Joomla! 1.5, 1.6 and 2.5 two-factor authentication (T-FA) plugin for use with the Yubico Yubikey one-time-password device. An accompanying component is included to manage Yubikey users.
With this authentication plugin you may selectively choose which Joomla users will require a Yubikey device to log into Joomla!.
Two-factor authentication is achieved by requiring a Yubikey-enabled user to first input their Joomla! password and then adding their Yubikey one-time-password to the end of that forming a single password consisting of the user's Joomla! password and their Yubikey one-time-password concatenated to the end of that. The username/password screens of Joomla! remain the same.
You can read more about the Yubikey one-time-password authentication device from the Yubico homepage at: http://www.yubico.com