The gpg-agent should provide populate the comment field when providing a SSH
formatted key, ideally identifying the user (primary uid) or the gpg
authentication subkey's id or fingerprint.
The following detail was originally posted to the mailing list
(https://lists.gnupg.org/pipermail/gnupg-users/2016-October/056911.html)
The SSH public key format contains a comment field (RFC4716, s3.3.2):
The comment header contains a user-specified comment. user@example.com
From "man sshd":
Public keys consist of the following space-separated fields: options, keytype, base64-encoded key, comment. The comment field is not used for anything (but may be convenient for the user to identify the key).
If I load an SSH key from a file using 'ssh-add' the comment field is
populated with the file name (i.e. "alice.pem") if the gpg-agent does
not already contain that key.
If I do "ssh-add -L" I will see "alice.pem" at the end of the output:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQAHT...IfFoxh2j13b3 alice.pem
If the key is in the agent because of the gpg keyring then it is known
as "(none)". If I do "ssh-add -L" I will see "(none)" at the end of the
output:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQAHT...IfFoxh2j13b3 (none)
The reason that I stumbled upon this was because I was debugging a ssh
connection that used the gpg-agent and the ssh debugging output
displayed the following misleading output:
debug1: Offering RSA public key: (none)
which means the public key called "(none)" rather than, as I initially
interpreted it, no public key.
It's also useful client-side to see who a public key belongs to.
It would be good if the comment field reflected the key source, perhaps
the short (or long) key id. For example:
ssh-rsa AAAAB3NzaC1yc2EAAAADAQAHT...IfFoxh2j13b3 (3A808C39)
Or even the primary uid of the key
ssh-rsa AAAAB3NzaC1yc2EAAAADAQAHT...IfFoxh2j13b3 alice@example.org