In certain applications, it is important for a remote server to securely determine whether or not two mobile devices are in close physical proximity. In particular, in the context of an NFC transaction, the bank server can validate the transaction if both the NFC phone and reader are precisely at the same location thereby preventing a form of a devastating relay attack against such systems.In this paper, we develop secure proximity detection techniques based on the information collected by ambient sensors available on NFC mobile phones, such as audio and light data. These techniques can work under the current payment infrastructure, and offer many advantages. First, they do not require the users to perform explicit actions, or make security decisions, during the transaction -just bringing the devices close to each other is sufficient. Second, being based on environmental attributes, they make it very hard, if not impossible, for the adversary to undermine the security of the system. Third, they provide a natural protection to users' location privacy as the explicit location information is never transmitted to the server. Our experiments with the proposed techniques developed on off-the-shelf mobile phones indicate them to be quite effective in significantly raising the bar against known attacks, without affecting the NFC usage model. Although the focus of this work is on NFC phones, our approach will also be broadly applicable to RFID tags or related payment cards equipped with on-board audio or light sensors.

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