On 18/01/2023 11:10, Phillip Wood wrote:

quoted

the agent [1].  A fix is scheduled to be released in OpenSSH 9.1. All 
that
needs to be done is to pass an additional backward-compatible option 
-U to
'ssh-keygen -Y sign' call.  With '-U', ssh-keygen always interprets 
the file
as public key and expects to find the private key in the agent.

The documentation for user.signingKey says

  If gpg.format is set to ssh this can contain the path to either your 
private ssh key or the public key when ssh-agent is used.

If I've understood correctly passing -U will prevent users from setting 
this to a private key.

If there is an easy way to tell if the user has given us a public key 
then we could pass "-U" in that case.

Best Wishes

Phillip