(Node.js) Verify Signature of Alexa Custom Skill Request

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

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // 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:
    var signature = "dSUmPwxc9...aKAf8mpEXg==";
    var certChainUrl = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem";
    var 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 
    var http = new chilkat.Http();
    var sbPem = new chilkat.StringBuilder();
    var success = http.QuickGetSb(certChainUrl,sbPem);
    if (success == false) {
        console.log(http.LastErrorText);
        return;
    }

    var pem = new chilkat.Pem();
    success = pem.LoadPem(sbPem.GetAsString(),"passwordNotUsed");
    if (success == false) {
        console.log(pem.LastErrorText);
        return;
    }

    // The 1st certificate should be the signing certificate.
    // cert: Cert
    var cert = pem.GetCert(0);
    if (pem.LastMethodSuccess == false) {
        console.log(pem.LastErrorText);
        return;
    }

    // Get the public key from the cert.
    // pubKey: PublicKey
    var pubKey = cert.ExportPublicKey();
    if (cert.LastMethodSuccess == false) {
        console.log(cert.LastErrorText);

        return;
    }

    // Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
    var rsa = new chilkat.Rsa();
    success = rsa.ImportPublicKeyObj(pubKey);
    if (success == false) {
        console.log(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.EncodingMode = "base64";
    var bVerified = rsa.VerifyStringENC(jsonBody,"sha1",signature);
    if (bVerified == true) {
        console.log("The signature is verified against the JSON body of the request. Yay!");
    }
    else {
        console.log("Sorry, not verified.  Crud!");
    }


}

chilkatExample();