A system identifies a user through a user identifier, commonly abbreviated to user ID. The process by which a system confirms that a user really is who the user claims to be is called authentication. We saw in Chapter 5 that there are three types of credentials through which a user can be authenticated: knowledge factor (what a user knows), ownership factor (what a user owns), and inherence factor (what a user is).
When you have an authentication mechanism that leverages a combination of two of these factors, it is called two-factor authentication (TFA, T-FA, or 2FA). A real-life example for TFA is an automated ...