Cryptography is the practice and study of hiding information. Modern cryptography intersects the disciplines of mathematics, computer science, and electrical engineering. Applications of cryptography include ATM cards, computer passwords, and electrical commerce. In this book, the authors present current research in the study of the protocols, design and application of cryptography. Topics discussed include quantum cryptography protocols and quantum security; visual cryptography for halftone images; mathematical cryptography of the RSA cryptosystem; multi-layer QKD protocol using correlated photon of dark soliton array in a wavelength router and low-cost mutual authentication protocols.