WinSCP cannot negotiate a secure key exchange with OpenSSH 6.9 (Fixed in PuTTY 0.65)

WinSCP supports the following key exchange mechanisms:
While OpenSSH 6.9 supports these by default:
Which means that the following are common:
However, WinSCP is still using the old SSH2_MSG_KEX_DH_GEX_REQUEST_OLD message. Support for the old deprecated message was removed in OpenSSH 6.9:
Which means that WinSCP cannot successfully negotiate diffie-hellman-group-exchange-* ciphers. For this reason, they were blacklisted for WinSCP clients in OpenSSH 6.9:
This leaves WinSCP with diffie-hellman-group14-sha1. SHA-1 is deprecated and insecure, and security-conscious SSH server administrators disable SHA-1 key exchange mechanisms. WinSCP can no longer negotiate any key exchange mechanism with OpenSSH 6.9 that is considered secure.

PuTTY 0.65 supports the newer message type. Additionally, PuTTY snapshots support newer protocols like curve25519 kex and the chacha20 cipher, which are a really good idea. WinSCP should update to at least 0.65, and the WinSCP team should notify the OpenSSH developers of the minimum version number that will support the new message type, so they can limit the blacklist to older versions.

Thanks for your message.

We will look into this.

+1 for this!
Can't connect from Far Manager Netbox plugin (based on latest WinSCP) to server with recent OpenSSH 6.9 but successfully connect to it from latest beta PuTTY.

RFC 4419 added to the tracker:
https://winscp.net/tracker/1345

Hello,

as far as I understand, the problem is fixed in PuTTY 0.65, and therefore only in non-leased-yet WinSCP 5.8. I guess the information in (https://winscp.net/tracker/1345) is misleading, because it says that the problem has been fixed in WinSCP version 5.7.5.

When is the 5.8 version due to be released?

Wojtek

The tracker is correct. The RFC 4419 is implemented in 5.7.5.

See also version history
https://winscp.net/eng/docs/history?a=5.7.5

@martin, latest WinSCP 5.7.5 (Build 5665) still get error "Couldn't agree a host key algorithm (available: ssh-ed25519)" when trying to connect to server with OpenSSH_6.9p1.

Hello again, @martin,

whatever it is that has been corrected in 5.7.5, the error that was mentioned earlier (problem connecting to OpenSSH 6.9) still persists:

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and as has been written somewhere, the problem is in PuTTY, and has been fixed in PuTTY version 0.65, so until WinSCP moves to PuTTY 0.65 code, it probably won't work. I've read, that WinSCP v5.8 will be based on PuTTY 0.65.

So, back to my question – when is WinSCP 5.8 going to be released? Or is this bug going to be fixed in an earlier version of WinSCP?

Best regards,
Wojtek

Please attach a full log file showing the problem (using the latest version of WinSCP).

I have the same problem.
Log file attached...

@Omyl: Your server offers only curve25519-sha256@libssh.org KEX to WinSCP.
It's probably because OpenSSH 6.9 (yours) and 7.0 won't offer DH KEX to WinSCP, as OpenSSH since 6.9 dropped support for pre-RFC4419 implementations.

OpenSSH 7.1 will offer DH KEX to WinSCP 5.7.5 and newer (which supports RFC4419). So please upgrade your OpenSSH to the latest version.

martin wrote:

OpenSSH 7.1 will offer DH KEX to WinSCP 5.7.5 and newer (which supports RFC4419). So please upgrade your OpenSSH to the latest version.

"OpenSSH_7.1p1, without OpenSSL" – the same error

Couldn't agree a key exchange algorithm (available: curve25519-sha256@libssh.org)

@mvs: Did you use the latest version of WinSCP (5.7.5)?

Please attach a full log file showing the problem (using the latest version of WinSCP).


Can I have a test account on the server?

Hello @martin, sorry for the late reply. I attach the log file, both from WinSCP (as you instructed), and from the OpenSSH server (run in debug mode), both in one file, zipped.

@Woitek: See my answer to @Omyl's post above:
https://winscp.net/forum/viewtopic.php?p=58774#58774

Oh, I get it – OpenSSH not only disabled the KEX, they also stopped advertising it to WinSCP based on the name/version of the client. So indeed, either one has to upgrade to OpenSSH 7.1, or probably only apply this patch:
https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/compat.c.diff?r1=1.96&r2=1.97

I'll try it in the evening.

@Woitek: Correct, applying the patch helps too.

Correct, applying the patch helps too.
It works! Thank you, @martin.

Wojtek