title: How to authenticate a user's identity using their phone
body: <p>I am making an app with firebase that should allow a user to authenticate their identity when they get to a certain physical place in order to be let into the place. I want staff at the place to be able to use their own phones to securely verify the user's identity and let them in.
  My first idea was to make the user's phone present a QR code with the uid of the user for the designated staff members to scan. The problem with that is that the uid is not a valid means of authorization in firebase because it is not a secret, which makes me worry that this approach may not be secure.
  Is there any secure way for one phone to authenticate the identity of a user who is logged in on another phone?</p>
  <p>EDIT: My question has nothing to do with logging in to the same user account using multiple authentication methods, it is about using a physical device connected to one user account to authenticate that a different physical device is connected to a different user account.</p>