How to call API Gateway with Cognito Credentials through retrofit2 on Android?

Question

I use retrofit2 in my android apps for any http/rest call. Now I need to call an api generated with Amazon AWS API Gateway.

The AWS documentation say I should generate the client code throw the API Gateway console and use the class ApiClientFactory to build the request:

ApiClientFactory factory = new ApiClientFactory();

// Use CognitoCachingCredentialsProvider to provide AWS credentials
// for the ApiClientFactory
AWSCredentialsProvider credentialsProvider = new CognitoCachingCredentialsProvider(
        context,          // activity context
        "identityPoolId", // Cognito identity pool id
        Regions.US_EAST_1 // region of Cognito identity pool
};

factory.credentialsProvider(credentialsProvider);

// Create an instance of your SDK (this should come from the generated code).
final MyApiClient client = factory.build(MyApiClient.class);

// Invoke a method (e.g., 'parentPath1Get(param1,body)') exposed by your SDK. 
// Here the method's return type is OriginalModel.
OriginalModel output = client.parentPath1Get(param1,body);

// You also have access to your API's models.
OriginalModel myModel = new OriginalModel();
myModel.setStreetAddress(streetAddress);
myModel.setCity(city);
myModel.setState(state);
myModel.setStreetNumber(streetNumber);
myModel.setNested(nested);
myModel.setPoBox(poBox);

Instead I would like to define the API like I would with retrofit: with an interface I write, connect it to RxJava, OkHttp etc...

My question is: how can I sign the retrofit requests with Cognito Identity Provider?

Answer 1

It took me several days to figure out how to make it work. Don't know why they don't point out the class instead of dozen of document pages. There are 4 steps in total, you must call in worker thread, I am using Rxjava but you can use AsyncTask instead:

    Observable.create((Observable.OnSubscribe<String>) subscriber -> {
//Step 1: Get credential, ask server team for Identity pool id and regions            
CognitoCachingCredentialsProvider credentialsProvider = new CognitoCachingCredentialsProvider(
                this, // Context
                "Identity Pool ID", // Identity Pool ID
                Regions.US_EAST_1 // Region
            );

//Step 2: Get these 3 three keys, test with postman v4.9.3 to see if identity is correct  
            String identityId = credentialsProvider.getIdentityId();
            Log.show("identityId = " + identityId);

            String AccessKey = credentialsProvider.getCredentials().getAWSAccessKeyId();
            String SecretKey = credentialsProvider.getCredentials().getAWSSecretKey();
            String SessionKey = credentialsProvider.getCredentials().getSessionToken();

            Log.show("AccessKey = " + AccessKey);
            Log.show("SecretKey = " + SecretKey);
            Log.show("SessionKey = " + SessionKey);
//Step 3: Create an aws requets and sign by using AWS4Signer class
            AmazonWebServiceRequest amazonWebServiceRequest = new AmazonWebServiceRequest() {
            };

            ClientConfiguration clientConfiguration = new ClientConfiguration();

            String API_GATEWAY_SERVICE_NAME = "execute-api";

            Request request = new DefaultRequest(amazonWebServiceRequest,API_GATEWAY_SERVICE_NAME);
            request.setEndpoint(URI.create("YOUR_URI"));
            request.setHttpMethod(HttpMethodName.GET);

            AWS4Signer signer = new AWS4Signer();
            signer.setServiceName(API_GATEWAY_SERVICE_NAME);
            signer.setRegionName(Region.getRegion(Regions.US_EAST_1).getName());
            signer.sign(request, credentialsProvider.getCredentials());

            Log.show("Request header " + request.getHeaders().toString());
//Step 4: Create new request with authorization headers 

            OkHttpClient httpClient = new OkHttpClient();
            Map<String, String> headers = request.getHeaders();
            List<String> key = new ArrayList<String>();
            List<String> value = new ArrayList<String>();

            for (Map.Entry<String, String> entry : headers.entrySet())
            {
                key.add(entry.getKey());
                value.add(entry.getValue());
            }

            try {
                okhttp3.Request request2 = new okhttp3.Request.Builder()
                        .url("Your_url") // remember to add / to the end of the url, otherwise the signature will be different 
                        .addHeader(key.get(0), value.get(0))
                        .addHeader(key.get(1), value.get(1))
                        .addHeader(key.get(2), value.get(2))
                        .addHeader(key.get(3), value.get(3))

                        .addHeader("Content-Type", "application/x-www-form-urlencoded")
                        .build();
                Response response = null;

                response = httpClient.newCall(request2).execute();
                String body = response.body().string();
                Log.show("response " + body);
            } catch (Exception e) {
                Log.show("error " + e);
            }

            subscriber.onNext(identityId);

        }).subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe(new Subscriber<String>() {
            @Override
            public void onCompleted() {

            }

            @Override
            public void onError(Throwable e) {
                Log.show("Throwable = " + e.getMessage());
            }

            @Override
            public void onNext(String s) {

            }
        });

The key here is the AWS4Signer class does 4 steps as documented here, you don't need to build one from scratch. In order to use AWS4Signer and AmazonWebServiceRequest, you need to import aws sdk in gradle:

compile 'com.amazonaws:aws-android-sdk-cognito:2.3.9'

Answer 2

Created an OkHttp interceptor based on @thanhbinh84 answer. Give it a try: https://github.com/Ghedeon/AwsInterceptor