The Base64-encoded content in this file has been encrypted via AES-128 in ECB mode under the key
"YELLOW SUBMARINE".
(case-sensitive, without the quotes; exactly 16 characters; I like "YELLOW SUBMARINE" because it's exactly 16 bytes long, and now you do too).
Decrypt it. You know the key, after all.
Easiest way: use OpenSSL::Cipher and give it AES-128-ECB as the cipher.
You can obviously decrypt this using the OpenSSL command-line tool, but we're having you get ECB working in code for a reason. You'll need it a lot later on, and not just for attacking ECB.