Here is an interactive Online Message Encryptor / Decryptor Tool (utilizing AES Encryption). By generating a key from a secret passphrase, you can encrypt and decrypt messages below. You can enter any (standard) text in the message areas and the Encryption or Decryption will be done, on-the-fly as you type.
Be careful to only use standard Characters for consistent results (not extended international characters).
Exact copies including spaces and carriage returns are required to get identical results.
All the work is done in your browser, so I do not have access to the data you enter.
If you save the data in a text file, be sure to save plain text (notepad etc.) rather than a wordprocessor that might change or add characters.