  Recent growth in popularity of mobile and wearable devices has re-ignited the
need for reliable and stealthy communication side-channels to enable
applications such as secret/PIN sharing, co-location proofs and user
authentication. Existing short-range wireless radio technology such as
Bluetooth/BLE and NFC, although mature and robust, is prone to eavesdropping,
jamming and/or interference, and is not very useful as a covert communication
side-channel. This paper designs and implements SkinSense, a vibration-based
communication protocol which uses human body/skin as a communication medium to
create a low-bandwidth and covert communication channel between user-held
mobile and wearable devices. SkinSense employs a novel frequency modulation
technique for encoding bits as vibration pulses and a spectrogram-based
approach to decode the sensed motion data (corresponding to the encoded
vibration pulses) to reconstruct the transmitted bits. SkinSense is
comprehensively evaluated for a variety of operational parameters, hardware
setups and communication settings by means of data collected from human subject
participants. Results from these empirical evaluations demonstrate that
SkinSense is able to achieve a stable bandwidth of upto 6.6 bps, with bit error
rates below 0.1 in our custom hardware setup, and can be employed as a
practical communication side-channel.
