Examples of the challenges proposed at https://cryptohack.org/. It contains the python scripts made to overcome the challenges. If you are interested in crypto it is a good way to learn (in a simple way).
For registration you have to decrypt an encrypted phrase in the emperor's encryption, read about it: https://en.wikipedia.org/wiki/Caesar_cipher Keep in mind if the alphabet has 26 or 27 letters.
You have online utilities to decipher, but I recommend (if you really want to learn) that you try to understand how it works and apply it, it is simple.