(PHP Extension) Verify Signature of Alexa Custom Skill Request

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

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<?php

// The version number (9_5_0) should match version of the Chilkat extension used, omitting the micro-version number.
// For example, if using Chilkat v9.5.0.48, then include as shown here:
include("chilkat_9_5_0.php");

// 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:
$signature = 'dSUmPwxc9...aKAf8mpEXg==';
$certChainUrl = 'https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem';
$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 
$http = new CkHttp();
$sbPem = new CkStringBuilder();
$success = $http->QuickGetSb($certChainUrl,$sbPem);
if ($success == false) {
    print $http->lastErrorText() . "\n";
    exit;
}

$pem = new CkPem();
$success = $pem->LoadPem($sbPem->getAsString(),'passwordNotUsed');
if ($success == false) {
    print $pem->lastErrorText() . "\n";
    exit;
}

// The 1st certificate should be the signing certificate.
// cert is a CkCert
$cert = $pem->GetCert(0);
if ($pem->get_LastMethodSuccess() == false) {
    print $pem->lastErrorText() . "\n";
    exit;
}

// Get the public key from the cert.
// pubKey is a CkPublicKey
$pubKey = $cert->ExportPublicKey();
if ($cert->get_LastMethodSuccess() == false) {
    print $cert->lastErrorText() . "\n";

    exit;
}

// Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
$rsa = new CkRsa();
$success = $rsa->ImportPublicKeyObj($pubKey);
if ($success == false) {
    print $cert->lastErrorText() . "\n";

    exit;
}

// 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');
$bVerified = $rsa->VerifyStringENC($jsonBody,'sha1',$signature);
if ($bVerified == true) {
    print 'The signature is verified against the JSON body of the request. Yay!' . "\n";
}
else {
    print 'Sorry, not verified.  Crud!' . "\n";
}


?>