feat: add support for Impersonated Service Account Credentials

src/ApplicationDefaultCredentials.php

@@ -20,6 +20,7 @@
 use DomainException;
 use Google\Auth\Credentials\AppIdentityCredentials;
 use Google\Auth\Credentials\GCECredentials;
+use Google\Auth\Credentials\ImpersonatedServiceAccountCredentials;
 use Google\Auth\Credentials\ServiceAccountCredentials;
 use Google\Auth\Credentials\UserRefreshCredentials;
 use Google\Auth\HttpHandler\HttpClientCache;
@@ -301,6 +302,7 @@ public static function getIdTokenCredentials(
 
             $creds = match ($jsonKey['type']) {
                 'authorized_user' => new UserRefreshCredentials(null, $jsonKey, $targetAudience),
+                'impersonated_service_account' => new ImpersonatedServiceAccountCredentials(null, $jsonKey, $targetAudience),
                 'service_account' => new ServiceAccountCredentials(null, $jsonKey, null, $targetAudience),
                 default => throw new InvalidArgumentException('invalid value in the type field')
             };

src/Credentials/ImpersonatedServiceAccountCredentials.php

@@ -35,6 +35,7 @@ class ImpersonatedServiceAccountCredentials extends CredentialsLoader implements
     use IamSignerTrait;
 
     private const CRED_TYPE = 'imp';
+    private const IAM_SCOPE = 'https://www.googleapis.com/auth/iam';
 
     /**
      * @var string
@@ -71,10 +72,12 @@ class ImpersonatedServiceAccountCredentials extends CredentialsLoader implements
      *     @type int                            $lifetime The lifetime of the impersonated credentials
      *     @type string[]                       $delegates The delegates to impersonate
      * }
+     * @param string|null $targetAudience The audience to request an ID token.
      */
     public function __construct(
         $scope,
-        $jsonKey
+        $jsonKey,
+        private ?string $targetAudience = null
     ) {
         if (is_string($jsonKey)) {
             if (!file_exists($jsonKey)) {
@@ -93,10 +96,23 @@ public function __construct(
         if (!array_key_exists('source_credentials', $jsonKey)) {
             throw new LogicException('json key is missing the source_credentials field');
         }
+        if ($scope && $targetAudience) {
+            throw new InvalidArgumentException(
+                'Scope and targetAudience cannot both be supplied'
+            );
+        }
         if (is_array($jsonKey['source_credentials'])) {
             if (!array_key_exists('type', $jsonKey['source_credentials'])) {
                 throw new InvalidArgumentException('json key source credentials are missing the type field');
             }
+            if (
+                $targetAudience !== null
+                && $jsonKey['source_credentials']['type'] === 'service_account'
+            ) {
+                // Service account tokens MUST request a scope, and as this token is only used to impersonate
+                // an ID token, the narrowest scope we can request is `iam`.
+                $scope = self::IAM_SCOPE;
+            }
             $jsonKey['source_credentials'] = CredentialsLoader::makeCredentials($scope, $jsonKey['source_credentials']);
         }
 
@@ -171,13 +187,19 @@ public function fetchAuthToken(?callable $httpHandler = null)
             'Content-Type' => 'application/json',
             'Cache-Control' => 'no-store',
             'Authorization' => sprintf('Bearer %s', $authToken['access_token'] ?? $authToken['id_token']),
-        ], 'at');
-
-        $body = [
-            'scope' => $this->targetScope,
-            'delegates' => $this->delegates,
-            'lifetime' => sprintf('%ss', $this->lifetime),
-        ];
+        ], $this->isIdTokenRequest() ? 'it' : 'at');
+
+        $body = match ($this->isIdTokenRequest()) {
+            true => [
+                'audience' => $this->targetAudience,
+                'includeEmail' => true,
+            ],
+            false => [
+                'scope' => $this->targetScope,
+                'delegates' => $this->delegates,
+                'lifetime' => sprintf('%ss', $this->lifetime),
+            ]
+        };
 
         $request = new Request(
             'POST',
@@ -189,10 +211,13 @@ public function fetchAuthToken(?callable $httpHandler = null)
         $response = $httpHandler($request);
         $body = json_decode((string) $response->getBody(), true);
 
-        return [
-            'access_token' => $body['accessToken'],
-            'expires_at' => strtotime($body['expireTime']),
-        ];
+        return match ($this->isIdTokenRequest()) {
+            true => ['id_token' => $body['token']],
+            false => [
+                'access_token' => $body['accessToken'],
+                'expires_at' => strtotime($body['expireTime']),
+            ]
+        };
     }
 
