The OverTheWire Krypton wargame focuses on teaching cryptography concepts. Players solve various cryptographic challenges ranging from basic ciphers to more advanced encryption techniques, gaining access to the next level by decrypting passwords.
Learning encyrption things such as: level 1: Simple Rotation Cipher level 2: Caesar cipher (shifted by known amount) level 3: Caesar Cipher (shifted by unknown amount) level 4: Polyalphabetic Vigenère cipher (with known key length) level 5: Polyalphabetic Vigenère cipher (with unknown key length) level 6: Stream Cipher with Weak Randomness