PCASA: Proximity Based Continuous and Secure Authentication of Personal Devices

User's personal portable devices such as smartphone, tablet and laptop require continuous authentication of the user to prevent against illegitimate access to the device and personal data. Current authentication techniques require users to enter password or scan fingerprint, making frequent access to the devices inconvenient. In this work, we propose to exploit user's on-body wearable devices to detect their proximity from her portable devices, and use the proximity for continuous authentication of the portable devices. We present \ksystem which utilizes acoustic communication for secure proximity estimation with sub-meter level accuracy. \ksystem uses Differential Pulse Position Modulation scheme that modulates data through varying the silence period between acoustic pulses to ensure energy efficiency even when authentication operation is being performed once every second. It yields an secure and accurate distance estimation even when user is mobile by utilizing Doppler effect for mobility speed estimation. We evaluate \ksystem using smartphone and smartwatches, and show that it supports up to 34 hours of continuous authentication with a fully charged battery.