  Lensless cameras multiplex the incoming light before it is recorded by the
sensor. This ability to multiplex the incoming light has led to the development
of ultra-thin, high-speed, and single-shot 3D imagers. Recently, there have
been various attempts at demonstrating another useful aspect of lensless
cameras - their ability to preserve the privacy of a scene by capturing
encrypted measurements. However, existing lensless camera designs suffer
numerous inherent privacy vulnerabilities. To demonstrate this, we develop the
first comprehensive attack model for encryption cameras, and propose OpEnCam --
a novel lensless OPtical ENcryption CAmera design that overcomes these
vulnerabilities. OpEnCam encrypts the incoming light before capturing it using
the modulating ability of optical masks. Recovery of the original scene from an
OpEnCam measurement is possible only if one has access to the camera's
encryption key, defined by the unique optical elements of each camera. Our
OpEnCam design introduces two major improvements over existing lensless camera
designs - (a) the use of two co-axially located optical masks, one stuck to the
sensor and the other a few millimeters above the sensor and (b) the design of
mask patterns, which are derived heuristically from signal processing ideas. We
show, through experiments, that OpEnCam is robust against a range of attack
types while still maintaining the imaging capabilities of existing lensless
cameras. We validate the efficacy of OpEnCam using simulated and real data.
Finally, we built and tested a prototype in the lab for proof-of-concept.
