(Java) Verify Signature of Alexa Custom Skill Request

This example verifies the signature of an Alexa Custom Skill Request.

Chilkat Java Downloads Java Libs for Windows, MacOS, Linux, Alpine Linux, Solaris Java Libs for Android

import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // This example assumes you have a web service that will receive requests from Alexa.
    // A sample request sent by Alexa will look like the following:

    // Connection: Keep-Alive
    // Content-Length: 2583
    // Content-Type: application/json; charset=utf-8
    // Accept: application/json
    // Accept-Charset: utf-8
    // Host: your.web.server.com
    // User-Agent: Apache-HttpClient/4.5.x (Java/1.8.0_172)
    // Signature: dSUmPwxc9...aKAf8mpEXg==
    // SignatureCertChainUrl: https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem
    // 
    // {"version":"1.0","session":{"new":true,"sessionId":"amzn1.echo-api.session.433 ... }}

    // First, assume we've written code to get the 3 pieces of data we need:
    String signature = "dSUmPwxc9...aKAf8mpEXg==";
    String certChainUrl = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem";
    String jsonBody = "{\"version\":\"1.0\",\"session\":{\"new\":true,\"sessionId\":\"amzn1.echo-api.session.433 ... }}";

    // To validate the signature, we do the following:

    // First, download the PEM-encoded X.509 certificate chain that Alexa used to sign the message 
    CkHttp http = new CkHttp();
    CkStringBuilder sbPem = new CkStringBuilder();
    boolean success = http.QuickGetSb(certChainUrl,sbPem);
    if (success == false) {
        System.out.println(http.lastErrorText());
        return;
        }

    CkPem pem = new CkPem();
    success = pem.LoadPem(sbPem.getAsString(),"passwordNotUsed");
    if (success == false) {
        System.out.println(pem.lastErrorText());
        return;
        }

    // The 1st certificate should be the signing certificate.
    CkCert cert = pem.GetCert(0);
    if (pem.get_LastMethodSuccess() == false) {
        System.out.println(pem.lastErrorText());
        return;
        }

    // Get the public key from the cert.
    CkPublicKey pubKey = cert.ExportPublicKey();
    if (cert.get_LastMethodSuccess() == false) {
        System.out.println(cert.lastErrorText());

        return;
        }

    // Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
    CkRsa rsa = new CkRsa();
    success = rsa.ImportPublicKeyObj(pubKey);
    if (success == false) {
        System.out.println(cert.lastErrorText());

        return;
        }

    // RSA "decrypt" the signature.
    // (Amazon's documentation is confusing, because we're simply verifiying the signature against the SHA-1 hash
    // of the request body.  This happens in a single call to VerifyStringENC...)
    rsa.put_EncodingMode("base64");
    boolean bVerified = rsa.VerifyStringENC(jsonBody,"sha1",signature);
    if (bVerified == true) {
        System.out.println("The signature is verified against the JSON body of the request. Yay!");
        }
    else {
        System.out.println("Sorry, not verified.  Crud!");
        }
  }
}