Why is everyone just authoritatively dismissing this, when this has been supported for >7 years and is easily found with a google search?

There is the HostKeyAgent configuration directive, which communicates over a unix domain socket to make signing requests.

https://framkant.org/2017/10/strong-authentication-openssh-h...

https://github.com/openssh/openssh-portable/blob/12492c0abf1...

> How can openssh documentation be vendor-specific?

I isn't, because the cryptography is (in case of HSM) not handled by OpenSSH itself. So OpenSSH's configuration has nothing to do with the HSM.

Usually, the actual cryptographic functions are not performed user-space, but handled by the kernel, which in turn can offload this to dedicated hardware. Basically if you compile OpenSSH for it to use kernel level cryptographic function, then OpenSSH can work with a HSM without it even knowing it.

Disclaimer: this is simplified explanation, there is a lot more to this, and I am by no means an expert on this matter.

Edit: meant to say kernel level cryptographic functions, not TLS.

So you‘re saying that OpenSSH has an interface for that on the host key side?

I‘m aware of the PKCS#11 integration in the OpenSSH client and have dabbled a bit with it but was not aware of any server side equivalent.

And how does TLS fit in here? SSH is a very different protocol from that, no?

Update: I can't find any OpenSSH documentation of either (server-side) PCKS#11/HSM support or kernel-mode cryptography (which also in the case of Linux only addresses symmetric encryption to my knowledge, at least the mainline kernel version I know of).

Maybe you're thinking of some other SSH implementation? The protocol definitely allows for server-side HSM usage, and Github at their scale is not bound to OpenSSH in any way.

> I can't find any OpenSSH documentation of either (server-side) PCKS#11/HSM support

OpenSSHd talks to an ssh-agent that then talks to the HSM:

> Identifies the UNIX-domain socket used to communicate with an agent that has access to the private host keys. If the string "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK environment variable.

* https://man.openbsd.org/sshd_config#HostKeyAgent

Interesting, I didn't know that OpenSSHd supported the agent protocol. Thank you!

It's just the agent protocol, used by sshd instead of ssh, to make signing requests with a host key (instead of a user's identity key).

That's cool, I wasn't aware that the server supports the agent protocol as well. Thank you for the pointer!

It makes a lot of sense, since it avoids having to link the HSM/PCKS#11 stuff against sshd.