fix(derSerializePublicKey): Support KMS-backed private keys

Needed for relaycorp/cloud-gateway#93

src/lib/crypto/keys/serialisation.spec.ts

@@ -31,10 +31,20 @@ describe('Key serializers', () => {
       expect(publicKeyDer).toEqual(Buffer.from(stubPublicKeyDer));
     });
 
-    test('Public key should be extracted first if input is PrivateKey', async () => {
-      const publicKeyDer = await derSerializePublicKey(stubKeyPair.privateKey);
-
-      expect(publicKeyDer).toEqual(Buffer.from(stubPublicKeyDer));
+    test('Provider should be used directly first if input is CryptoKeyWithProvider', async () => {
+      const mockPublicKey = Buffer.from('mock public key');
+      const mockExportKey = jest.fn().mockResolvedValue(mockPublicKey);
+      const privateKeyWithProvider = {
+        ...stubKeyPair.privateKey,
+        provider: {
+          exportKey: mockExportKey,
+        },
+      };
+
+      const publicKeyDer = await derSerializePublicKey(privateKeyWithProvider);
+
+      expect(publicKeyDer).toEqual(mockPublicKey);
+      expect(mockExportKey).toBeCalledWith('spki', privateKeyWithProvider);
     });
   });
 

src/lib/crypto/keys/serialisation.ts

@@ -1,6 +1,7 @@
 import bufferToArray from 'buffer-to-arraybuffer';
 
-import { getEngineForKey, NODE_ENGINE } from '../pkijs';
+import { NODE_ENGINE } from '../pkijs';
+import { CryptoKeyWithProvider } from './CryptoKeyWithProvider';
 
 const DEFAULT_RSA_KEY_PARAMS: RsaHashedImportParams = {
   hash: { name: 'SHA-256' },
@@ -12,9 +13,19 @@ const DEFAULT_RSA_KEY_PARAMS: RsaHashedImportParams = {
  *
  * @param publicKey
  */
-export async function derSerializePublicKey(publicKey: CryptoKey): Promise<Buffer> {
-  const engine = getEngineForKey(publicKey);
-  const publicKeyDer = await engine.exportKey('spki', publicKey);
+export async function derSerializePublicKey(
+  publicKey: CryptoKey | CryptoKeyWithProvider,
+): Promise<Buffer> {
+  let publicKeyDer: ArrayBuffer;
+  if ((publicKey as CryptoKeyWithProvider).provider) {
+    // This is likely a KMS-backed private key, so use the provider directly to prevent the
+    // engine from exporting the key to JWK first.
+    // https://github.com/relaycorp/cloud-gateway/issues/93
+    const provider = (publicKey as CryptoKeyWithProvider).provider;
+    publicKeyDer = (await provider.exportKey('spki', publicKey as CryptoKey)) as ArrayBuffer;
+  } else {
+    publicKeyDer = await NODE_ENGINE.exportKey('spki', publicKey as CryptoKey);
+  }
   return Buffer.from(publicKeyDer);
 }