ABAC for AWS KMS

Attribute-based access control (ABAC) is an authorization strategy that defines permissions based on attributes. AWS KMS supports ABAC by allowing you to control access to your customer managed keys based on the tags and aliases associated with the KMS keys. The tag and alias condition keys that enable ABAC in AWS KMS provide a powerful and flexible way to authorize principals to use KMS keys without editing policies or managing grants. But you should use these feature with care so principals aren't inadvertently allowed or denied access.

If you use ABAC, be aware that permission to manage tags and aliases is now an access control permission. Be sure that you know the existing tags and aliases on all KMS keys before you deploy a policy that depends on tags or aliases. Take reasonable precautions when adding, deleting, and updating aliases, and when tagging and untagging keys. Give permissions to manage tags and aliases only to principals who need them, and limit the tags and aliases they can manage.

Notes

When using ABAC for AWS KMS, be cautious about giving principals permission to manage tags and aliases. Changing a tag or alias might allow or deny permission to a KMS key. Key administrators who don't have permission to change key policies or create grants can control access to KMS keys if they have permission to manage tags or aliases.

It might take up to five minutes for tag and alias changes to affect KMS key authorization. Recent changes might be visible in API operations before they affect authorization.

To control access to a KMS key based on its alias, you must use a condition key. You cannot use an alias to represent a KMS key in the Resource element of a policy statement. When an alias appears in the Resource element, the policy statement applies to the alias, not to the associated KMS key.

Learn more

• For details about AWS KMS support for ABAC, including examples, see Use aliases to control access to KMS keys and Use tags to control access to KMS keys.

• For more general information about using tags to control access to AWS resources, see What is ABAC for AWS? and Controlling Access to AWS Resources Using Resource Tags in the IAM User Guide.

ABAC condition keys for AWS KMS

To authorize access to KMS keys based on their tags and aliases, use the following condition keys in a key policy or IAM policy.

ABAC condition key	Description	Policy type	AWS KMS operations
aws:ResourceTag	Tag (key and value) on the KMS key matches the tag (key and value) or tag pattern in the policy	IAM policy only	KMS key resource operations 2
aws:RequestTag/tag-key	Tag (key and value) in the request matches the tag (key and value) or tag pattern in the policy	Key policy and IAM policies1	TagResource, UntagResource
aws:TagKeys	Tag keys in the request match the tag keys in the policy	Key policy and IAM policies1	TagResource, UntagResource
kms:ResourceAliases	Aliases associated with the KMS key match the aliases or alias patterns in the policy	IAM policy only	KMS key resource operations 2
kms:RequestAlias	Alias that represents the KMS key in the request matches the alias or alias patterns in the policy.	Key policy and IAM policies1	Cryptographic operations, DescribeKey, GetPublicKey

¹Any condition key that can be used in a key policy can also be used in an IAM policy, but only if the key policy allows it.

²A KMS key resource operation is an operation authorized for a particular KMS key. To identify the KMS key resource operations, in the AWS KMS permissions table, look for a value of KMS key in the Resources column for the operation.

For example, you can use these condition keys to create the following policies.

• An IAM policy with kms:ResourceAliases that allows permission to use KMS keys with a particular alias or alias pattern. This is a bit different from policies that rely on tags: Although you can use alias patterns in a policy, each alias must be unique in an AWS account and Region. This allows you to apply a policy to a select set of KMS keys without listing the key ARNs of the KMS keys in the policy statement. To add or remove KMS keys from the set, change the alias of the KMS key.

• A key policy with kms:RequestAlias that allows principals to use a KMS key in a Encrypt operation, but only when the Encrypt request uses that alias to identify the KMS key.

• An IAM policy with aws:ResourceTag/tag-key that denies permission to use KMS keys with a particular tag key and tag value. This lets you apply a policy to a select set of KMS keys without listing the key ARNs of the KMS keys in the policy statement. To add or remove KMS keys from the set, tag or untag the KMS key.

• An IAM policy with aws:RequestTag/tag-key that allows principals to delete only "Purpose"="Test" KMS key tags.

