The Java and .NET frameworks contain all the algorithms you need to keep your users' data secret from prying eyes. Web servers like Apache, Tomcat, and IIS, combined with tools like OpenSSL, keep your users secure online. But to use these tools correctly, and to avoid mistakes of the past, you must understand how cryptography works. Learn the math behind encryption and digital signatures. Study examples of how it has been misused, and explore the possibilities that cryptography enables in digital currency and collaboration.