private function _verifySignatureWithVersionOneAlgorithm(Message $snsMessage)
    {      
        $result = $this->_verifySignature->verifySignatureIsCorrect(
            $this->_constructSignatureFromSnsMessage($snsMessage),
            base64_decode($snsMessage->getMandatoryField("Signature")),
            $snsMessage->getMandatoryField("SigningCertURL")
        );
        
        if (!$result) {
            throw new OffAmazonPaymentsNotifications_InvalidMessageException(
                "Unable to match signature from remote server: signature of " .
                $this->_constructSignatureFromSnsMessage($snsMessage) . 
                " , SigningCertURL of " . 
                $snsMessage->getMandatoryField("SigningCertURL") . 
                " , SignatureOf " . 
                $snsMessage->getMandatoryField("Signature")
            );
        }
    }