Permissions 0664 for '/home/centos/.ssh/crystal.ppk' are too open. It is required that your private key files are NOT accessible by others. This private key will be ignored. code example

Example 1: private key is too open

chmod 600 ~/.ssh/id_rsa

Example 2: It is required that your private key files are NOT accessible by others ubuntu

chmod 400 {keyfile}.pem is what amazon instructed and it works.