On 20/01/2023 09:03, Fabian Stelzer wrote:

On 18.01.2023 16:29, Phillip Wood wrote:

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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:

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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?

Hi,
I have encountered the mentioned problem before as well and tried to fix 
it but did not find a good / reasonable way to do so. Git just passes 
the user.signingKey to ssh-keygen which states:
`The key used for signing is specified using the -f option and may refer 
to either a private key, or a public key with the private half available 
via ssh-agent(1)`

I don't think it's a good idea for git to parse the key and try to 
determine if it's public or private. The fix should probably be in 
openssh (different error message) but when looking into it last time i 
remember that the logic for using the key is quite deeply embedded into 
the ssh code and not easily adjusted for the signing use case. At the 
moment I don't have the time to look into it but the openssh code for 
signing is quite readable so feel free to give it a try. Maybe you find 
a good way.

Thanks Fabian, perhaps the easiest way forward is for us to only pass 
"-U" when we have a literal key in user.signingKey as we know it must a 
be public key in that case.

Best Wishes

Phillip

Best regards,
Fabian

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Best Wishes

Phillip

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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:

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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