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

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.