Use both secret keys for both directions. Encode turns plain text into encrypted output, and decode restores the original text from the encrypted payload.
Encode
Enter both secret keys and plain text to generate encrypted output.
Decode
Enter both secret keys and encrypted text to restore plain text.
Encryption uses AES-GCM with a key derived from combining both secret keys. The encoded value includes the IV, so you need both secret keys and encrypted text to decode it.