RFID systems are different from other means of identification because RF communication is non-contact and non-line-of-sight, whereas other means of identification are either contact-based or require line-of-sight. In other words, it is more difficult for the owner of the RF tag to physically impede communication with the tag. The promiscuity of RF tags is not unique; magnetic stripe cards, for example, are promiscuous, but we assume that the owner of the card takesthephysicalresponsibilityofpreventingunauthorizedusersfromphysically accessing the card. Of course, the propagation characteristics of electromagnetic fields do limit the range from which passive RFID cards can be read.