After upgrading some bastion hosts to Debian 10, connections to some older network gear failed.

Connecting to some ASA firewalls generated the error:

Unable to negotiate with port 22: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1

This was a simple fix:

lab-5585-1# conf t
lab-5585-1(config)# ssh key-exchange group dh-group14-sha1
lab-5585-1(config)# end

Some older devices, Catalyst 3750 switches and ASA 5540 firewalls, complained of no matching cipher:

%SSH-3-NO_MATCH: No matching cipher found: client,aes128-ctr,aes192-ctr,aes256-ctr,, server aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc

This could be fixed on the device with the ip ssh server algorithm encryption ... (3750) and ssh cipher encryption ... (ASA) commands, but I decided to fix this on the bastion host instead by adding Ciphers +aes256-cbc to /etc/ssh/ssh_config.