This book introduces the reader to the most advanced topics of physical-layer security (PLS), cryptography, covert/stealth communications, and quantum key distribution (QKD), also known as the quantum cryptography, and post-quantum cryptography (PQC). So far, these topics have been considered as separate disciplines, even though they are targeting the same security problems we are facing today. The book integrates modern cryptography, physical-layer security, QKD, covert communication, PQC, and cyber security technologies. 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. Unifies conventional cryptography, physical-layer security, QKD, and PQC; Requires no prerequisite material; all background material is provided in the Appendix chapter; Prepares readers to perform independent research in any of the areas listed above. .