How to disable normal username/password login authentication to your Ubuntu server, only allowing SSH key authentication. This gives you the advantages of SSH’s whilst securing your server that little bit more.
A guide on creating SSH keys for your server here.
Open the SSH config to edit its values:
sudo nano /etc/ssh/sshd_config
Find the parameter PasswordAuthentication and set it to no
Next find ChallengeResponseAuthentication and set it to no
Save these changes with ctrl + x and then enter.
Finally, reload the SSH server
sudo systemctl reload ssh
You will now no longer be able to long with a username and password, Only SSH keys will work.