Node.js

Verify Signature of Alexa Custom Skill Request

See more HTTP Misc Examples

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

NODEJS_PRELUDE

function chilkatExample() {

    var success = false;

    // 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();
    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.
    var pubKey = new chilkat.PublicKey();
    cert.GetPublicKey(pubKey);

    // 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.UsePublicKey(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();