Visual FoxPro

Verify Signature of Alexa Custom Skill Request

See more HTTP Misc Examples

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

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