- `kms_key_id` (string) - ID, alias or ARN of the KMS key to use for boot
  volume encryption. This only applies to the main `region`, other regions
  where the AMI will be copied will be encrypted by the default EBS KMS key.
  For valid formats see _KmsKeyId_ in the [AWS API docs -
  CopyImage](https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_CopyImage.html).
  This field is validated by Packer, when using an alias, you will have to
  prefix `kms_key_id` with `alias/`.
