Using the OpenSSH Certificate format, many of the common features from X.509, like intermediates, cross signing, key usage attributes or restricting access based on an attribute in the certificate are not part of the spec:
https://github.com/openssh/openssh-portable/blob/master/PROT...
When I first learned about OpenSSH Certskeys, I was really excited; Spent awhile trying to make it useful, but you end up building all the CA infrastructure, and this time instead of distributing certificates to servers once a year, you want to distribute certificates to your _users_ every month or two -- so the pain is higher, there is less automation, and everyone on your team feels it....
Exploring OpenSSH certs is what led me to founding ScaleFT. There had to be a better way.
ScaleFT Access leverages these SSH Certificates, but we expire them every 5 minutes to provide other features that are hard with the limited capabilities of the format:
https://www.scaleft.com/products/access/
There are patches to make OpenSSH use X.509, but they are not widely adopted... and asking people to patch a sshd is a non-starter for many environments.
