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.
Open the SSH config to edit its values:
sudo nano /etc/ssh/sshd_config
Find the parameter PasswordAuthentication and set it to no
PasswordAuthentication no
Next find ChallengeResponseAuthentication and set it to no
ChallengeResponseAuthentication 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.