This updated textbook introduces the reader to the most advanced topics of physical-layer security, cryptography, covert/stealth communications, quantum key distribution (QKD), also known as quantum cryptography, as well as postquantum cryptography. The book, which does not require a prerequisite, integrates conventional cryptography, physical-layer security, QKD, and postquantum cryptography. The book offers in-depth exposition on cryptography, information-theoretic approach to cryptography, physical-layer security, covert/stealth/low-probability of detection communications, quantum information-theory, QKD, and postquantum cryptography; to mention few. The author provides a unified description of the most advanced topics related to: (i) modern cryptography, (ii) physical-layer security, (iii) QKD, (iv) covert and low probability of intercepting (LPI) communications, and (v) postquantum cryptography. Each chapter of the book contains comprehensive problems to help deeper understanding and develop problem solving skills. Relevant chapters also contain detail on various experimental demonstrations. The book is intended for a very diverse group of readers in communications engineering, optical engineering, wireless communications, free-space optical communications, optical wireless communications, mathematics, physics, communication theory, information theory, photonics, as well as computer science
