@defer.inlineCallbacks
    def _submit_captcha(self, ip_addr, private_key, challenge, response):
        data = yield self.captcha_client.post_urlencoded_get_raw(
            "http://www.google.com:80/recaptcha/api/verify",
            args={
                'privatekey': private_key,
                'remoteip': ip_addr,
                'challenge': challenge,
                'response': response
            }
        )
        defer.returnValue(data)
Used only by c/s api v1
dataset/ETHPy150Open matrix-org/synapse/synapse/handlers/register.py/RegistrationHandler._submit_captcha