Google Cloud Platform - cloud functions API - 401 Unauthorized

Question

I'm struggling with invoking GCP cloud functions via REST API using Java.

The steps that I've performed to do it were:

create a service account with role "Cloud Functions Invoker"
download JSON key file for the newly created service account
in my code, obtain an access token using the following method:

private String getAuthToken() {
  File credentialsPath = new File(PATH_TO_JSON_KEY_FILE);

  GoogleCredentials credentials;
  try (FileInputStream serviceAccountStream = new FileInputStream(credentialsPath)) {
    credentials = ServiceAccountCredentials.fromStream(serviceAccountStream);
    return credentials
           .createScoped(Lists.newArrayList("https://www.googleapis.com/auth/cloud-platform"))
           .refreshAccessToken()
           .getTokenValue();
  } catch (IOException e) {
    throw new RuntimeException("Action could not be performed");
  }
}

perform a REST call, using the created token:

public <Payload, Response> ResponseEntity<Response> callCloudFunction(
    String endpoint,
    Payload payload,
    Class<Response> klazz
) {
  RestTemplate restTemplate = new RestTemplate();
  HttpHeaders headers = new HttpHeaders();
  headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON));
  headers.setContentType(MediaType.APPLICATION_JSON);
  String url = gCloudUrl + endpoint;
  String token = getAuthToken();
  String payloadString = null;

  if (payload != null) {
    try {
      ObjectMapper objectMapper = new ObjectMapper();
      payloadString = objectMapper.writeValueAsString(payload);
    } catch (JsonProcessingException e) {
      System.out.println(e.getMessage());
      throw new RuntimeException("Could not perform action");
    }
  }

  headers.add("Authorization", String.format("Bearer %s", token));
  HttpEntity<String> entity = new HttpEntity<>(payloadString, headers);

  return restTemplate.exchange(url, HttpMethod.POST, entity, klazz);
}

The implementation looks fine, but in response I'm getting 401 Unauthorized.

Unfortunately, GCP documentation is not really helpful. I think I've searched through all the possible places.

Here is my solution: stackoverflow.com/a/64720883/14592586 See if it helps in solving the problem. — Daniel Zanelatto

guillaume blaquiere guillaume blaquiere · Accepted Answer · 2020-07-12T20:05:55

First of all, agree, it's not clear...

Then, you have to know (and it's not clear again) that you need an access token to call Google Cloud API, but and identity token to call IAP (on App Engine for example) or private Cloud Function and Cloud Run. And this identity token need to be signed by Google.

And, as mentioned in the code, you need to have a service account on your computer, but I recommend you to avoid this on GCP, it's not required if you use default authentication (see my code, on your computer set the GOOGLE_APPLICATION_CREDENTIALS env var that points to the service account key file). The best way is to not use service account key file on your computer also, but it's not yet possible (that is a security issue IMO, and I'm discussing with Google on this...)

Anyway, here a code snippet which works in Java (nowhere in the documentation...)

  String myUri = "https://path/to/url";
  // You can use here your service account key file. But, on GCP you don't require a service account key file.
  // However, on your computer, you require one because you need and identity token and you can generate it with your user account (long story... I'm still in discussion with Google about this point...)
  Credentials credentials = GoogleCredentials.getApplicationDefault().createScoped("https://www.googleapis.com/auth/cloud-platform");
  IdTokenCredentials idTokenCredentials = IdTokenCredentials.newBuilder()
    .setIdTokenProvider((IdTokenProvider) credentials)
    .setTargetAudience(myUri).build();

  HttpRequestFactory factory = new NetHttpTransport().createRequestFactory(new HttpCredentialsAdapter(idTokenCredentials));

  HttpRequest request = factory.buildGetRequest(new GenericUrl(myUri));
  HttpResponse httpResponse = request.execute();
  System.out.println(CharStreams.toString(new InputStreamReader(httpResponse.getContent(), Charsets.UTF_8)));

NOTE If you want to continue to use RestTemplate object and set manually your token, you can generate it like this

  String token = ((IdTokenProvider) credentials).idTokenWithAudience(myUri, Collections.EMPTY_LIST).getTokenValue();
  System.out.println(token);

Google Cloud Platform - cloud functions API - 401 Unauthorized

1 Answers