code: function (jsonS) {
      var jws = KJUR.jws.JWS;
      if(jws.isSafeJSONString(jsonS)) {
        return jws.readSafeJSONString(jsonS);
      }
      return null;
    }
docs: Get the JSON object from the JSON string
  @param {string} jsonS    JSON string
  @returns {object|null}   JSON object or null