code: function getToken (token, callback) {
  
      if (token != null &&
          token != '') {
  
          // Fetch token from database or cache
  
          if (token == 'h328hdkasjd3') {
  
              var record = { algorithm: 'hmac-sha-256',   // Required
                  secret: '23hdkdho2893hdkjd',            // Required
                  userId: 'johndoe'                       // Application-specific field
              };
  
              callback(record);
          }
          else {
  
              // Token not found
  
              callback({});
          }
      }
      else {
  
          // Invalid token request
  
          callback({});
      }
  }
docs: MAC Authentication