Great spotting! This was it. Quite embarrassing that I've looked at this code so many time yet it didn't cross my mind to double check arguments order.

@jpalus You are right.

computed by ssh_signature_encoder_rsa, including additional 0, reach:

Note that exactly same data and length computed by ssh_signature_encoder_rsa, including additional 0, reach:
https://github.com/openssh/openssh-portable/blob/V_10_2_P1/sshkey.c#L517-L537

Let's see whether Niibe-san still remembers the T7882 case.

Added to some debug logging and whenever login issue occurs new logic is applied:
https://github.com/gpg/gnupg/blob/bc7c91bee521e4adf3506ca32bf34177b84ce1c5/agent/command-ssh.c#L1482

Looks like indeed related to T7882. After reverting c7e0ec12609b401ea81c4851522d86eb5ec27170 I was able to make 2000 connections without any issue. Bringing the change back and retrying issue appeared within first 300.

I've already tried with verbose which gave no errors. That's why I moved to debug logging. With double verbose I don't see anything wrong either. Excerpt from log for relevant 100 connections among which 1 failed:

$ cat gpg.log | 
    sed 's/.*gpg-agent\[[0-9]*\] //'  | # remove date, time and process id                            
    grep -v 'ssh handler .* \(started\|terminated\)' | # appears to be mostly noise wit hex address
    sort|uniq -c
     80 new connection to /usr/libexec/gnupg2/scdaemon daemon established
     20 new connection to /usr/libexec/gnupg2/scdaemon daemon established (reusing)
    100 received ssh request of length 1
    100 received ssh request of length 208
    100 received ssh request of length 748
    100 sending ssh response of length 1
    100 sending ssh response of length 281
    100 sending ssh response of length 626
    100 ssh request handler for extension (27) ready
    100 ssh request handler for extension (27) started
    100 ssh request handler for request_identities (11) ready
    100 ssh request handler for request_identities (11) started
    100 ssh request handler for sign_request (13) ready
    100 ssh request handler for sign_request (13) started
    100 ssh-agent extension 'session-bind@openssh.com' not supported
    100 ssh-agent extension 'session-bind@openssh.com' received

You need to get a log form gpg-agent. Put this into ~/.gnupg/gpg-agent/conf

Frankly, he OpenSSH support for Windows was experimental and I have never tested it. If it can be confirmed that this really works and is useful, it will be easy to add the opeion to gpgconf.

Frankly, he OpenSSH support for Windows was experimental and I have never tested it. If it can be confirmed that this really works and is useful, it will be easy to add the opeion to gpgconf. Note that the gpgconf option feature handles only a subset of all options on purpose.

The problem was: In scdaemon, PKSIGN with OPENPGP.3 didn't work well for Ed25519 (done by do_auth function in app-openpgp.c), when --hash=sha512 (not SHA1).

I located the bug in scdaemon.

Using the primary key for ssh was not intended and thus not tested. I have not yet found the time too look closer at your report. Just one remark:

Thanks for that info. I tag it as FAQ and change the subject in case someone searches for such a problem.

Fixed in 2.5.2.

Closed, since this was documentation for the workaround, four years ago.

Just a reminder: with Gnuk 1.2.15 and an ed25519 key PubkeyAuthentication unbound is required for hosts using the new feature.

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.