One thing that always has scared me a bit with using CAs for SSH is how you protect the signing certificates? After all, if an attacker gets that cert then they get full access to everything, and can masquerade as anyone. You end up with a choice between a) have lots of SSH keys out in the wild, each with varying degrees of access, or b) have a single cert that is on your infrastructure but has access to everything. (Not to mention how you deal with the operating the site, what happens if it crashes? How do you log in without the site to sign your ssh key? Using standard trusted SSH keys to access feels like its somewhat undermining the point of using CAs).

Has anyone solved this, or got a write up of some best practices for running this? All I've managed to find are articles about how to run such apps, rather than how it fits into the broader security architecture.

Ideally ideally, what I would actually like is the ability to configure OpenSSH to require multiple things to log in, i.e. both that they SSH key is trusted and that it has recently been signed by the signing service. That way gaining access to the signing certificate doesn't help without also gaining a trusted SSH key (it's still bad, but not quite Game Over levels of bad). I had a quick look to see if I could hack together a patch to do this, but alas I had forgotten how weak my C foo is :(