Using certificates for SSH authentication

I think it's easier than running an X.509 CA properly, but harder than running an SSH CA properly. The difficulty of running a trusted third party is based on the complexity of what it has to judge (plus implementation awkwardness), and SSH is at least as simple on that as Kerberos, while X.509 is really complicated. The fixed structure of a SSH certificate makes it pretty much equivalent in semantic complexity to a Kerberos ticket, and far simpler than most things called "certificates".