(Excel) Verify Signature of Alexa Custom Skill Request

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

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' 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 
Dim http As Chilkat.Http
Set http = Chilkat.NewHttp
Dim sbPem As Chilkat.StringBuilder
Set sbPem = Chilkat.NewStringBuilder

success = http.QuickGetSb(certChainUrl,sbPem)
If (success = False) Then
    Debug.Print http.LastErrorText
    Exit Sub
End If

Dim pem As Chilkat.Pem
Set pem = Chilkat.NewPem
success = pem.LoadPem(sbPem.GetAsString(),"passwordNotUsed")
If (success = False) Then
    Debug.Print pem.LastErrorText
    Exit Sub
End If

' The 1st certificate should be the signing certificate.

Set cert = pem.GetCert(0)
If (pem.LastMethodSuccess = False) Then
    Debug.Print pem.LastErrorText
    Exit Sub
End If

' Get the public key from the cert.

Set pubKey = cert.ExportPublicKey()
If (cert.LastMethodSuccess = False) Then
    Debug.Print cert.LastErrorText

    Exit Sub
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 Chilkat.Rsa
Set rsa = Chilkat.NewRsa
success = rsa.ImportPublicKeyObj(pubKey)
If (success = False) Then
    Debug.Print cert.LastErrorText

    Exit Sub
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"

bVerified = rsa.VerifyStringENC(jsonBody,"sha1",signature)
If (bVerified = True) Then
    Debug.Print "The signature is verified against the JSON body of the request. Yay!"
Else
    Debug.Print "Sorry, not verified.  Crud!"
End If