• An IAM policy with aws:TagKeys that denies permission to tag or untag a KMS key with a Restricted tag key.

ABAC makes access management flexible and scalable. For example, you can use the aws:ResourceTag/tag-key condition key to create an IAM policy that allows principals to use a KMS key for specified operations only when the KMS key has a Purpose=Test tag. The policy applies to all KMS keys in all Regions of the AWS account.

When attached to a user or role, the following IAM policy allows principals to use all existing KMS keys with a Purpose=Test tag for the specified operations. To provide this access to new or existing KMS keys, you don't need to change the policy. Just attach the Purpose=Test tag to the KMS keys. Similarly, to remove this access from KMS keys with a Purpose=Test tag, edit or delete the tag.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AliasBasedIAMPolicy",
      "Effect": "Allow",
      "Action": [
        "kms:Decrypt",
        "kms:Encrypt",
        "kms:GenerateDataKey*",
        "kms:DescribeKey"
      ],
      "Resource": "arn:aws:kms:*:111122223333:key/*",
      "Condition": {
        "StringEquals": {
          "aws:ResourceTag/Purpose": "Test"
        }
      }
    }
  ]
}

However, if you use this feature, be careful when managing tags and aliases. Adding, changing, or deleting a tag or alias can inadvertently allow or deny access to a KMS key. Key administrators who don't have permission to change key policies or create grants can control access to KMS keys if they have permission to manage tags and aliases. To mitigate this risk, consider limiting permissions to manage tags and aliases. For example, you might want to allow only select principals to manage Purpose=Test tags. For details, see Use aliases to control access to KMS keys and Use tags to control access to KMS keys.

Tags or aliases?

AWS KMS supports ABAC with tags and aliases. Both options provide a flexible, scalable access control strategy, but they're slightly different from each other.

You might decide to use tags or use aliases based on your particular AWS use patterns. For example, if you have already given tagging permissions to most administrators, it might be easier to control an authorization strategy based on aliases. Or, if you are close to the quota for aliases per KMS key, you might prefer an authorization strategy based on tags.

The following benefits are of general interest.

Benefits of tag-based access control

• Same authorization mechanism for different types of AWS resources.

  You can use the same tag or tag key to control access to multiple resource types, such as an Amazon Relational Database Service (Amazon RDS) cluster, an Amazon Elastic Block Store (Amazon EBS) volume, and a KMS key. This feature enables several different authorization models that are more flexible than traditional role-based access control.

• Authorize access to a group of KMS keys.

  You can use tags to manage access to a group of KMS keys in the same AWS account and Region. Assign the same tag or tag key to the KMS keys that you choose. Then create a simple, easy-to-maintain policy statement that is based on the tag or tag key. To add or remove a KMS key from your authorization group, add or remove the tag; you don't need to edit the policy.

Benefits of alias-based access control

• Authorize access to cryptographic operations based on aliases.

  Most request-based policy conditions for attributes, including aws:RequestTag/tag-key, affect only operations that add, edit, or delete the attribute. But the kms:RequestAlias condition key controls access to cryptographic operations based on the alias used to identify the KMS key in the request. For example, you can give a principal permission to use a KMS key in a Encrypt operation but only when the value of the KeyId parameter is alias/restricted-key-1. To satisfy this condition requires all of the following:

  • The KMS key must be associated with that alias.

  • The request must use the alias to identify the KMS key.

  • The principal must have permission to use the KMS key subject to the kms:RequestAlias condition.

  This is particularly useful if your applications commonly use alias names or alias ARNs to refer to KMS keys.

• Provide very limited permissions.

  An alias must be unique in an AWS account and Region. As a result, giving principals access to a KMS key based on an alias can be much more restrictive than giving them access based on a tag. Unlike aliases, tags can be assigned to multiple KMS keys in the same account and Region. If you choose, you can use an alias pattern, such as alias/test*, to give principals access to a group of KMS keys in the same account and Region. However, allowing or denying access to a particular alias allows very strict control on KMS keys.