Re: [PATCH] ssh signing: better error message when key not in agent

[Fabian Stelzer <fs@gigacodes.de>]

On 18.01.2023 16:29, Phillip Wood wrote:
>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?

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.

Best regards,
Fabian

>
>Best Wishes
>
>Phillip
>
>>Best
>>— Adam
>>
>>
>>On Wed, Jan 18, 2023 at 3:34 PM Phillip Wood <phillip.wood123@gmail.com> wrote:
>>>
>>>On 18/01/2023 11:10, Phillip Wood wrote:
>>>>>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