Creates the required URL parameters for a signed URL.

def _create_signing_params(self, url, keypair_id,
                          expire_time=None, valid_after_time=None,
                          ip_address=None, policy_url=None,
                          private_key_file=None, private_key_string=None):
        """
        Creates the required URL parameters for a signed URL.
        """
        params = {}
        # Check if we can use a canned policy
        if expire_time and not valid_after_time and not ip_address and not policy_url:
            # we manually construct this policy string to ensure formatting
            # matches signature
            policy = self._canned_policy(url, expire_time)
            params["Expires"] = str(expire_time)
        else:
            # If no policy_url is specified, default to the full url.
            if policy_url is None:
                policy_url = url
            # Can't use canned policy
            policy = self._custom_policy(policy_url, expires=expire_time,
                                         valid_after=valid_after_time,
                                         ip_address=ip_address)

            encoded_policy = self._url_base64_encode(policy)
            params["Policy"] = encoded_policy
        #sign the policy
        signature = self._sign_string(policy, private_key_file, private_key_string)
        #now base64 encode the signature (URL safe as well)
        encoded_signature = self._url_base64_encode(signature)
        params["Signature"] = encoded_signature
        params["Key-Pair-Id"] = keypair_id
        return params