Wearable devices -- like the FitBit, MOTOACTV, and Jawbone UP -- are increasingly becoming more pervasive whether for monitoring health and fitness, personal assistance, or home automation. While pervasive wearable devices have long been researched, we are now beginning to see the fruits of this research in the form of commercial offerings. Today, many of these commercial wearable devices are closed systems that do not interoperate with other devices a person might carry. We believe, however, these commercial offerings signal the coming of wireless body-area networks that will connect these pervasive wearable devices and leverage existing devices a user already owns (e.g., a smartphone). Such wireless body-area networks will allow devices to specialize and utilize the capabilities of other devices in the network. A sensor, for example, might harness the internet connectivity of a smartphone to store its data in the cloud. Utilized in this way, devices will become cheaper because they will only require the components necessary for their speciality, and they will also become more pervasive because they can easily be shared between users.

In order for such a vision to be successful, these devices will need to seamlessly interoperate with no interaction required of the user. As difficult as it is for users to manage their wireless area networks, it will be even more difficult for a user to manage their wireless body-area network in a truly pervasive world. As such, we believe these wearable devices should form a wireless body-area network that is passive in nature. This means that these pervasive wearable devices will require no configuration, yet they will be able form a wireless body-area network by (1) discovering their peers, (2) recognizing they are attached to the same body, (3) securing their communications, and (4) identifying to whom they are attached. While we are interested in all aspects of these passive wireless body-area networks, we focus on the last requirement: identifying who is wearing a device.