(Visual FoxPro) Verify Signature of Alexa Custom Skill Request

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

Note: This example requires Chilkat v11.0.0 or greater.

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL lnSuccess
LOCAL lcSignature
LOCAL lcCertChainUrl
LOCAL lcJsonBody
LOCAL loHttp
LOCAL loSbPem
LOCAL loPem
LOCAL loCert
LOCAL loPubKey
LOCAL loRsa
LOCAL lnBVerified

lnSuccess = 0

* 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:
lcSignature = "dSUmPwxc9...aKAf8mpEXg=="
lcCertChainUrl = "https://s3.amazonaws.com/echo.api/echo-api-cert-6-ats.pem"
lcJsonBody = '{"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 
loHttp = CreateObject('Chilkat.Http')
loSbPem = CreateObject('Chilkat.StringBuilder')
lnSuccess = loHttp.QuickGetSb(lcCertChainUrl,loSbPem)
IF (lnSuccess = 0) THEN
    ? loHttp.LastErrorText
    RELEASE loHttp
    RELEASE loSbPem
    CANCEL
ENDIF

loPem = CreateObject('Chilkat.Pem')
lnSuccess = loPem.LoadPem(loSbPem.GetAsString(),"passwordNotUsed")
IF (lnSuccess = 0) THEN
    ? loPem.LastErrorText
    RELEASE loHttp
    RELEASE loSbPem
    RELEASE loPem
    CANCEL
ENDIF

* The 1st certificate should be the signing certificate.
loCert = loPem.GetCert(0)
IF (loPem.LastMethodSuccess = 0) THEN
    ? loPem.LastErrorText
    RELEASE loHttp
    RELEASE loSbPem
    RELEASE loPem
    CANCEL
ENDIF

* Get the public key from the cert.
loPubKey = CreateObject('Chilkat.PublicKey')
loCert.GetPublicKey(loPubKey)

RELEASE loCert

* Use the public key extracted from the signing certificate to decrypt the encrypted signature to produce the asserted hash value.
loRsa = CreateObject('Chilkat.Rsa')
lnSuccess = loRsa.UsePublicKey(loPubKey)
IF (lnSuccess = 0) THEN
    ? loCert.LastErrorText
    RELEASE loHttp
    RELEASE loSbPem
    RELEASE loPem
    RELEASE loPubKey
    RELEASE loRsa
    CANCEL
ENDIF

* 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...)
loRsa.EncodingMode = "base64"
lnBVerified = loRsa.VerifyStringENC(lcJsonBody,"sha1",lcSignature)
IF (lnBVerified = 1) THEN
    ? "The signature is verified against the JSON body of the request. Yay!"
ELSE
    ? "Sorry, not verified.  Crud!"
ENDIF

RELEASE loHttp
RELEASE loSbPem
RELEASE loPem
RELEASE loPubKey
RELEASE loRsa