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

From: Phillip Wood <phillip.wood123@gmail.com>
Date: Jan 18, 2023, 16:29 UTC
Message-ID: <55282dec-825f-8c4b-1fb0-6e26ec326db1@dunelm.org.uk>
In-Reply-To: <CAEroKagqxC86X0SD8=tK0w+yXL7QecZ+z_7sja-K6ajs0=Z=BQ@mail.gmail.com>

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:

Show 6 quoted lines

> 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

Show 29 quoted lines

> 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