Cannot connect to modern ssh servers because only legacy key exchanges are supported

[costi]

When trying to initiate a transfer with an Ubuntu 20.04 box it gives this error:

Caused by: java.io.IOException: Cannot negotiate, proposals do not match.
	at ch.ethz.ssh2.transport.KexManager.handleMessage(KexManager.java:340)
	at ch.ethz.ssh2.transport.TransportManager.receiveLoop(TransportManager.java:571)
	at ch.ethz.ssh2.transport.TransportManager$1.run(TransportManager.java:338)
	at java.base/java.lang.Thread.run(Thread.java:834)

I enabled debug logging for the ssh server and I see this error:

Unable to negotiate with 127.0.0.1 port 59398: no matching key exchange method found. 
Their offer: diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1,diffie-hellman-group1-sha1 [preauth]

The current default key exchange algorithms are:

server-sig-algs=<ssh-ed25519,[email protected],ssh-rsa,rsa-sha2-256,rsa-sha2-512,ssh-dss,
ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,[email protected]>

Updating the config on the ssh server to support the legacy KEX is not a good idea. The KEX supported by fdt are old and insecure.