Hi Adam

I've cc'd Fabian who knows more about the ssh signing code that I do.

On 18/01/2023 15:28, Adam Szkoda wrote:

Hi Phillip,

Good point!  My first thought is to try doing a stat() syscall on the
path from 'user.signingKey' to see if it exists and if not, treat it
as a public key (and pass the -U option).  If that sounds reasonable,
I can update the patch.

My reading of the documentation is that user.signingKey may point to a 
public or private key so I'm not sure how stat()ing would help. Looking 
at the code in sign_buffer_ssh() we have a function is_literal_ssh_key() 
that checks if the config value is a public key. When the user passes 
the path to a key we could read the file check use is_literal_ssh_key() 
to check if it is a public key (or possibly just check if the file 
begins with "ssh-"). Fabian - does that sound reasonable?

Best Wishes

Phillip

Best
— Adam


On Wed, Jan 18, 2023 at 3:34 PM Phillip Wood [off-list ref] wrote:

quoted

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

quoted

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