code: def sender(self, body, stamp, url, sig):
          
          if not timestamp(stamp):
              return False
  
          if self.url != url:
              if not signature_cert_chain_url(url):
                  return False
  
              certs = retrieve(url)
              if not certs:
                  return False
  
              if not cert_chain(certs):
                  return False
  
              self.url = url
              self.cert = certs[0]
  
          if not signature(self.cert, sig, body):
              return False
  
          return True
docs: Validate request is from Alexa.
  
          Verifying that the Request was Sent by Alexa: https://goo.gl/AcrzB5.
          Checking the Signature of the Request: https://goo.gl/FDkjBN.
          Checking the Timestamp of the Request: https://goo.gl/Z5JhqZ
  
          Args:
              body: str. HTTPS request body.
              stamp: str. Value of timestamp within request object of HTTPS
                  request body.
              url: str. SignatureCertChainUrl header value sent
                  by request.
              sig: str. Signature header value sent by request.
  
          Returns:
              bool: True if valid, False otherwise.