Returns a query string which is sent to the validation servers.

def generate_query_string(self, otp, nonce, timestamp=False, sl=None,
                              timeout=None):
        """
        Returns a query string which is sent to the validation servers.
        """
        data = [('id', self.client_id),
                ('otp', otp),
                ('nonce', nonce)]

        if timestamp:
            data.append(('timestamp', '1'))

        if sl is not None:
            if sl not in range(0, 101) and sl not in ['fast', 'secure']:
                raise Exception('sl parameter value must be between 0 and '
                                '100 or string "fast" or "secure"')

            data.append(('sl', sl))

        if timeout:
            data.append(('timeout', timeout))

        query_string = urlencode(data)

        if self.key:
            hmac_signature = self.generate_message_signature(query_string)
            hmac_signature = hmac_signature
            query_string += '&h=%s' % (hmac_signature.replace('+', '%2B'))

        return query_string