opie : One-time Passwords In Everything ( http://www.inner.net/opie )
pam_mobile_otp : Mobile-OTP: Strong, two-factor authentication with mobile phones ( http://motp.sourceforge.net/ )
yubipam : YubiPAM: PAM module for Yubikeys ( http://www.securixlive.com/yubipam/ )
Add an ebuild in portage :
The ebuild is now in the portage tree.
You can also use layman : emerge layman then layman -a srcshelton
For Paludis use this rsync : rsync://gentoo.zugaina.org/srcshelton-portage
If you have a problem : ycarus(-at-)zugaina.org