When login to yous amazon ec2 instance via putty or command line it is possible that you get this error.
Authenticating with public key "imported-openssh-key"
Please login as the ec2-user user rather than root user.
Then login as user "ec2-user" instead of root user and if you want to run commands as root user, use sudo command or switch the user to root
#sudo vi /etc/dhcpd.conf
#sudo su -
Then you'll become root user.