public function validateMessage(Message $snsMessage)
    {
        switch($snsMessage->getMandatoryField("SignatureVersion")) {
        case "1":
            $this->_verifySignatureWithVersionOneAlgorithm($snsMessage);
            break;
        default:
            throw new OffAmazonPaymentsNotifications_InvalidMessageException(
                "Error with signature verification - " .
                "unable to handle signature version " .
                $snsMessage->getMandatoryField("SignatureVersion")
            );
        }
    }