CrypTool

This is the official app of CrypTool, an organization that is supposed to explain and implement ciphers in several programming languages, to help people understand and create new ciphers. We don't teach security but the art of hiding the content of a message.
With this app, you'll be able to learn cryptography in a fun way, by completing challenges that are just the right level to make you think a little bit.
Most of our ciphers are already implemented in this app, and we'll keep implementing new ones.
Our app is not, in any case, a way of communicating securely, security is not our goal. We simply teach the basics, and we let users experiment with ciphers.