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

[Phillip Wood <phillip.wood123@gmail.com>]

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

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
> 
>>
>> 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