What worked for me was the root key generated on my hmc deployment node with ssh-keygen.
you can perform a get opertion after connecing to the node with sftp.
blue@Stallion:~$ sftp -i blue.pem firstname.lastname@example.org
Connected to ec2-50-17-141-9.compute-1.amazonaws.com.
sftp> get .ssh/id_rsa
Fetching /root/.ssh/id_rsa to id_rsa
/root/.ssh/id_rsa 100% 1675 1.6KB/s 00:00
I have recently tried providing the amazon key, in my case blue.pem and since they set it up so that you need this private key to access your nodes it should work. Ill get back with you if its successful or not…