This plugin adds the option to use a YubiKey as second-factor
authentication.

The YubiKey is a USB key emulating a generic keyboard and make
use of One-time Passwords to provide two factor authentication.

Activate the plugin  by adding the following to Roundcube's
config/config.inc.php:
$config['plugins'] = array('roundcube_yubikey_authentication');

WWW: https://github.com/northox/roundcube-yubikey-plugin