Simple Two Factor Authentication
Much noise is made about two factor authentication (2FA).
Generally, 2FA is defined as a security authentication method that includes something you possess and something you know.
SSH provides both. Key pairs is something you possess. Protecting the private key with a pass phrase is something you know. Without knowing the pass phrase, the key pairs are impotent.
The usual disclaimer applies. I am not a security expert and do not play one on TV.