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Date: Thu, 8 Oct 2015 21:01:49 +0200
Message-ID: <CAN-5ny=PC8wpw1kd_pngVjXAB1J4tEZt2P37UprxfhKaVa8sfQ@mail.gmail.com>
Subject: Re: Kex Error in X2Go client
From: rpr nospam <rpr.nospam@gmail.com>
To: 765@bugs.x2go.org
Content-Type: text/plain; charset=UTF-8

Hi!

I also saw this issue while trying to connect with X2Go Client for
Windows (v. 4.0.5.0) to x2goserver (v. 4.0.1.19-0~1064~ubuntu14.04.1)
on a Linux Mint 17 machine.

The connection worked fine until the SSH server on the Linux machine
was reconfigured in order to harden it (see the recommendations in
https://stribika.github.io/2015/01/04/secure-secure-shell.html).

That reconfiguration allowed only the following host keys and key
exchange algorithms:

HostKey /etc/ssh/ssh_host_ed25519_key
HostKey /etc/ssh/ssh_host_rsa_key

KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256

The Elliptic Curve Digital Signature Algorithm (ECDSA) (and the
corresponding /etc/ssh/ssh_host_ecdsa_key) is not enabled because its
implementation may use a faulty pseudo-random number generator (i.e.
containing a back door created by NSA) as explained in some pages
referred by the aforementioned article.

After some investigation I found how to get rid of the error: in
C:\Users\username\ssh\known_hosts remove the line corresponding to the
x2goserver host and ecdsa-sha2-nistp256 key exchange algorithm:

x2goserver_host_address ecdsa-sha2-nistp256 ...

-- rpr.