Xojo Plugin

Verify Signature of Alexa Custom Skill Request

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

Dim success As Boolean
success = False

// 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
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 New Chilkat.PublicKey
success = cert.GetPublicKey(pubKey)

// 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.UsePublicKey(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