client host certificates and receiving host configuration

[Damien Miller]

On Wed, 17 Jun 2020, Rory Campbell-Lange wrote:

> > > Secondly, would there be any alteration to the requirement for a
> > > "HostCertificate" CA-signed public key (from a private "HostKey") on
> > > sshd receiving servers?
> > 
> > I don't understand what you mean here. Could you elabourate?
> 
> My apologies for the poor explanation. Let me try again.
> 
> Adding a user certificate to a client forwarded agent allows that client
> to use that certificate to authenticate to servers with
> TrustedUserCAKeys set to the public key used to sign the certificate.
> 
> What would host certificates added to a client forwarded agent give one
> (if any),

Hmm, the agent deals with private keys only at present. Do you mean
modifying the agent to support adding the public part of host
certificates?

If so, then I guess it would be possible to use the agent as a kind of
known_hosts file but there is a lot of glue needed to make that happen
and I'm not sure the agent is an ideal place for it.

If you do mean adding the private host keys then it would additionally
make host-based authentication possible via the agent but, less
beneficially, give the user and anyone with access to their agent socket
the ability to impersonate those hosts.

> and what part of the normal set of configuration requirements*
> does it help with?
> 
> * normal config : @cert-authority in the client's ~/.ssh/known_hosts;
>   setup of appropriate HostCertificate directives on receiving hosts

I don't think it really helps with this. What you might want to be able
to forward around with you is your database of known_hosts, including
the @cert-authority markers. These are public keys only, plus the host
name(s) they apply to, plus a couple of bits per key of metadata (revoked,
cert-authority, etc.)

The agent, as it stands, only handles private keys (not public), has
no ancilliary metadata storage and no protocol verbs to perform the
actions needed by host key verification. All these would need to be
added for the agent to be capable of subsuming known_hosts operations.

However these are sufficiently different to the agent's current role
that it would be worth considering doing it in a different service,
as the agent really needs to be as simple and to present as little
attack surface as possible so it can protect its private keys.

-d