If you want to update your EC2 instance to enable root login via key file:
1. Login as user "EC2-User"
2. sudo su -
3. cp ~/.ssh/authorized_keys ~/.ssh/authorized_keys.bak
4. vim ~/.ssh/authorized_keys
5. remove everything before "ssh-rsa"
6. esc -> :wq
7. vim /etc/ssh/sshd_config
8. Please ensure these are enabled:
PermitRootLogin yes
# Only allow root to run commands over ssh, no shell
# PermitRootLogin forced-commands-only
RSAAuthentication yes
PubkeyAuthentication yes
AuthorizedKeysFile .ssh/authorized_keys
8. esc -> :wq
9. service sshd reload
10. Profit?!