     /**
@@ -220,4 +245,9 @@ protected function getCredType(): string
     {
         return self::CRED_TYPE;
     }
+
+    private function isIdTokenRequest(): bool
+    {
+        return !is_null($this->targetAudience);
+    }
 }

src/FetchAuthTokenInterface.php

@@ -26,6 +26,7 @@ interface FetchAuthTokenInterface
      * Fetches the auth tokens based on the current state.
      *
      * @param callable|null $httpHandler callback which delivers psr7 request
+     *
      * @return array<mixed> a hash of auth tokens
      */
     public function fetchAuthToken(?callable $httpHandler = null);

tests/ApplicationDefaultCredentialsTest.php

@@ -496,6 +496,13 @@ public function testGetIdTokenCredentialsFailsIfNotOnGceAndNoDefaultFileFound()
         );
     }
 
+    public function testGetIdTokenCredentialsWithImpersonatedServiceAccountCredentials()
+    {
+        putenv('HOME=' . __DIR__ . '/fixtures5');
+        $creds = ApplicationDefaultCredentials::getIdTokenCredentials('123@456.com');
+        $this->assertInstanceOf(ImpersonatedServiceAccountCredentials::class, $creds);
+    }
+
     public function testGetIdTokenCredentialsWithCacheOptions()
     {
         $keyFile = __DIR__ . '/fixtures' . '/private.json';

tests/Credentials/ImpersonatedServiceAccountCredentialsTest.php

@@ -23,7 +23,9 @@
 use Google\Auth\Credentials\ServiceAccountCredentials;
 use Google\Auth\Credentials\UserRefreshCredentials;
 use Google\Auth\FetchAuthTokenInterface;
+use Google\Auth\Middleware\AuthTokenMiddleware;
 use Google\Auth\OAuth2;
+use GuzzleHttp\Psr7\Request;
 use GuzzleHttp\Psr7\Response;
 use LogicException;
 use PHPUnit\Framework\TestCase;
@@ -133,6 +135,44 @@ public function testGetAccessTokenWithServiceAccountAndUserRefreshCredentials($j
         $this->assertEquals(2, $requestCount);
     }
 
+    /**
+     * Test ID token impersonation for Service Account and User Refresh Credentials.
+     *
+     * @dataProvider provideAuthTokenJson
+     */
+    public function testGetIdTokenWithServiceAccountAndUserRefreshCredentials($json, $grantType)
+    {
+        $requestCount = 0;
+        // getting an id token will take two requests
+        $httpHandler = function (RequestInterface $request) use (&$requestCount, $json, $grantType) {
+            if (++$requestCount == 1) {
+                // the call to swap the refresh token for an access token
+                $this->assertEquals(UserRefreshCredentials::TOKEN_CREDENTIAL_URI, (string) $request->getUri());
+                parse_str((string) $request->getBody(), $result);
+                $this->assertEquals($grantType, $result['grant_type']);
+            } elseif ($requestCount == 2) {
+                // the call to swap the access token for an id token
+                $this->assertEquals($json['service_account_impersonation_url'], (string) $request->getUri());
+                $this->assertEquals(self::TARGET_AUDIENCE, json_decode($request->getBody(), true)['audience'] ?? '');
+                $this->assertEquals('Bearer test-access-token', $request->getHeader('authorization')[0] ?? null);
+            }
+
+            return new Response(
+                200,
+                ['Content-Type' => 'application/json'],
+                json_encode(match ($requestCount) {
+                    1 => ['access_token' => 'test-access-token'],
+                    2 => ['token' => 'test-impersonated-id-token']
+                })
+            );
+        };
+
+        $creds = new ImpersonatedServiceAccountCredentials(null, $json, self::TARGET_AUDIENCE);
+        $token = $creds->fetchAuthToken($httpHandler);
+        $this->assertEquals('test-impersonated-id-token', $token['id_token']);
+        $this->assertEquals(2, $requestCount);
+    }
+
     public function provideAuthTokenJson()
     {
         return [
@@ -180,6 +220,72 @@ public function testGetAccessTokenWithExternalAccountCredentials()
         $this->assertEquals(3, $requestCount);
     }
 
