1 Capacitive Touch Communication: A Technique to Input Data Through Devices’ Touchscreen Tam Vu, Akash Baid, Simon Gao, Marco Gruteser, Richard Howard, Janne Lindqvist, Predrag Spasojevic, Jeffrey Walling Abstract—As we are surrounded by an ever-larger variety of post-PC devices, the traditional methods for identifying and authenticating users have become cumbersome and time- consuming. In this paper, we present a capacitive communication method through which a device can recognize who is interacting with it. This method exploits the capacitive touchscreens, which are now used in laptops, phones, and tablets, as a signal receiver. The signal that identifies the user can be generated by a small transmitter embedded into a ring, watch, or other artifact carried on the human body. We explore two example system designs with a low-power continuous transmitter that communicates through the skin and a signet ring that needs to be touched to the screen. Experiments with our prototype transmitter and tablet receiver show that capacitive communication through a touchscreen is possible, even without hardware or firmware modifications on a receiver. This latter approach imposes severe limits on the data rate, but the rate is sufficient for differentiating users in multiplayer tablet games or parental control applications. Controlled experiments with a signal generator also indicate that future designs may be able to achieve datarates that are useful for providing less obtrusive authentication with similar assurance as PIN codes or swipe patterns commonly used on smartphones today. I. I NTRODUCTION Mobile devices now provide us ubiquitous access to a vast array of media content and digital services. They can access our emails and personal photos, open our cars [42] or our garage doors [15], pay bills and transfer funds between our bank accounts, order merchandise, as well as control our homes [10]. Arguably, they now provide the de-facto single- sign on access to all our content and services, which has proven so elusive on the web. As we increasingly rely on a variety of such devices, we tend to quickly switch between them and temporarily share them with others [27]. We may let our children play games on our smartphones or share a tablet with colleagues or family members. Sometimes a device may be used by several persons simultaneously, as when playing a multi-player game on a tablet, and occasionally, a device might fall into the hands of strangers. In all these situations, it would be of great benefit for the device to know who is interacting with it and occasionally to authenticate the user. We may want to limit access to age- appropriate games and media for our children or prevent them from charging our credit card. 1 We desire to hide sensitive 1 Apple is facing a law suit over children’s in-app credit card purchases [19]. personal information from strangers, colleagues, or perhaps even an curious spouse [24], [27]. Or, we may simply want to enjoy an enhanced user experience from the multi-player game that can tell who touched the screen. Unfortunately, user identification and authentication mech- anisms available on today’s mobile devices have been largely adopted from PC software and have not followed the ver- satility of the usage and sharing possibilities. For example, several mobile devices (e.g. iPad or iOS devices) do allow to restrict access to device functions, but the devices do not provide any easy way to quickly change, let alone authenticate, users. They provide PIN codes, passwords, for authentication, and a number of other techniques have been proposed by researchers [11]. Yet they remain cumbersome and very few people enable these security features on their phones. In this paper, we will explore a form of “wireless” com- munication, that we term capacitive touch communication to address this issue. The key idea is to exploit the pervasive capacitive touch screen and touchpad input devices as receivers for an identification code transmitted by a hardware identifica- tion token. While the token can take many forms, we consider here an example realization as a ring, inspired by the signet rings used since ancient times. The token transmits electrical signals on contact with the screen, either direct contact or indirect contact through the human skin. The major contributions of this paper are as follows: • Painting a vision to use the near-ubiquitous capacitative touch sensors to distinguish and possibly authenticate users. • Introducing and exploring the concept of capacitive touch communication as one mechanism to distinguish users. • Showing how the output of an off-the-shelf touchscreen system can be affected by electrical signals generated in a token that is in contact with the screen. We also show how such signals can be transmitted through the human skin. • Designing and implementing a prototype transmitter in the form of a signet ring and receiver software for communicating short codes through an off-the-shelf ca- pacitative touch screen II. BACKGROUND Touchscreen technology was first developed in the 1960’s for air traffic control systems [26] and is now a popular