Verify the server response signature. 
        
        :param signature: 
        :param nonce: 
        :param timestamp: 
        :param signed_id: either transactionid, documentid or marketplacemerchantid
        :return: true of false

def verify_signature(self, signature, nonce, timestamp, signed_id):
        """
        Verify the server response signature. 
        
        :param signature: 
        :param nonce: 
        :param timestamp: 
        :param signed_id: either transactionid, documentid or marketplacemerchantid
        :return: true of false
        """
        message = "%s%s%s%s" % (self.api_key, nonce, timestamp, signed_id)

        if self._pub_key is None:
            warnings.warn("Skipping RSA signature verification. Please pass public key on class initialization to ensure highest level of security!")
            return True

        try:
            rsa.verify(message.encode('utf-8'), signature, self._pub_key)
        except rsa.VerificationError:
            return False

        return True