Je vertelt niet welk OS je gebruikt, maar misschien heb je wat aan deze links:
(1)
http://www.howtoforge.com/securing-openvpn-with-a-one-time-password-otp-on-ubuntuSo, you got yourself a nice OpenVPN box. People need to login with their certificates but... if their laptop is stolen anyone could login. Sure, you could add password login but thats a bit outdated. The solution for this is using a OTP (one time password).
This technique is being used by a lot of large corporations including banks. This tutorial shows you how to configure OpenVPN to add additional security.
(2)
http://www.taferno.com/ (free software vanaf
http://taferno.sourceforge.net/)
Taferno is a frugal Two-factor Authentication system for network operations like VPN, SSH, Web Single Sign-On, etc. The system uses standard USB flash drives, a suite of web and backend programs that work in conjunction with a stateful firewall, dynamically modifying the firewall rules, setting required secure cookies, generation of the OTP (One Time Password), etc. to allow the requested access for each successfully authenticated user.