In case if someone finds it through a search:

Closing due to age and because gpg4win 4 started to using the much improved GnuPG 2.4

Here's another data point.

See the the commit for a description of the changes.

@MathiasMagnus This change is to support Win32-OpenSSH by gpg-agent emulation of ssh-agent; You can use gpg-agent emulation of ssh-agent when you use Win32-OpenSSH. That is, you can use GPG auth subkey for Win32-OpenSSH.

@gniibe Am I misunderstanding something? I thought that with this change one is able to connect from a Windows box to a Linux box and have GPG agent forwarding work. I am still hitting pretty much the same issue described here: https://github.com/PowerShell/Win32-OpenSSH/issues/1564
On my Windows endpoint I'm running gpg.exe version 2.4.0.49237 and in C:\Users\mate\AppData\Roaming\gnupg\gpg-agent.conf I have a single line enable-win32-openssh-support. Running gpg-connect-agent.exe reloadagent /bye I have a gpg-agent running. Get-Process gpg-agent shows that it's running. In my Windows env I have SSH_AUTH_SOCK set to \\.\pipe\openssh-ssh-agent and my Linux endpoint is configured in SSH config with

ForwardAgent yes
AddKeysToAgent yes
RemoteForward /run/user/1015/gnupg/S.gpg-agent C\:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra

As the remote end reports /run/user/1015/gnupg/S.gpg-agent that socket for agent-socket when issuing gpgconf --list-dirs and my local gpgconfg.exe --list-dirs reports C%3a\Users\mate\AppData\Local\gnupg\S.gpg-agent.extra where I transform %3a to \: manually. SSH authentication works perfectly, when connecting pinentry-qt pops up to unlock my key and when connecting to yet another machine, my SSH agent is forwarded again. However, gpg fails to use my agent. Issuing gpg --list-secret-keys --verbose prints the following to the console:

gpg --list-secret-keys --verbose
gpg: using pgp trust model
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
gpg: no running gpg-agent - starting '/usr/bin/gpg-agent'
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
gpg: waiting for the agent to come up ... (5s)
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
getsockopt SO_ERROR failed
connect_to C:/Users/mate/AppData/Local/gnupg/S.gpg-agent.extra port -2: failed.
gpg: waiting for the agent to come up ... (4s)
gpg: waiting for the agent to come up ... (3s)
gpg: waiting for the agent to come up ... (2s)
gpg: waiting for the agent to come up ... (1s)
gpg: can't connect to the agent: End of file

What is missing to tie the knot on both ends without having to resort to 3rd party tools like @rupor-github 's agent-gui? The remote gpg version is 2.2.19, is that the issue? Must that also be 2.3.9+?

Let's first collect all keys, assign a priority, sort, and only then send them back to ssh.

Thanks all. It is a bug in Win32 OpenSSH. https://github.com/PowerShell/Win32-OpenSSH/issues/1953 it is already fixed. I think the issue will be resolved after the update is shipped. I could use ssh -T git@github.com as a workaround.

Well, not our bug... it's a kind of support question and answer:
This might help: https://stackoverflow.com/questions/3844393/what-to-do-about-pty-allocation-request-failed-on-channel-0

This does not look like a problem in GnuPG/gpg4win because gnupg implements the ssh-agent protocol and not the ssh server or client functionality. ssh tells sshd whether it shall allocate a PTY (Pseudo TTY). I don't use ssh with github but it is likely that you may only run commands (which don't require a PTY). Usually you would invoke a "git" command cia ssh.

Authentication succeed if I pressed enter after:PTY allocation request failed on channel 0

I try WinGPG 4.1.0, and I receive an error:
ssh git@github.com
PTY allocation request failed on channel 0

Implications are... you won't be possible to use new protocols introduced by newer OpenSSH:

Thanks. Adding 'PubkeyAuthentication unbound' to my ~/.ssh/config seems to workaround it for me on openssh-9.1p1-3 (arch). I don't quite follow what the implications of that setting are though.

In my cases (tested with 9.1), here are the length of data to be signed by ssh-agent (emulation by gpg-agent).

• 164 bytes: Both features disabled by: ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com -o PubkeyAuthentication=unbound
• 192 bytes: Unbound only by: ssh -o PubkeyAuthentication=unbound
• 298 bytes: No Post Quantum only by: ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com
• 330 bytes: Both features enabled (no options)

I tested with openssh 9.1. When I add -o PubkeyAuthentication=unbound, I can make the length of data smaller.

In T5931#165009, @alexk wrote:

A workaround you can add the following line to ~/.ssh/config or /etc/ssh/ssh_config:

KexAlgorithms -sntrup761x25519-sha512@openssh.com

For me ssh -o KexAlgorithms=-sntrup761x25519-sha512@openssh.com ... does work as well.

A workaround you can add the following line to ~/.ssh/config or /etc/ssh/ssh_config:

The problem here is how large the data to be signed is. It is an issue of protocol design. The protocols are explained in openssh/PROTOCOL.certkeys and openssh/PROTOCOL. Unfortunately, it seems that it was designed with not much consideration for smartcard use case, so, data to be signed may be longer (than the capability of smartcard).

Sadly, it doesn't work for me. But thank you.

I managed to find a way to minimize the data (less than the one on Oct 25).
And it somehow works for me.

So what should I do now? Should I report it to OpenSSH team?

Will go into 2.3.9 and gpg4win 4.0.5

Pushed to master.

We want to get rid of sshcontrol but we could keep it as an optional configuration to sort keys. I won't say it is a bug, though.

I hacked configure.ac of gnupg to force it build with libgpg-error 1.45, and OpenSSH works with the created pipe. Maybe the libgpg-error fix is only necessary in some certain circumstances?

It's not yet pushed, because it requires new release of libgpg-error (for T6112: libgpg-error,w32: bidirectional Pipe support for estream).

I was looking for this when writing the update NEWS for the latest release and noticed that this has not been pushed yet. I really think that it would be nice to have that. Especially for Smartcard use cases.