Google Authenticator only offers a subset of standard TOTP, and it also offers a very similar subset of HOTP. - Option of TOTP or HOTP ("time-based" or "coun

Google Authenticator only offers a subset of standard TOTP, and it also offers a very similar subset of HOTP. - Option of TOTP or HOTP ("time-based" or "counter-based") - SHA1 only - 6-digit codes only - 30-second time step only (TOTP) - Base32-encoded keys only "Google Authenticator" also implies that the secret is stored on your phone, where it's far more susceptible to remote compromise than if it were stored in a dedicated hardware TOTP token. Personally, my preference for SSH two-factor authentication is to just use standard pubkey authentication, but only allow keys that have been placed into PIN-protected, tamper-resistant hardware. The crypto is stronger, it doesn't involve some obscure PAM module, and the attack surface is smaller (since you can avoid invoking PAM at all until after authentication succeeds).