Secure device pairing is a widely studied problem. Local wireless connections such as Bluetooth and WiFi typically rely on user-entered secret keys or manually verified authentication codes. Several recent proposals replace these with contextual or location-dependent sensor inputs, which are assumed to be secret from anyone not present at the location where the pairing takes place. These protocols have to cope with a fuzzy secret, i.e. noisy secret input that differs between the devices. In this paper, we overview such protocols and propose a new variation using time-based opening of commitments. Our protocol has the advantage of treating the fuzzy secret as one piece of data rather than requiring it to be partitioned into time intervals, and being more robust against variations in input entropy than those based on error correction codes. The protocol development is motivated by the discovery of a novel human source for the fuzzy secret: synchronized drawing with two fingers of the same hand on two touch screens or surfaces. Metrics for measuring the distance between the drawings are described and evaluated. We implement a prototype of this surprisingly simple and natural pairing mechanism and show that it accurately differentiates between true positives and man-in-the-middle attackers.
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