class plgAuthenticationEmail extends JPlugin {

    /**
     * This method should handle any authentication and report back to the subject
     */
    function onUserAuthenticate(&$credentials, $options, &$response) {
        // Get a database object
        $db = JFactory::getDbo();
        $query = $db->getQuery(true);

        $query->select('username, password');
        $query->from('#__users inner join #__comprofiler ON #__users.id=#__comprofiler.id');
        $query->where('cb_FOO LIKE ' . $db->Quote($credentials['username']));

        $db->setQuery($query);
        $result = $db->loadObject();

        if ($result) {
            // why mess with re-creating authentication - just use the system.
            $credentials['username'] = $result->username;
            require_once JPATH_PLUGINS . '/authentication/joomla/joomla.php';
            PlgAuthenticationJoomla::onUserAuthenticate($credentials, $options, $response);
        } else {
            $response->status = JAuthentication::STATUS_FAILURE;
            $response->error_message = JText::_('JGLOBAL_AUTH_INVALID_PASS');            
        }
    }

}