e items detected are logged in a database every time a person passes a reader, or if it is done for nefarious reasons, such as a mugger using a handheld scanner to obtain an instant assessment of the wealth of potential victims. With permanent RFID serial numbers, an item leaks unexpected information about a person even after disposal-for example, items being resold or given away enable mapping of a person's social network. Another privacy issue has to do with RFID's support for a singulation (i.e., anticollision) protocol. This is the means by which a reader enumerates all the tags Chapter 15 WMANs, WLANs, and WPANs responding to it without them mutually interfering. The structure of the most com- mon version of this protocol is such that all but the last bit of each tag's serial num- ber can be deduced by passively eavesdropping on just the reader's part of the protocol. Because of this, whenever RFID tags are near readers, the distance at which a tag's signal can be eavesdropped is irrelevant; what counts is the distance at which the much more powerful reader can be received. Just how far this is depends on the type of the reader, but in the extreme case, some readers have a maximum power output of 4 watts, which could be received from tens of kilo- meters away. Rarely is information encrypted between a tag and the reader. This creates opportunities for malicious people to leavesdrop on communications and reuse them in nefarious ways-for instance, quickly and easily duplicating a passport. Similarly, there is no standard authentication protocol between a tag and the reader. Again, considering the passport as an example, it is currently possible to conduct a man-in-the-middle attack between a tag-equipped passport and the reader on the desk of the passport control officer. An attacker could substitute information on-the-fly, possibly circumventing detention in one case while making life very difficult for some other innocent citizen standing in line. Fortunately, gov- ernments are starting to realize the risks 