title: Firebase Authentication with multi-factor authentication
body: <p>Is there a way to implement multi-factor authentication on Firebase Authentication? From my research it seems impossible since Firebase authentication has public REST api of which, assumingly, someone knowing API_KEY and user's email/pass could directly sign in to that user's account using</p>
  
  <p><code>https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyPassword?key=[API_KEY]</code></p>
  
  <p>Thank you</p>