This guide uncovers the technology behind network security: its strengths, weaknesses, past, and future. It answers fundamental questions like:   As a tutorial, it explains sophisticated concepts in a friendly and intuitive manner. As a reference, it covers concepts and techniques rigorously and in depth. The authors cover a wide spectrum of topics essential for securing web-based transactions, including public and secret key cryptography, hashes/message digests, signatures, authentication, blockchains, electronic money, secret sharing, and multiparty computation. They also address exciting emerging issues such as quantum computing, post-quantum algorithms, homomorphic encryption, and secure multiparty computation. Wherever math beyond high school algebra is needed,   covers what students need to know, making it a self-contained solution suitable for undergraduate students, graduate students, and working engineers alike. To support learning and mastery, it also includes extensive homework problems, fully updated to reflect current concepts and technologies.