code: function signature(consumerSecret, tokenSecret, requestMethod, url, body, protocolParams) {
      var signatureMethod = protocolParams.oauth_signature_method;
      if (signatureMethod === 'HMAC-SHA1') {
          return crypto.createHmac('sha1', signingKey(consumerSecret, tokenSecret))
              .update(signatureBaseString(requestMethod, url, body, protocolParams))
              .digest('base64');
      }
  }
docs: get signature