openssh unix dev - Oct 2024 - [PATCH] Specify signature algorithm during server hostkeys prove

Hello,

I've found that when using the ssh agent and sshd together, there is an
issue
when using multiple host keys. Specifically, after the key exchange phase,
when a client requests proof of ownership for the host keys via the
"hostkeys-prove-00 at openssh.com" request, the server prepares the
response
without specifying the signature algoorithm in case of non-RSA keys.

This leads to "SSH_ERR_INVALID_ARGUMENT" when verifying the signature
in :

openssh-portable/authfd.c line 
if ((r = sshkey_check_sigtype(sig, len, alg)) != 0)

To resolve this, I explicitly sets the signature
algorithm, ensuring proper verification for all key types.

I would appreciate any feedback or suggestions regarding this issue.

Best regards,
Maxime

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On Mon, 28 Oct 2024, Maxime Rey wrote:
> 
> Hello,
> 
> I've found that when using the ssh agent and sshd together, there is an
issue
> when using multiple host keys. Specifically, after the key exchange phase,
> when a client requests proof of ownership for the host keys via the
> "hostkeys-prove-00 at openssh.com" request, the server prepares
the response
> without specifying the signature algoorithm in case of non-RSA keys.
> 
> This leads to "SSH_ERR_INVALID_ARGUMENT" when verifying the
signature in :
> 
> openssh-portable/authfd.c line 
> if ((r = sshkey_check_sigtype(sig, len, alg)) != 0)
> 
> To resolve this, I explicitly sets the signature
> algorithm, ensuring proper verification for all key types.
> 
> I would appreciate any feedback or suggestions regarding this issue.
Hi,

I'm having trouble replicating this failure by making changes to the
existing hostkey-agent.sh regress test.

Can you share a bit more about how it happens? Debug traces from the
client and server would be very helpful.

Thanks,
Damien Miller