The paper deals with the problem of making secure data transmission inside Home and Building Automation environment; as known, here data exchanged may be critical, as they may regard commands to actuators and/or private and secret information. The need to protect critical data has led to introduce secure transmissions inside communication systems used for home/building automation applications. The paper deals with this problem taking into account the KNX communication system, which, at this moment, doesn't foresee any security and authentication mechanisms. A security and authentication solution will be presented and assessed comparing it with the current state of the art. © 2009 IEEE.