  Copresence verification based on context can improve usability and strengthen
security of many authentication and access control systems. By sensing and
comparing their surroundings, two or more devices can tell whether they are
copresent and use this information to make access control decisions. To the
best of our knowledge, all context-based copresence verification mechanisms to
date are susceptible to context-manipulation attacks. In such attacks, a
distributed adversary replicates the same context at the (different) locations
of the victim devices, and induces them to believe that they are copresent. In
this paper we propose DoubleEcho, a context-based copresence verification
technique that leverages acoustic Room Impulse Response (RIR) to mitigate
context-manipulation attacks. In DoubleEcho, one device emits a wide-band
audible chirp and all participating devices record reflections of the chirp
from the surrounding environment. Since RIR is, by its very nature, dependent
on the physical surroundings, it constitutes a unique location signature that
is hard for an adversary to replicate. We evaluate DoubleEcho by collecting RIR
data with various mobile devices and in a range of different locations. We show
that DoubleEcho mitigates context-manipulation attacks whereas all other
approaches to date are entirely vulnerable to such attacks. DoubleEcho detects
copresence (or lack thereof) in roughly 2 seconds and works on commodity
devices.
