What is a 2FA restore key and why should I protect it?

The restore key allows:

• regain access to your account on a new mobile device by duplicating your code generation privileges - it's useful if the phone from which you enter 2FA codes is damaged or lost,
• setting up several devices to enable authentication, 

NOTE: The recovery key should be protected from access by third parties. Otherwise someone can authenticate the login with your data and take control of your account.