# AWS Key Management Service Construct Library Define a KMS key: ```go kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ EnableKeyRotation: jsii.Boolean(true), }) ``` 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. ```go key := kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ PendingWindow: awscdk.Duration_Days(jsii.Number(10)), }) ``` Add a couple of aliases: ```go key := kms.NewKey(this, jsii.String("MyKey")) key.addAlias(jsii.String("alias/foo")) key.addAlias(jsii.String("alias/bar")) ``` Define a key with specific key spec and key usage: Valid `keySpec` values depends on `keyUsage` value. ```go key := kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ KeySpec: kms.KeySpec_ECC_SECG_P256K1, // Default to SYMMETRIC_DEFAULT KeyUsage: kms.KeyUsage_SIGN_VERIFY, }) ``` ## 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: ```go /** * Stack that defines the key */ type keyStack struct { stack key key } func newKeyStack(scope app, id *string, props stackProps) *keyStack { this := &keyStack{} cdk.NewStack_Override(this, scope, id, props) this.key = kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ RemovalPolicy: cdk.RemovalPolicy_DESTROY, }) return this } type useStackProps struct { stackProps key iKey } /** * Stack that uses the key */ type useStack struct { stack } func newUseStack(scope app, id *string, props useStackProps) *useStack { this := &useStack{} cdk.NewStack_Override(this, scope, id, props) // Use the IKey object here. // Use the IKey object here. kms.NewAlias(this, jsii.String("Alias"), &AliasProps{ AliasName: jsii.String("alias/foo"), TargetKey: props.key, }) return this } keyStack := NewKeyStack(app, jsii.String("KeyStack")) NewUseStack(app, jsii.String("UseStack"), &useStackProps{ key: keyStack.key, }) ``` ## 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)`: ```go myKeyImported := kms.Key_FromKeyArn(this, jsii.String("MyImportedKey"), jsii.String("arn:aws:...")) // you can do stuff with this imported key. myKeyImported.AddAlias(jsii.String("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. ```go import cloudtrail "github.com/aws/aws-cdk-go/awscdk" myKeyAlias := kms.Alias_FromAliasName(this, jsii.String("myKey"), jsii.String("alias/aws/s3")) trail := cloudtrail.NewTrail(this, jsii.String("myCloudTrail"), &TrailProps{ SendToCloudWatchLogs: jsii.Boolean(true), KmsKey: myKeyAlias, }) ``` 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. ### 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: ```go myKeyLookup := kms.Key_FromLookup(this, jsii.String("MyKeyLookup"), &KeyLookupOptions{ AliasName: jsii.String("alias/KeyAlias"), }) role := iam.NewRole(this, jsii.String("MyRole"), &RoleProps{ AssumedBy: iam.NewServicePrincipal(jsii.String("lambda.amazonaws.com")), }) myKeyLookup.GrantEncryptDecrypt(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. ## 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: ```go kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ TrustAccountIdentities: jsii.Boolean(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: ```json { "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. ```go key := kms.NewKey(this, jsii.String("MyKey")) user := iam.NewUser(this, jsii.String("MyUser")) key.grantEncrypt(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. ```go myTrustedAdminRole := iam.Role_FromRoleArn(this, jsii.String("TrustedRole"), jsii.String("arn:aws:iam:....")) key := kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ Admins: []iPrincipal{ myTrustedAdminRole, }, }) secondKey := kms.NewKey(this, jsii.String("MyKey2")) secondKey.GrantAdmin(myTrustedAdminRole) ``` 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. ```go myTrustedAdminRole := iam.Role_FromRoleArn(this, jsii.String("TrustedRole"), jsii.String("arn:aws:iam:....")) // Creates a limited admin policy and assigns to the account root. myCustomPolicy := iam.NewPolicyDocument(&PolicyDocumentProps{ Statements: []policyStatement{ iam.NewPolicyStatement(&PolicyStatementProps{ Actions: []*string{ jsii.String("kms:Create*"), jsii.String("kms:Describe*"), jsii.String("kms:Enable*"), jsii.String("kms:List*"), jsii.String("kms:Put*"), }, Principals: []iPrincipal{ iam.NewAccountRootPrincipal(), }, Resources: []*string{ jsii.String("*"), }, }), }, }) key := kms.NewKey(this, jsii.String("MyKey"), &KeyProps{ Policy: myCustomPolicy, }) ``` > **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. ### 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 `grantGenerateMac` and `grantVerifyMac` methods. ```go key := kms.NewKey(this, jsii.String("MyKey")) user := iam.NewUser(this, jsii.String("MyUser")) key.grantGenerateMac(user) // Adds 'kms:GenerateMac' to the principal's policy key.grantVerifyMac(user) ```