Understanding AWS Key Management Service (KMS) Policies
Tags: aws, kms, key, devops
Published: 2023-08-21 11:26:04

Amazon Web Services Key Management Service (KMS) is a vital component of AWS's security infrastructure, offering a robust way to manage cryptographic keys and encrypt sensitive data. KMS provides a versatile and granular control mechanism known as KMS policies. These policies dictate who can access and manage KMS keys and what actions they can perform with them.



## What is a KMS Policy?
![What is a AWS KMS Policy](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/wazretyhnuzjy9tty8rm.jpg)
In AWS, a KMS policy is a JSON document that defines permissions for a KMS key. These permissions specify which AWS identities (such as IAM users, roles, or AWS services) can perform specific operations on a KMS key. A KMS policy essentially answers the following questions:

- Who: Specifies which AWS identities can use the KMS key.
- What: Specifies what actions those identities can perform on the key (e.g., encrypt, decrypt, generate data keys, etc.).
- When: Optionally, defines conditions under which the policy is applied.




## Key Elements of a KMS Policy
![Key Elements of a AWS KMS Policy](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/kss5rwhps5ho9e11v6oe.jpg)
- Principal: This element defines the AWS identities that are allowed to access the KMS key. It could be an IAM user, role, or even an AWS service.

- Actions: Actions define what operations are allowed on the KMS key. These actions can range from basic ones like kms:Encrypt and kms:Decrypt to more advanced key management actions like kms:CreateKey and kms:DeleteKey.

- Resource: The resource element specifies which KMS key the policy applies to. You can attach a policy to a specific key, a key alias, or even a wildcard to apply the policy to multiple keys.

- Condition (Optional): Conditions allow you to define circumstances under which the policy is applied. For example, you can restrict key access to a specific IP address range, AWS services or AWS caller account.

## Best Practices for Managing KMS Policies
![Best Practices for AWS KMS Policies](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/3oo1cxw4a8ol11v7yzis.jpg)
- Least Privilege: Follow the principle of least privilege. Only grant the minimum permissions necessary for users and services to perform their tasks.

- Regular Review: Periodically review and audit your KMS policies to ensure they align with your security requirements. Remove unnecessary permissions.

- Use IAM Roles: Whenever possible, associate KMS keys with IAM roles rather than IAM users. Roles are more secure and flexible.

- Resource-Level Policies: Implement resource-level policies when dealing with multiple KMS keys to maintain control and organization.

- Key rotation: It's a good idea to enable automatic key rotation, which makes the solution more secure.

Sometimes the service does not work as we expected. In my [latest video](https://youtu.be/z_90vZaEnqk?feature=shared) you will see 3 ways to deal with error **KMS Access Denied Exception**.

{% embed https://www.youtube.com/watch?v=z_90vZaEnqk %}

## Types of keys
The KMS keys that you create are customer managed keys. AWS services that use KMS keys to encrypt your service resources often create keys for you. This is a very big topic, if you are interested, take a look at the [documentation](https://docs.aws.amazon.com/kms/latest/developerguide/overview.html). Maybe I'll write an article about it soon. For now, I wanted to focus on KMS Policies.

I also encourage you to take a look at my blog [lepczynski.it](https://lepczynski.it/en) and to subscribe to my [YouTube channel](https://www.youtube.com/@WojciechLepczynski).