private function sign($data, $algorithm)
 {
     if ($algorithm === 'HmacSHA1') {
         $hash = 'sha1';
     } else {
         if ($algorithm === 'HmacSHA256') {
             $hash = 'sha256';
         } else {
             throw new \Exception("Non-supported signing method specified");
         }
     }
     return base64_encode(hash_hmac($hash, $data, $this->config->getSecretKey(), true));
 }