code: function verifyCallback(callbackBody, contentSignature) {
    return new Promise((resolve, reject) => {
      if (!callbackBody || !contentSignature) {
        throw new Error('callbackBody and contentSignature must be defined.');
      }
  
      const key_url = callbackBody.Metadata && callbackBody.Metadata.key_url;
  
      request(key_url, (err, res, body) => {
        if (err) throw err;
        try {
          const parsed = JSON.parse(body);
          const signingKey = parsed.signing_key;
  
          return resolve(verifyMessage(callbackBody, contentSignature, signingKey));
        } catch (parseErr) {
          throw parseErr;
        }
      });
    });
  }
docs: @typedef {object} CWCallback
  @property verifyCallback
  @property verifyMessage
  @property middleware
  
   
  Load signing key and verify the message
  @param {CallbackPayload} callbackBody
  @param {string} contentSignature base64 content signature req.headers[x-content-signature]
  @return {Promise<boolean>}