An introduction to one of the backbones of the digital world. Cryptography is one of the most important aspects of information technology security, central to the protection of digital assets and the mitigation of risks that come with increased global connectivity. The digital world is wholly reliant on secure algorithms and protocols for establishing identity, protecting user data, and more. Groundbreaking recent developments in network communication and a changing digital landscape have been accompanied by similar advances in cryptography, which is more central to digital life than ever before. Cryptography is the discipline at the intersection of Computer Science and mathematics. It provides algorithms for guaranteeing confidentiality, integrity, authentication, and non-repudiation for parties that share data or exchange messages to perform operations and transactions in cyberspace. This book constitutes a comprehensive yet accessible introduction to the algorithms, protocols, and standards which protect the modern internet. Built around both foundational theories and hundreds of specific algorithms, it also incorporates the required skills in complex mathematics. The result is an indispensable introduction to the protocols and systems which should define cryptography for decades to come.