The Yubikey authentication standard is proprietary – but it is completely open. So it would be perfectly possible for anybody to implement their own authenticator from the ground up.
However: why bother? Yubico makes a lot of source code available at its github site including:
- Source code for an authentication server.
- Source code for a PAM module which interfaces with the authentication server.
In addition, Yubico provides a free ‘cloud-based’ authentication service, ‘Yubicloud’.
So in fact experimenting with Yubikey authentication is easy: for a start, anyway, we can just use the ‘Yubicloud’ service.