Support for sntrup761x25519-sha512@openssh.com in 8.9 missing?

[aa-matthias]

"OpenSSH for Windows" version
V8.9.1.0p1-Beta

Server OperatingSystem
Ubuntu 22.04

Client OperatingSystem
Windows 11 Pro

What is failing
Windows 11 Pro Client has V8.9.1.0P1-Beta installed
.ssh/config has KexAlgorithms config option set and is including sntrup761x25519-sha512@openssh.com
trying to connect to a server shows a config failure. ssh-Q kex is not listing sntrup761x25519-sha512@openssh.com

Expected output
Shouldn't sntrup761x25519-sha512@openssh.com be supported starting with OpenSSH 8.9 out of the box? It's working just fine on my 22.04 ubuntu install.

Actual output

[Harvester57]

OpenSSH v9.0 switched to using this key exchange cipher, but it is still not supported by the Windows client (even the latest release: https://github.com/PowerShell/Win32-OpenSSH/releases/tag/v9.1.0.0p1-Beta, even though the changelog explicitely mention it includes upstream v9.0 and v9.1 changes)

[appletalk]

sntrup761x25519-sha512@openssh.com still seems to be missing from the latest v9.2 as well.

[jasperweiss]

It is still missing from v9.8 as well

[lilyanatia]

#2140 (comment)

[Binarus]

We must SSH all the day to Linux servers as well as to Windows servers. The Linux servers are mostly configured so that they offer only sntrup761x25519-sha512@openssh.com and nothing else for safety reasons.

At the client side, we work around the issue by using Linux or (on Windows) using Cygwin, whose SSH client (and server) of course support sntrup761x25519-sha512@openssh.com.

For the Windows server side, things are not that easy. Once upon a time, when Windows did not yet provide its own native port of the OpenSSH daemon, we installed Cygwin on the servers and used its OpenSSH implementation, which always was satisfying in the sense that we barely couldn't tell it apart from the original. All configuration directives and all ciphers that Linux systems provided were available and worked flawlessly and without taking special effort in Cygwin's OpenSSH daemon, too.

However, we had a few problems with permission handling in Cygwin vs. Windows, and installing Cygwin just to run the OpenSSH daemon was only a stopgap.

Therefore we switched to the native Windows OpenSSH daemon on our Windows servers as soon as we could use it reasonably. But now it looks that we should go back to Cygwin due to the native Windows OpenSSH daemon not supporting sntrup761x25519-sha512@openssh.com.

Does anybody know if there are plans to integrate it? We could go for a few further weeks without it, but if there are no such plans at all, we'll have to go back to Cygwin on the Windows servers. That's a thing we would like to avoid, though.

Thank you very much in advance!