public function verifySignature()
    {
        if ( isset($this->data['CreditCardTransactionResults']['ResponseCode']) && (
            '1' == $this->data['CreditCardTransactionResults']['ResponseCode'] ||
            '2' == $this->data['CreditCardTransactionResults']['ResponseCode']) )
        {
            $signature  = $this->request->getMerchantPassword();
            $signature .= $this->request->getMerchantId();
            $signature .= $this->request->getAcquirerId();
            $signature .= $this->request->getTransactionId();

            $signature  = base64_encode( sha1($signature, true) );

            if ( $signature !== $this->data['Signature'] ) {
                throw new InvalidResponseException('Signature verification failed');
            }
        }
    }