Securing SSH with two factor authentication using Google Authenticator

Two-step verification (also known as Two-factor authentication, abbreviated to TFA) is a process involving two stages to verify the identity of an entity trying to access services in a computer or in a network. This is a special case of a multi-factor authentication which might involve only one of the three authentication factors (a knowledge factor, a possession factor, and an inheritance factor) for both steps. If each step involves a different authentication factor then the two-step verification is additionally two-factor authentication.