Though often invisible, cryptography plays a critical role in our everyday lives. Broadly defined as a set of tools for establishing security in cyberspace, cryptography enables us to protect our information and share it securely. It underpins the security of mobile phone calls, card payments, web connections, internet messaging, Bitcoin transactions—in short, everything we do online. Clearly and concisely, Keith Martin reveals the many crucial ways we all rely on cryptographic technology and demystifies its controversial applications and the nuances behind alarming headlines about data breaches or Edward Snowden. Essential reading for anyone with a password, Cryptography offers a profound perspective on personal security, online and off.