+    /**
+     * Test ID token impersonation for Exernal Account Credentials.
+     */
+    public function testGetIdTokenWithExternalAccountCredentials()
+    {
+        $json = self::EXTERNAL_ACCOUNT_TO_SERVICE_ACCOUNT_JSON;
+        $httpHandler = function (RequestInterface $request) use (&$requestCount, $json) {
+            if (++$requestCount == 1) {
+                // the call to swap the refresh token for an access token
+                $this->assertEquals(
+                    $json['source_credentials']['credential_source']['url'],
+                    (string) $request->getUri()
+                );
+            } elseif ($requestCount == 2) {
+                $this->assertEquals($json['source_credentials']['token_url'], (string) $request->getUri());
+            } elseif ($requestCount == 3) {
+                // the call to swap the access token for an id token
+                $this->assertEquals($json['service_account_impersonation_url'], (string) $request->getUri());
+                $this->assertEquals(self::TARGET_AUDIENCE, json_decode($request->getBody(), true)['audience'] ?? '');
+                $this->assertEquals('Bearer test-access-token', $request->getHeader('authorization')[0] ?? null);
+            }
+
+            return new Response(
+                200,
+                ['Content-Type' => 'application/json'],
+                json_encode(match ($requestCount) {
+                    1 => ['access_token' => 'test-access-token'],
+                    2 => ['access_token' => 'test-access-token'],
+                    3 => ['token' => 'test-impersonated-id-token']
+                })
+            );
+        };
+
+        $creds = new ImpersonatedServiceAccountCredentials(null, $json, self::TARGET_AUDIENCE);
+        $token = $creds->fetchAuthToken($httpHandler);
+        $this->assertEquals('test-impersonated-id-token', $token['id_token']);
+        $this->assertEquals(3, $requestCount);
+    }
+
+    /**
+     * Test ID token impersonation for an arbitrary credential fetcher.
+     */
+    public function testGetIdTokenWithArbitraryCredentials()
+    {
+        $httpHandler = function (RequestInterface $request) {
+            $this->assertEquals('https://some/url', (string) $request->getUri());
+            $this->assertEquals('Bearer test-access-token', $request->getHeader('authorization')[0] ?? null);
+            return new Response(200, [], json_encode(['token' => 'test-impersonated-id-token']));
+        };
+
+        $credentials = $this->prophesize(FetchAuthTokenInterface::class);
+        $credentials->fetchAuthToken($httpHandler, Argument::type('array'))
+            ->shouldBeCalledOnce()
+            ->willReturn(['access_token' => 'test-access-token']);
+
+        $json = [
+            'type' => 'impersonated_service_account',
+            'service_account_impersonation_url' => 'https://some/url',
+            'source_credentials' => $credentials->reveal(),
+        ];
+        $creds = new ImpersonatedServiceAccountCredentials(null, $json, self::TARGET_AUDIENCE);
+
+        $token = $creds->fetchAuthToken($httpHandler);
+        $this->assertEquals('test-impersonated-id-token', $token['id_token']);
+    }
+
     /**
      * Test access token impersonation for an arbitrary credential fetcher.
      */
@@ -211,6 +317,34 @@ public function testGetAccessTokenWithArbitraryCredentials()
         $this->assertEquals('test-impersonated-access-token', $token['access_token']);
     }
 
+    public function testIdTokenWithAuthTokenMiddleware()
+    {
+        $targetAudience = 'test-target-audience';
+        $credentials = new ImpersonatedServiceAccountCredentials(null, self::USER_TO_SERVICE_ACCOUNT_JSON, $targetAudience);
+
+        // this handler is for the middleware constructor, which will pass it to the ISAC to fetch tokens
+        $httpHandler = getHandler([
+            new Response(200, ['Content-Type' => 'application/json'], '{"access_token":"this.is.an.access.token"}'),
+            new Response(200, ['Content-Type' => 'application/json'], '{"token":"this.is.an.id.token"}'),
+        ]);
+        $middleware = new AuthTokenMiddleware($credentials, $httpHandler);
+
+        // this handler is the actual handler that makes the authenticated request
+        $requestCount = 0;
+        $httpHandler = function (RequestInterface $request) use (&$requestCount) {
+            $requestCount++;
+            $this->assertTrue($request->hasHeader('authorization'));
+            $this->assertEquals('Bearer this.is.an.id.token', $request->getHeader('authorization')[0] ?? null);
+        };
+
+        $middleware($httpHandler)(
+            new Request('GET', 'https://www.google.com'),
+            ['auth' => 'google_auth']
+        );
+
+        $this->assertEquals(1, $requestCount);
+    }
+
     // User Refresh to Service Account Impersonation JSON Credentials
     private const USER_TO_SERVICE_ACCOUNT_JSON = [
         'type' => 'impersonated_service_account',

tests/ObservabilityMetricsTest.php

@@ -131,6 +131,20 @@ public function testImpersonatedServiceAccountCredentials()
         $this->assertUpdateMetadata($impersonatedCred, $handler, 'imp', $handlerCalled);
     }
 
+    public function testImpersonatedServiceAccountCredentialsWithIdTokens()
+    {
+        $keyFile = __DIR__ . '/fixtures5/.config/gcloud/application_default_credentials.json';
+        $handlerCalled = false;
+        $responseFromIam = json_encode(['token' => '1/abdef1234567890']);
+        $handler = getHandler([
+            $this->getExpectedRequest('imp', 'auth-request-type/at', $handlerCalled, $this->jsonTokens),
+            $this->getExpectedRequest('imp', 'auth-request-type/it', $handlerCalled, $responseFromIam),
+        ]);
+
+        $impersonatedCred = new ImpersonatedServiceAccountCredentials(null, $keyFile, 'test-target-audience');
+        $this->assertUpdateMetadata($impersonatedCred, $handler, 'imp', $handlerCalled);
+    }
+
     /**
      * UserRefreshCredentials haven't enabled identity token support hence
      * they don't have 'auth-request-type/it' observability metric header check.