Privacy made easy
Just a note to keep that idea: sometimes it's so so simple to find a solution without using hi-tech. If it is always perfect or reasonable is just another question, however this one works fast and easy:
The Clipped Tag, developed at IBM's Watson Research Center, allows consumers to tear off the majority of an RFID tag's antenna, reducing the tag's read range to just a few inches, ensuring consumer privacy while maintaining the benefits of the technology, such as product authentication or recalls.
Via the RFID-Weblog