AWS Key Management Service Construct Library

Define a KMS key:

kms.Key(self, "MyKey",
    enable_key_rotation=True,
    rotation_period=Duration.days(180)
)

Define a KMS key with waiting period:

Specifies the number of days in the waiting period before AWS KMS deletes a CMK that has been removed from a CloudFormation stack.

key = kms.Key(self, "MyKey",
    pending_window=Duration.days(10)
)

Add a couple of aliases:

key = kms.Key(self, "MyKey")
key.add_alias("alias/foo")
key.add_alias("alias/bar")

Define a key with specific key spec and key usage:

Valid keySpec values depends on keyUsage value.

key = kms.Key(self, "MyKey",
    key_spec=kms.KeySpec.ECC_SECG_P256K1,  # Default to SYMMETRIC_DEFAULT
    key_usage=kms.KeyUsage.SIGN_VERIFY
)

Create a multi-Region primary key:

key = kms.Key(self, "MyKey",
    multi_region=True
)

Importing existing keys

Import key by ARN

To use a KMS key that is not defined in this CDK app, but is created through other means, use Key.fromKeyArn(parent, name, ref):

my_key_imported = kms.Key.from_key_arn(self, "MyImportedKey", "arn:aws:...")

# you can do stuff with this imported key.
my_key_imported.add_alias("alias/foo")

Note that a call to .addToResourcePolicy(statement) on myKeyImported will not have an affect on the key’s policy because it is not owned by your stack. The call will be a no-op.

Import key by alias

If a Key has an associated Alias, the Alias can be imported by name and used in place of the Key as a reference. A common scenario for this is in referencing AWS managed keys.

import aws_cdk.aws_cloudtrail as cloudtrail


my_key_alias = kms.Alias.from_alias_name(self, "myKey", "alias/aws/s3")
trail = cloudtrail.Trail(self, "myCloudTrail",
    send_to_cloud_watch_logs=True,
    encryption_key=my_key_alias
)

Note that calls to addToResourcePolicy method on myKeyAlias will be a no-op, addAlias and aliasTargetKey will fail. The grant methods (i.e., methods in KeyGrants) will not modify the key policy, as the imported alias does not have a reference to the underlying KMS Key. For the grant methods to modify the principal’s IAM policy, the feature flag @aws-cdk/aws-kms:applyImportedAliasPermissionsToPrincipal must be set to true. By default, this flag is false and grant calls on an imported alias are a no-op.

Lookup key by alias

If you can’t use a KMS key imported by alias (e.g. because you need access to the key id), you can lookup the key with Key.fromLookup().

In general, the preferred method would be to use Alias.fromAliasName() which returns an IAlias object which extends IKey. However, some services need to have access to the underlying key id. In this case, Key.fromLookup() allows to lookup the key id.

The result of the Key.fromLookup() operation will be written to a file called cdk.context.json. You must commit this file to source control so that the lookup values are available in non-privileged environments such as CI build steps, and to ensure your template builds are repeatable.

Here’s how Key.fromLookup() can be used:

my_key_lookup = kms.Key.from_lookup(self, "MyKeyLookup",
    alias_name="alias/KeyAlias"
)

role = iam.Role(self, "MyRole",
    assumed_by=iam.ServicePrincipal("lambda.amazonaws.com")
)
my_key_lookup.grant_encrypt_decrypt(role)

Note that a call to .addToResourcePolicy(statement) on myKeyLookup will not have an affect on the key’s policy because it is not owned by your stack. The call will be a no-op.

If the target key is not found in your account, an error will be thrown. To prevent the error in the case, you can receive a dummy key without the error by setting returnDummyKeyOnMissing to true. The dummy key has a keyId of 1234abcd-12ab-34cd-56ef-1234567890ab. The value of the dummy key id can also be referenced using the Key.DEFAULT_DUMMY_KEY_ID variable, and you can check if the key is a dummy key by using the Key.isLookupDummy() method.

dummy = kms.Key.from_lookup(self, "MyKeyLookup",
    alias_name="alias/NonExistentAlias",
    return_dummy_key_on_missing=True
)

if kms.Key.is_lookup_dummy(dummy):
    pass

Key Policies

Controlling access and usage of KMS Keys requires the use of key policies (resource-based policies attached to the key); this is in contrast to most other AWS resources where access can be entirely controlled with IAM policies, and optionally complemented with resource policies. For more in-depth understanding of KMS key access and policies, see

https://docs.aws.amazon.com/kms/latest/developerguide/control-access-overview.html
https://docs.aws.amazon.com/kms/latest/developerguide/key-policies.html

KMS keys can be created to trust IAM policies. This is the default behavior for both the KMS APIs and in the console. This behavior is enabled by the ‘@aws-cdk/aws-kms:defaultKeyPolicies’ feature flag, which is set for all new projects; for existing projects, this same behavior can be enabled by passing the trustAccountIdentities property as true when creating the key:

kms.Key(self, "MyKey", trust_account_identities=True)

With either the @aws-cdk/aws-kms:defaultKeyPolicies feature flag set, or the trustAccountIdentities prop set, the Key will be given the following default key policy:

