akn-cybersec/encrypt-decrypt ? reverse-engineered prompt

Build me a simple single page tool where I can type a short message, enter a secret key, and then encrypt it into a hidden looking code that I can copy and share with a friend.

I want the same page to also decrypt messages back into plain text when the correct key is entered. Keep it playful, clean, and easy to use, with clear boxes for the message, key, encrypted output, and decrypted output. Add buttons for encrypt and decrypt, plus copy and clear actions if they fit naturally.

This is just for small personal messages and learning, so it does not need to be fancy or high security, but it should feel smooth and friendly. Please make it work with plain HTML, CSS, and JavaScript only, with no frameworks or setup needed. If you need to check anything, look up current browser docs online first.