Which method is NOT suitable for keeping secrets out of Terraform configuration files?

The choice of "secure string" as a method that is not suitable for keeping secrets out of Terraform configuration files is accurate because Terraform itself does not have a built-in feature specifically called "secure string." While many programming languages and frameworks might offer a concept of secure strings, in the context of Terraform, it does not provide a designated mechanism or resource type that functions as a "secure string."

Instead, Terraform strongly encourages the use of other methods outlined in the options, such as the -var flag, environment variables, and utilizing external secrets management systems via providers.

Using the -var flag allows you to pass sensitive variables at runtime, reducing the risk of hardcoding sensitive information directly in the configuration files. This approach also allows you to avoid accidental exposure of secrets when the configuration files are shared.

Setting environment variables is another widely used method as it keeps sensitive information out of version control and allows for dynamic retrieval of secrets when running Terraform commands. This way, the sensitive values can be managed outside of the Terraform code itself.

Utilizing a Terraform provider tailored for secret management (like HashiCorp Vault) is a well-established practice. Such providers can securely fetch sensitive data at runtime, significantly enhancing security and opacity of sensitive information