(Xojo Plugin) Verify Signature of Alexa Custom Skill Request

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

Chilkat Xojo Plugin Download Xojo Plugin for Windows, Linux, Mac OS X, and ARM, ARM64

// 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:
Dim signature As String
signature = "dSUmPwxc9...aKAf8mpEXg=="
Dim certChainUrl As String
certChainUrl = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem"
Dim jsonBody As String
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 
Dim http As New Chilkat.Http
Dim sbPem As New Chilkat.StringBuilder
Dim success As Boolean
success = http.QuickGetSb(certChainUrl,sbPem)
If (success = False) Then
    System.DebugLog(http.LastErrorText)
    Return
End If

Dim pem As New Chilkat.Pem
success = pem.LoadPem(sbPem.GetAsString(),"passwordNotUsed")
If (success = False) Then
    System.DebugLog(pem.LastErrorText)
    Return
End If

// The 1st certificate should be the signing certificate.
Dim cert As Chilkat.Cert
cert = pem.GetCert(0)
If (pem.LastMethodSuccess = False) Then
    System.DebugLog(pem.LastErrorText)
    Return
End If

// Get the public key from the cert.
Dim pubKey As Chilkat.PublicKey
pubKey = cert.ExportPublicKey()
If (cert.LastMethodSuccess = False) Then
    System.DebugLog(cert.LastErrorText)

    Return
End If

// Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
Dim rsa As New Chilkat.Rsa
success = rsa.ImportPublicKeyObj(pubKey)
If (success = False) Then
    System.DebugLog(cert.LastErrorText)

    Return
End If

// 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"
Dim bVerified As Boolean
bVerified = rsa.VerifyStringENC(jsonBody,"sha1",signature)
If (bVerified = True) Then
    System.DebugLog("The signature is verified against the JSON body of the request. Yay!")
Else
    System.DebugLog("Sorry, not verified.  Crud!")
End If