The CDK Construct Library for AWS::KMS
Project description
AWS Key Management Service Construct Library
---Define a KMS key:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
import aws_cdk.aws_kms as kms
kms.Key(self, "MyKey",
enable_key_rotation=True
)
Add a couple of aliases:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
key = kms.Key(self, "MyKey")
key.add_alias("alias/foo")
key.add_alias("alias/bar")
Sharing keys between stacks
To use a KMS key in a different stack in the same CDK application, pass the construct to the other stack:
# Example automatically generated. See https://github.com/aws/jsii/issues/826
#
# Stack that defines the key
#
class KeyStack(cdk.Stack):
def __init__(self, scope, id, *, description=None, env=None, stackName=None, tags=None, synthesizer=None, terminationProtection=None, analyticsReporting=None):
super().__init__(scope, id, description=description, env=env, stackName=stackName, tags=tags, synthesizer=synthesizer, terminationProtection=terminationProtection, analyticsReporting=analyticsReporting)
self.key = kms.Key(self, "MyKey", removal_policy=cdk.RemovalPolicy.DESTROY)
#
# Stack that uses the key
#
class UseStack(cdk.Stack):
def __init__(self, scope, id, *, key, description=None, env=None, stackName=None, tags=None, synthesizer=None, terminationProtection=None, analyticsReporting=None):
super().__init__(scope, id, key=key, description=description, env=env, stackName=stackName, tags=tags, synthesizer=synthesizer, terminationProtection=terminationProtection, analyticsReporting=analyticsReporting)
# Use the IKey object here.
kms.Alias(self, "Alias",
alias_name="alias/foo",
target_key=key
)
key_stack = KeyStack(app, "KeyStack")
UseStack(app, "UseStack", key=key_stack.key)
Importing existing keys
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)
:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
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 .addToPolicy(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.
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.
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
my_key_alias = kms.Alias.from_alias_name(self, "myKey", "alias/aws/s3")
trail = cloudtrail.Trail(self, "myCloudTrail",
send_to_cloud_watch_logs=True,
kms_key=my_key_alias
)
Note that calls to addToResourcePolicy
and grant*
methods on myKeyAlias
will be
no-ops, and addAlias
and aliasTargetKey
will fail, as the imported alias does not
have a reference to the underlying KMS Key.
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:
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
kms.Key(stack, "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.
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
key = kms.Key(stack, "MyKey")
user = iam.User(stack, "MyUser")
key.grant_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 grantAdmin
method.
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
my_trusted_admin_role = iam.Role.from_role_arn(stack, "TrustedRole", "arn:aws:iam:....")
key = kms.Key(stack, "MyKey",
admins=[my_trusted_admin_role]
)
second_key = kms.Key(stack, "MyKey2")
second_key.grant_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.
# Example automatically generated without compilation. See https://github.com/aws/jsii/issues/826
my_trusted_admin_role = iam.Role.from_role_arn(stack, "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(stack, "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.
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