  When users want to establish wireless communication between/among their
devices, the channel has to be bootstrapped first. To prevent any malicious
control of or eavesdropping over the communication, the channel is desired to
be authenticated and confidential. The process of setting up a secure
communication channel between two previously unassociated devices is referred
to as "Secure Device Pairing". When there is no prior security context, e.g.,
shared secrets, common key servers or public key certificates, device pairing
requires user involvement into the process. The idea usually involves
leveraging an auxiliary human-perceptible channel to authenticate the data
exchanged over the insecure wireless channel.
  We observe that the focus of prior research has mostly been limited to
pairing scenarios where a single user controls both the devices. In this paper,
we consider more general and emerging "two-user" scenarios, where two different
users establish pairing between their respective devices. Although a number of
pairing methods exists in the literature, only a handful of those are
applicable to the two-user setting. We present the first study to identify the
methods practical for two-user pairing scenarios, and comparatively evaluate
the usability of these methods. Our results identify methods best-suited for
users, in terms of efficiency, error-tolerance and of course, usability. Our
work sheds light on the applicability and usability of pairing methods for
emerging two-user scenarios, a topic largely ignored so far.
