&gt; But it does protect against the exact threat model you proposed.<p>Encryption doesn&#x27;t protect against traffic analysis. Knowing someone is present or that some device is in use is significant information.<p>Does the device you are proposing authenticate the reader before transmitting anything? If not, it&#x27;s not particularly difficult (probaly by modifying a reader) to test if people have a security device on them. That only requires a ping, no crypto needed.<p>&gt; I&#x27;m a ham<p>I used to be, for many years. (I wish I had more time for such things these days)<p>&gt; large, awkward, and expensive antenna<p>That depends entirely on what you want to do. If you want to read the entire crypto transaction from the next building, then yes, an expensive antenna[1] will be required. If I just want to detect who is carrying a security device, you won&#x27;t need a particularly accurate antenna - it just needs to have a decent gain.<p>My point with that example is that it&#x27;s never a good idea to underestimate how much time and effort people will put into an attack. If criminals can add a man-in-the-middle chip piggybacked onto a chip-and-pin smartcard[2], they can made a decent cantenna.<p>&gt; So do you or don&#x27;t you think RF communication in a security device is an inherent problem in and of itself?<p>RF is an extra risk that should be avoided whenever possible for security devices, <i>especially</i> when effective alternatives are available.<p>In your linked video about the hospital, the smart card was slotted into a reader. This would work well and has no need for RF. It&#x27;s certainly a far better solution than memorizing bad passwords.<p>[1] The $10k antenna used for TempestSDR that was mentioned yesterday should work. <a href="https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10685504" rel="nofollow">https:&#x2F;&#x2F;news.ycombinator.com&#x2F;item?id=10685504</a><p>[2] <a href="http:&#x2F;&#x2F;hackaday.com&#x2F;2015&#x2F;10&#x2F;21&#x2F;smart-cards-used-to-hack-smart-cards&#x2F;" rel="nofollow">http:&#x2F;&#x2F;hackaday.com&#x2F;2015&#x2F;10&#x2F;21&#x2F;smart-cards-used-to-hack-smar...</a>