Fix ssh-agent certificate not recognized with IdentitiesOnly

[AlexpFr]

• Bugzilla – Bug 3681 - SSH Agent Certificate Not Recognized with 'IdentitiesOnly' Configured

The certificate present in the SSH agent is not added to the keys to be tested when 'IdentitiesOnly yes' is configured in the ~/.ssh/config file.

$ cat ~/.ssh/config
Host exemple.org
	IdentityFile ~/.ssh/id_ed25519.pub
	IdentitiesOnly Yes

$ ssh-add -l
256 SHA256:<FINGERPRINT> <COMMENT> (ED25519)
256 SHA256:<FINGERPRINT> <COMMENT> (ED25519-CERT)

$ ls ~/.ssh/
config    id_ed25519.pub

With the same configuration, 'ssh' defaults to looking for a certificate in ~/.ssh/id_ed25519.pub-cert::

$ ssh -vvv user@exemple.org
debug1: identity file ~/.ssh/id_ed25519.pub type 3
debug1: identity file ~/.ssh/id_ed25519.pub-cert type -1

I believe the expected behavior should also include searching for the certificate in the agent.

I have attempted a very simple fix.

The certificate is added from the agent with 'IdentitiesOnly Yes'. The code is redundant, but it works.

Yet another inconsistency: 'ssh-keygen' generates certificates in the form id_ed25519-cert.pub, whereas "ssh" searches for id_ed25519.pub-cert.