anchor.diff:            <filename>~/.config/Yubico/u2f_keys</filename> are able to log in
             with the associated U2F key.
           '';
  +      };
  +
  +      googleAuthenticator = {
  +        enable = mkOption {
  +          default = false;
  +          type = types.bool;
  +          description = ''
  +            If set, users with enabled Google Authenticator (created
  +            <filename>~/.google_authenticator</filename>) will be required
  +            to provide Google Authenticator token to log in.
  +          '';
  +        };
         };
   
         usbAuth = mkOption {
  
positive.code:           # prompts the user for password so we run it once with 'required' at an
            # earlier point and it will run again with 'sufficient' further down.
            # We use try_first_pass the second time to avoid prompting password twice
            (optionalString (cfg.unixAuth && (config.security.pam.enableEcryptfs || cfg.pamMount || cfg.enableKwallet || cfg.enableGnomeKeyring)) ''
                auth required pam_unix.so ${optionalString cfg.allowNullPassword "nullok"} likeauth
                ${optionalString config.security.pam.enableEcryptfs
                  "auth optional ${pkgs.ecryptfs}/lib/security/pam_ecryptfs.so unwrap"}
  
negative.code:                  " kwalletd=${pkgs.libsForQt5.kwallet.bin}/bin/kwalletd5")}
                ${optionalString cfg.enableGnomeKeyring
                  ("auth optional ${pkgs.gnome3.gnome_keyring}/lib/security/pam_gnome_keyring.so")}
                ${optionalString cfg.googleAuthenticator.enable
                    "auth required ${pkgs.googleAuthenticator}/lib/security/pam_google_authenticator.so no_increment_hotp"}
              '') + ''
            ${optionalString cfg.unixAuth
                "auth sufficient pam_unix.so ${optionalString cfg.allowNullPassword "nullok"} likeauth try_first_pass"}
  