Cryptography is a vital technology that underpins the security of information in computer networks. This book presents a comprehensive introduction to the role that cryptography plays in providing information security for everyday technologies such as the Internet mobile phones Wi-Fi networks payment cards Tor and Bitcoin. This book is intended to be introductory self-contained and widely accessible. It is suitable as a first read on cryptography. Almost no prior knowledge of mathematics is required since the book deliberately avoids the details of the mathematics techniques underpinning cryptographic mechanisms. Instead our focus will be on what a normal user or practitioner of information security needs to know about cryptography in order to understand the design and use of everyday cryptographic applications. By focusing on the fundamental principles of modern cryptography rather than the technical details of current cryptographic technology the main part this book is relatively timeless and illustrates the application of these principles by considering a number of contemporary applications of cryptography. Following the revelations of former NSA contractor Edward Snowden the book considers the wider societal impact of use of cryptography and strategies for addressing this. A reader of this book will not only be able to understand the everyday use of cryptography but also be able to interpret future developments in this fascinating and crucially important area of technology.