Questions you set for each of your Contacts (recipient of your messages).
Based on the answers to these questions the Volatile Encryption Keys will be created and used to encrypt (and later decrypt) the messages to each contact.
The answers to these questions should be known only to you and to the recipient of your message. Any difference, even the slightest one, between your answers and the answers given by the recipients will make it impossible for them to decrypt the messages.
Please check the Good Practice section to know more.