I’m writing this in February 2020, but I should have posted it last April. This is about using openssh certificates for authentication.
For many years I have used exclusively public-key authentication for ssh connections. None of the boxes I support ever allows password authentication for ssh. But as the number of boxes in the constellation has grown, the problem of keeping the public keys up to date has become nightmarish. If I really truly needed NxN capability it would be unmanageable. The truth is though that there are only 3 or 4 boxes from which I typically establish ssh connections to all the others.
Still it is – if not a full-blown nightmare, at least a bad dream. The folks at openssh implemented a mechanism to make it easier: certificates. These are not full blow x509 certificates, they are simpler, but the idea is the same. The problem: how do we arrange to get a public key on to some box with confidence that the public key belongs to the party it purports to represent? The certificate answer is to have the public-key signed by a party that the recipient trusts.
So how does it work. I designate a box as my “signer”. All the other boxes in my world trust the signer. Instead of putting public keys on every box, instead every box has its own public key signed by the “signer”. When it wishes to connect to another box (in my own constellation of boxes) it presents its certificate – i.e. its signed public key. Since the recipient trusts the signer, it will accept the signed public key as being the genuine public key, and it can then proceed to authenticate.
How it works mechanically is that when I initialize a new box, and create a new key pair for it, I have to send the new public key to the signer. Then, on the signer system I sign the key, and send it back to the new box. I’m actually using my repository for the moving of this stuff back and forth. I think this is ok, since the only thing that ever appears in the repository are public keys and public certificates. The public keys exist in the repo for mere moments. The certificates stay there. But the private keys of course never leave the box on which they originate.