Openssh 7_9: key exchange failed, expected SSH_MSG_KEX_GEX_GROUP

Question

I'm trying to connect from SmartCVS via ssh to a Fedora 29 system but connection fails with an error message "key exchange failed, expected SSH_MSG_KEX_GEX_GROUP".

Since SmartCVS is somewhat outdated, I think this is caused by it's limited capabilities (it supports RSA and DSA for key exchange). So my question: how can I re-enable these functionalities in my OpenSSH-server?

Thanks!

Answer 1

The key exchange algorithms diffie-hellman-group-exchange-sha1 and diffie-hellman-group-exchange-sha256 were changed in OpenSSH 6.9. Prior to that update they supported an old message that many other implementations were using. The OpenSSH developers removed support for that in 6.9. This is the error message you will see if your client tries to use the old message.

There is no way to fix it other than disabling those key exchange algorithms on the server (which I certainly do not advise). The alternative is to find a more up-to-date CVS client that uses today's stronger security settings.