘D’ and character ‘B’ was replaced by ‘E’ and so on. The characters would wrap around at the end, so character ‘X’ would be replaced by ‘A’. An example of the character substitution based on Caesar’s algorithm has been shown in figure 1.1.

      Figure