(Perl) Verify Signature of Alexa Custom Skill Request

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

Chilkat Perl Downloads Perl Module for Windows, MacOS, Linux, Alpine Linux, Solaris

use chilkat();

# 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 = chilkat::CkHttp->new();
$sbPem = chilkat::CkStringBuilder->new();
$success = $http->QuickGetSb($certChainUrl,$sbPem);
if ($success == 0) {
    print $http->lastErrorText() . "\r\n";
    exit;
}

$pem = chilkat::CkPem->new();
$success = $pem->LoadPem($sbPem->getAsString(),"passwordNotUsed");
if ($success == 0) {
    print $pem->lastErrorText() . "\r\n";
    exit;
}

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

# Get the public key from the cert.
# pubKey is a PublicKey
$pubKey = $cert->ExportPublicKey();
if ($cert->get_LastMethodSuccess() == 0) {
    print $cert->lastErrorText() . "\r\n";

    exit;
}

# Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
$rsa = chilkat::CkRsa->new();
$success = $rsa->ImportPublicKeyObj($pubKey);
if ($success == 0) {
    print $cert->lastErrorText() . "\r\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 == 1) {
    print "The signature is verified against the JSON body of the request. Yay!" . "\r\n";
}
else {
    print "Sorry, not verified.  Crud!" . "\r\n";
}