If you're the type of person that is fascinated by spies, covert operations, coded messages and the likes, you might want to consider going into the cybersecurity or even just security industry. And to train yourself for such a career, maybe you'd need an app for that. Cryptoy is one such app, teaching you the ways of encryption and decoding secret messages.
But lest you think that this sort of thing may be illegal, the app is actually from the Government Communication Headquarters (GCHQ), the British equivalent of the NSA and developed by STEM (science, technology, engineering and maths) students who were interning at the organization. It's targetted for 14-16 year olds, but who are we kidding, adults would also love to learn such things.
Basically, the app teaches you all about cryptography; its history, the different kinds of encryption (shift, substitution, vigenere, enigma) and of course how to actually do the encoding of messages. What makes it more fun is that you can send these encrypted messages to your friends, either through email or social networks, and they can try deciphering it on their own or with the help of the app too.
The app is not intended as a surveillance tool, or whatever our paranoid brain can come up with. A spokeperson from GCHQ said that part of their purpose for the app is to encourage younger people to be more interested in maths and computer science for a future in cybersecurity. But even if you're not the target audience, you can still have fun using Cryptoy. You can download it from the Google Play Store for free. It has only been tested on Android devices running on 4.1.2 up to 4.4.2.