Why use AWS Customer Managed Keys?

tfsec promotes CMK keys over AWS managed KMS, how come?

Published: Wednesday, Dec 14, 2022 Last modified: Sunday, Jan 29, 2023

tfsec promotes CMK usage as mentioned in my video. But why CMK and what’s the trade off?

The table on the official AWS documentation doesn’t spell out why you would want to use CMK over AWS-Managed.

I’ve asked around and the reasons are:

Cost of CMK

$1/key/month, but with yearly rotation you pay an extra $1/key/month for the previous versions too.

Key policy is the last line of defence for IAM policy errors

KMS is the only service where both the IAM principal policy AND the resource policy has to allow access. In S3, if either of those allowed access than the Principal would have access. That’s not true for KMS. The key policy must still allow it.

So if you accidentally gave * to a Principal the key policy could still prevent access.

Reasons not to use CMK

It’s not unusual for a key per app/service/team adding costly complexity for little benefit. Overzealous security reasoning includes: “Separation of concerns” and the “Principle of least privilege”. It’s like have an SSL certificate for every subdomain and application and team.

Having to orchestrate infrastructure with a specific key makes every *.tfvars to require an extra parameter like kms_master_key_id. If you don’t manage the key and policy carefully you might accidentally lock out your data.

The threat model is arguably just local to the AWS account, and likelyhood of AWS-managed and encrypted data being exposed is both unlikely and limited to essentially AWS. Hence the extra overhead of managing keys for internal communication outweigh the surface area of the threat. Especially considering there are probably far easier ways for AWS to intercept your data if you didn’t embrace the shared security model.

ChatGPT: Is there a downside to using AWS CMK over AWS managed keys?

There are a few potential downsides to using AWS Customer Managed Keys (CMKs) over AWS managed keys. First, using CMKs requires more work on the part of the customer. The customer is responsible for creating, rotating, disabling, and deleting the keys, as well as managing the key policies and grants. This can require a dedicated team or person to handle these tasks, which can add to the cost and complexity of using the service.

Second, using CMKs can also be more expensive than using AWS managed keys. While AWS managed keys are provided at no additional cost, customers must pay for the use of CMKs based on the number of keys and the number of cryptographic operations performed.

Overall, the decision to use AWS CMKs or AWS managed keys depends on the sto pecific needs and goals of the organization. AWS CMKs can provide greater control and flexibility, but they also require more work and can be more expensive.