(PowerShell) Verify Signature of Alexa Custom Skill Request

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

Chilkat .NET Downloads Chilkat .NET Assemblies

Add-Type -Path "C:\chilkat\ChilkatDotNet47-9.5.0-x64\ChilkatDotNet47.dll"

# 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-Object Chilkat.Http
$sbPem = New-Object Chilkat.StringBuilder
$success = $http.QuickGetSb($certChainUrl,$sbPem)
if ($success -eq $false) {
    $($http.LastErrorText)
    exit
}

$pem = New-Object Chilkat.Pem
$success = $pem.LoadPem($sbPem.GetAsString(),"passwordNotUsed")
if ($success -eq $false) {
    $($pem.LastErrorText)
    exit
}

# The 1st certificate should be the signing certificate.
$cert = $pem.GetCert(0)
if ($pem.LastMethodSuccess -eq $false) {
    $($pem.LastErrorText)
    exit
}

# Get the public key from the cert.
$pubKey = $cert.ExportPublicKey()
if ($cert.LastMethodSuccess -eq $false) {
    $($cert.LastErrorText)

    exit
}

# Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
$rsa = New-Object Chilkat.Rsa
$success = $rsa.ImportPublicKeyObj($pubKey)
if ($success -eq $false) {
    $($cert.LastErrorText)

    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.EncodingMode = "base64"
$bVerified = $rsa.VerifyStringENC($jsonBody,"sha1",$signature)
if ($bVerified -eq $true) {
    $("The signature is verified against the JSON body of the request. Yay!")
}
else {
    $("Sorry, not verified.  Crud!")
}