The purpose of this configuration is to disable ssh login using password and force user to use ssh-key pair instead. Open /etc/ssh/sshd_config file using text editor and change following option.

PermitRootLogin no          # prevent root login via ssh
PasswordAuthentication no   # deny ssh login using password
PubkeyAuthentication yes    # enable public key authentication

# Explicitly define which user can login via ssh (optional)
# AllowUsers root otheruser

Note: if the option is commented, delete ‘#’ at the start of line to enable the option.

After we done editing config file, we need to restart sshd service. On ubuntu/debian based distro, we can use command

sudo service sshd restart