Auth systems don't key off of usernames... When we added FIDO support there's still a backing user store that records user x has FIDO y, with a recording of the

Auth systems don't key off of usernames... When we added FIDO support there's still a backing user store that records user x has FIDO y, with a recording of the public key. That's how the pairing works. You can see it today on login.live.com