  Sensitive data protection is essential for mobile users. Plausibly Deniable
Encryption (PDE) systems provide an effective manner to protect sensitive data
by hiding them on the device. However, existing PDE systems can lose data due
to overriding the hidden volume, waste physical storage because of the reserved
area used for avoiding data loss, and require device reboot when using the
hidden volume. This paper presents MobiGyges, a hidden volume-based mobile PDE
system, to fill the gap. MobiGyges addresses the problem of data loss by
restricting each storage block used only by one volume, and it improves storage
utilization by eliminating the reserved area. MobiGyges can also avoid device
reboot by mounting the hidden volume dynamically on-demand with the Dynamic
Mounting service. Moreover, we identify two novel PDE oriented attacks, the
capacity comparison attack and the fill-to-full attack. MobiGyges can defend
them by jointly leveraging the Shrunk U-disk method and multi-level
deniability. We implement the MobiGyges proof-of-concept system on a real
mobile phone Google Nexus 6P with LineageOS 13. Experimental results show that
MobiGyges prevents data loss, avoids device reboot, improves storage
utilization by over 30% with acceptable performance overhead compared with
current works.
