&gt; RFID<p>Adding <i>radio</i> as a primary component in a security system is always going to be a bad idea. Security is hard enough without adding in the possibility of 3rd parties hearing the protoocol - or worse.<p>&gt; directional<p>A common misunderstanding of radio is the belief that it can be contained in an area. Unless you&#x27;re building a proper Faraday cage (which is <i>hard</i>), the ability to hear a transmmission often depends on the receiving antenna.<p>For convenience without involving radio, one simply has to get creative. Something like the (defunct) Java Ring[1] would allow most of the ease-of-use of RFID (possibly with a simple proximity sensor for auto-logout, if needed).<p>[1] <a href="http:&#x2F;&#x2F;electronics.howstuffworks.com&#x2F;gadgets&#x2F;home&#x2F;digital-jewelry3.htm" rel="nofollow">http:&#x2F;&#x2F;electronics.howstuffworks.com&#x2F;gadgets&#x2F;home&#x2F;digital-je...</a><p>edit: fixed typo