RFID, which stands for Radio Frequency Identification, is ubiquitous in our lives. We find RFID tags in our library books, grocery, consumer goods, printer cartridges, and are even implanted into people’s bodies.
The basic principle behind RFID is that a simple, passive device responds to a burst of RF with a unique number, which can be used to identify the object to which the device is attached. There are many types of tags, some of them can even be written to. When I have the time, I will write an in-depth article on this subject.
RenderMan, Thorn and Audit have written a book on this topic, titled RFID Security. You can get this book at Amazon.com. RenderMan is very active in the Church of WiFi, Thorn has participated in other books, such as Wardriving: Drive, Detect, Defend. Audit is a very active moderator of the Netstumbler forums, hosts personalwireless.org, and also participates in many WiFi-related projects.