rest - How to generate Signature in AWS from Java

Question

When I invoke API endpoints from REST client, I got error by concerning with Signature.

Request:

Host: https://xxx.execute-api.ap-southeast-1.amazonaws.com/latest/api/name

Authorization: AWS4-HMAC-SHA256 Credential={AWSKEY}/20160314/ap-southeast-1/execute-api/aws4_request,SignedHeaders=host;range;x-amz-date,Signature={signature}

X-Amz-Date: 20160314T102915Z

Response:

{
"message": "The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details. The Canonical String for this request should have been 'xxx' "
}

From Java code, I followed AWS reference of how to generate Signature.

    String secretKey = "{mysecretkey}";
    String dateStamp = "20160314";
    String regionName = "ap-southeast-1";
    String serviceName = "execute-api";

    byte[] signature = getSignatureKey(secretKey, dateStamp, regionName, serviceName);
    System.out.println("Signature : " + Hex.encodeHexString(signature));

    static byte[] HmacSHA256(String data, byte[] key) throws Exception  {
         String algorithm="HmacSHA256";
         Mac mac = Mac.getInstance(algorithm);
         mac.init(new SecretKeySpec(key, algorithm));
         return mac.doFinal(data.getBytes("UTF8"));
    }

    static byte[] getSignatureKey(String key, String dateStamp, String regionName, String serviceName) throws Exception  {
         byte[] kSecret = ("AWS4" + key).getBytes("UTF8");
         byte[] kDate    = HmacSHA256(dateStamp, kSecret);
         byte[] kRegion  = HmacSHA256(regionName, kDate);
         byte[] kService = HmacSHA256(serviceName, kRegion);
         byte[] kSigning = HmacSHA256("aws4_request", kService);
         return kSigning;
    }

May I know what I was wrong while generating Signature?

Reference how to generate Signature : http://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-java

Answer 1

You can use classes from aws-java-sdk-core: https://github.com/aws/aws-sdk-java/tree/master/aws-java-sdk-core

More specifically, Request, Aws4Signer and a few other ones:

//Instantiate the request
Request<Void> request = new DefaultRequest<Void>("es"); //Request to ElasticSearch
request.setHttpMethod(HttpMethodName.GET);
request.setEndpoint(URI.create("http://..."));

//Sign it...
AWS4Signer signer = new AWS4Signer();
signer.setRegionName("...");
signer.setServiceName(request.getServiceName());
signer.sign(request, new AwsCredentialsFromSystem());

//Execute it and get the response...
Response<String> rsp = new AmazonHttpClient(new ClientConfiguration())
    .requestExecutionBuilder()
    .executionContext(new ExecutionContext(true))
    .request(request)
    .errorResponseHandler(new SimpleAwsErrorHandler())
    .execute(new SimpleResponseHandler<String>());

If you want a cleaner design, you can use the Decorator pattern to compose some elegant classes and hide the above mess. An example for that here: http://www.amihaiemil.com/2017/02/18/decorators-with-tunnels.html