add plain interface (#595)

Author: nvobilis

modules/kms-keyring/src/branch_key_id_supplier.ts

@@ -0,0 +1,12 @@
+// Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { EncryptionContext } from '@aws-crypto/material-management'
+
+//= aws-encryption-sdk-specification/framework/aws-kms/aws-kms-hierarchical-keyring.md#branch-key-supplier
+//# The Branch Key Supplier is an interface containing the GetBranchKeyId
+//# operation. This operation MUST take in an encryption context as input, and
+//# return a branch key id (string) as output.
+export interface BranchKeyIdSupplier {
+  getBranchKeyId(encryptionContext: EncryptionContext): string
+}

modules/kms-keyring/src/index.ts

@@ -16,3 +16,4 @@ export * from './region_from_kms_key_arn'
 export * from './kms_mrk_strict_multi_keyring'
 export * from './kms_mrk_discovery_multi_keyring'
 export { AwsEsdkKMSInterface } from './kms_types'
+export * from './branch_key_id_supplier'

modules/kms-keyring/test/branch_key_id_supplier.test.ts

@@ -0,0 +1,23 @@
+// Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { EncryptionContext } from '@aws-crypto/material-management'
+import { BranchKeyIdSupplier } from '../src'
+import { expect } from 'chai'
+
+describe('Branch key id supplier', () => {
+  //= aws-encryption-sdk-specification/framework/aws-kms/aws-kms-hierarchical-keyring.md#branch-key-supplier
+  //= type=test
+  //# The Branch Key Supplier is an interface containing the GetBranchKeyId
+  //# operation. This operation MUST take in an encryption context as input, and
+  //# return a branch key id (string) as output.
+  it('Can implement the interface', () => {
+    class Example implements BranchKeyIdSupplier {
+      getBranchKeyId(encryptionContext: EncryptionContext): string {
+        return '' in encryptionContext ? '' : ''
+      }
+    }
+
+    expect(new Example().getBranchKeyId({})).to.equal('')
+  })
+})