{
  "Effect": "Allow",
  "Principal": {"AWS": "arn:aws:iam::111122223333:root"},
  "Action": "kms:*",
  "Resource": "*"
}

This policy grants full access to the key to the root account user. This enables the root account user – via IAM policies – to grant access to other IAM principals. With the above default policy, future permissions can be added to either the key policy or IAM principal policy.

key = kms.Key(self, "MyKey")
user = iam.User(self, "MyUser")
key.grants.encrypt(user)

Adopting the default KMS key policy (and so trusting account identities) solves many issues around cyclic dependencies between stacks. Without this default key policy, future permissions must be added to both the key policy and IAM principal policy, which can cause cyclic dependencies if the permissions cross stack boundaries. (For example, an encrypted bucket in one stack, and Lambda function that accesses it in another.)

Appending to or replacing the default key policy

The default key policy can be amended or replaced entirely, depending on your use case and requirements. A common addition to the key policy would be to add other key admins that are allowed to administer the key (e.g., change permissions, revoke, delete). Additional key admins can be specified at key creation or after via the key.grants.admin method.

my_trusted_admin_role = iam.Role.from_role_arn(self, "TrustedRole", "arn:aws:iam:....")
key = kms.Key(self, "MyKey",
    admins=[my_trusted_admin_role]
)

second_key = kms.Key(self, "MyKey2")
second_key.grants.admin(my_trusted_admin_role)

Alternatively, a custom key policy can be specified, which will replace the default key policy.

Note: In applications without the ‘@aws-cdk/aws-kms:defaultKeyPolicies’ feature flag set and with trustedAccountIdentities set to false (the default), specifying a policy at key creation appends the provided policy to the default key policy, rather than replacing the default policy.

my_trusted_admin_role = iam.Role.from_role_arn(self, "TrustedRole", "arn:aws:iam:....")
# Creates a limited admin policy and assigns to the account root.
my_custom_policy = iam.PolicyDocument(
    statements=[iam.PolicyStatement(
        actions=["kms:Create*", "kms:Describe*", "kms:Enable*", "kms:List*", "kms:Put*"
        ],
        principals=[iam.AccountRootPrincipal()],
        resources=["*"]
    )]
)
key = kms.Key(self, "MyKey",
    policy=my_custom_policy
)

Warning: Replacing the default key policy with one that only grants access to a specific user or role runs the risk of the key becoming unmanageable if that user or role is deleted. It is highly recommended that the key policy grants access to the account root, rather than specific principals. See https://docs.aws.amazon.com/kms/latest/developerguide/key-policies.html for more information.

Signing and Verification key policies

Creating signatures and verifying them with KMS requires specific permissions. The respective policies can be attached to a principal via the key.grants.sign and key.grants.verify methods.

key = kms.Key(self, "MyKey")
user = iam.User(self, "MyUser")
key.grants.sign(user) # Adds 'kms:Sign' to the principal's policy
key.grants.verify(user)

If both sign and verify permissions are required, they can be applied with one method in KeyGrants called signVerify.

key = kms.Key(self, "MyKey")
user = iam.User(self, "MyUser")
key.grants.sign_verify(user)

HMAC specific key policies

HMAC keys have a different key policy than other KMS keys. They have a policy for generating and for verifying a MAC. The respective policies can be attached to a principal via the generateMac and verifyMac methods in KeyGrants.

key = kms.Key(self, "MyKey")
user = iam.User(self, "MyUser")
key.grants.generate_mac(user) # Adds 'kms:GenerateMac' to the principal's policy
key.grants.verify_mac(user)

Granting permissions for L1s

The examples above show how to use the KeyGrants methods to grant permissions to principals. If you are using L1 constructs that require permissions to be granted to a principal, you can use the KeyGrants utility class:

# principal: iam.IPrincipal
# key: kms.IKeyRef
# can be either an L1 or L2

kms.KeyGrants.from_key(key).sign(principal)

If key is an instance of CfnKey, and the grants process involves adding statements to the key policy, then the KeyGrants class will, by default, do the same thing it would do for an instance of Key: add statements to the keyPolicy property.

But if you want to customize this behavior, you can register an instance of IResourcePolicyFactory for the AWS::KMS::Key CloudFormation type:

from aws_cdk.aws_iam import AddToResourcePolicyResult
from aws_cdk import CfnResource
from aws_cdk.aws_iam import IResourcePolicyFactory, IResourceWithPolicyV2, PolicyStatement, ResourceWithPolicies
from constructs import Construct, IConstruct

# scope: Construct
@jsii.implements(IResourcePolicyFactory)
class MyFactory:
    def for_resource(self, resource):
        return {
            "env": resource.env,
            def add_to_resource_policy(self, statement):
                # custom implementation to add the statement to the resource policy
                return AddToResourcePolicyResult("statement_added"=True, "policy_dependable"=resource)
        }

ResourceWithPolicies.register(scope, "AWS::KMS::Key", MyFactory())

IResourcePolicyFactory is responsible for converting a construct into a IResourceWithPolicyV2, effectively providing an ad-hoc way to extend the behavior of L1s to support grants the same way as L2s do.