private function createStringToSign($self_key, $client_id, $scope, $context)
    {
        $s2s = sprintf(
            "SIGNER-HMAC-%s\n%s\n%s\n%s\n%s",
            strtoupper($this->hash_algo),
            $self_key,
            $client_id,
            hash($this->hash_algo, $scope),
            hash($this->hash_algo, $context)
        );

        $this->logger->debug(__FUNCTION__, [
            'string_to_sign' => $s2s,
        ]);

        /** @var string */
        return $s2s;
    }