openssh unix dev - Sep 2021 - Loading (Only) a Cert Into the Agent

A quick question (I hope): I built an SSH user CA that would allow users
to SSH in (using their keypair) and thus trigger creation of a matching
cert. What I would *like* to do is to (add agent forwarding to the login
and) have the CA load the cert straight into the agent.

What happens is that doing an ssh-add on the CA fails because it cannot
find the *private* key in a local file, and even when I download the
cert and do the ssh-add locally, I need to enter the passphrase into the
terminal, presumably because it does read the privkey from its file as
well - in spite of the fact that the privkey is already loaded in the
agent all the time.

Is this a principal limitation of the code/protocol/security model,
something I can work around (though I don't yet see how), a feature
request with a chance of getting implemented, ... ?

Kind regards,
-- 
Jochen Bern
Systemingenieur

Binect GmbH

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On 9/10/21 9:53 AM, Jochen Bern wrote:
> A quick question (I hope): I built an SSH user CA that would allow users
> to SSH in (using their keypair) and thus trigger creation of a matching
> cert. What I would *like* to do is to (add agent forwarding to the login
> and) have the CA load the cert straight into the agent.
> 
> What happens is that doing an ssh-add on the CA fails because it cannot
> find the *private* key in a local file, and even when I download the
> cert and do the ssh-add locally, I need to enter the passphrase into the
> terminal, presumably because it does read the privkey from its file as
> well - in spite of the fact that the privkey is already loaded in the
> agent all the time.
> 
> Is this a principal limitation of the code/protocol/security model,
> something I can work around (though I don't yet see how), a feature
> request with a chance of getting implemented, ... ?
Yes,
this was discussed since 2015 in context of PKCS #11 backed keys in 
hardware, but the protocol was not yet updated to support loading 
separate certificates:

https://bugzilla.mindrot.org/show_bug.cgi?id=2472

Regards,
-- 
Jakub Jelen
Crypto Team, Security Engineering
Red Hat, Inc.