  Near Field Communication (NFC) has enabled mobile phones to emulate
contactless smart cards. Similar to contactless smart cards, they are also
susceptible to relay attacks. To counter these, a number of methods have been
proposed that rely primarily on ambient sensors as a proximity detection
mechanism (also known as an anti-relay mechanism). In this paper, we, for the
first time in academic literature, empirically evaluate a comprehensive set of
ambient sensors for their effectiveness as a proximity detection mechanism. We
selected 15 out of a total of 17 sensors available via the Google Android
platform for evaluation, with the other two sensors unavailable on widely-used
handsets. In existing academic literature, only 5 sensors have been proposed
with positive results as a potential proximity detection mechanism. Each
sensor, where feasible, was used to record the measurements of 1000 contactless
transactions at four different physical locations. A total of 252 random users,
random sample of the university student population, were involved during the
field trails. The analysis of these transactions provides an empirical
foundation to categorically answer whether ambient sensors provide a strong
proximity detection mechanism for security sensitive applications like banking,
transport and high-security access control. After careful analysis, we conclude
that no single evaluated mobile ambient sensor is suitable for such critical
applications in realistic deployment scenarios. Lastly, we identify a number of
potential avenues that may improve their effectiveness.
