Decrypt a secret encrypted with AES-ECB, where arbitrary data is appended to the secret and the key is reused. This level is quite a step up in difficulty (and future levels currently do not build on this level), so if you are completely stuck feel free to move ahead. Check out this lecture video on how to approach level 5.
Perform a simplified TLS handshake as the server, completing a Diffie-Hellman key exchange and establishing an encrypted channel to provide a user certificate and prove private key ownership