To access secret conversation codes, you must know how to open a secret conversation on messenger.  First, open any regular message and then select the ‘i’ icon. Click the message there, then tap on the person's profile picture.  Afterward, you'll see that there are two choices at the top.You have your keys, and they have their keys.  When you tap on them, your key code and theirs will appear. You may confirm that your communications are end-to-end encrypted and the device key corresponds.  Cryptography, which describes the encryption format of the specific line, is where you'll find a key code.