Using certificates for SSH authentication [LWN.net]

Using certificates for SSH authentication

Posted Nov 15, 2022 7:06 UTC (Tue) by tilt12345678 (subscriber, #126336)
Parent article: Using certificates for SSH authentication

“How does a CA get a user's public key and how does the user get the CA's public key in a secure way? There's no generic, scalable solution for this, but the problem is somewhat simplified because it only needs to be done rarely.”

“OpenSSH does support revocations, but revocation information can be difficult to communicate to all parties in a fast, reliable way.”

To me this reads "we propose you manage SSH access in a PKI fashion, only that there is no PKI."

“An SSH public key does not specify which host or user it belongs to, for example, whereas a certificate has a list of "principals" that it vouches for. A principal is a host or user name.”

I fail to find this disclosure desirable.

Using certificates for SSH authentication

Posted Nov 15, 2022 16:46 UTC (Tue) by anton (subscriber, #25547) [Link] (1 responses)

“An SSH public key does not specify which host or user it belongs to, for example, whereas a certificate has a list of "principals" that it vouches for. A principal is a host or user name.”
I fail to find this disclosure desirable.

Apparently neither do the OpenSSH maintainers. I have old user public keys that contain username@hostname (which is helpful when weeding out authorized_keys files), and old entries in known_hosts files that contain the FQDN and IP of the host. That's gone or (for known_hosts) encrypted in newer public keys and known_hosts entries.

Makes me wonder if the reasons why they were eliminated also hold for certificates. I guess that in the presence of certificate servers one would need to encrypt the public keys in the files, otherwise one could recreate the information that were intentionally left away from these files by looking the public keys up in a certificate server.

Using certificates for SSH authentication

Posted Nov 19, 2022 2:27 UTC (Sat) by rra (subscriber, #99804) [Link]

Hostnames in known_hosts files were hashed because they were widely used by attackers to build target lists of places where the credentials they just stole were likely to work. I would argue that it's security via obscurity, not a real security control, but it does tend to slow them down.

If you prefer the format without the hashing, you can turn it off in your config (HashKnownHosts).