  We present a comprehensive investigation into the complexity of a new private
key storage apparatus: a novel silicon photonic physical unclonable function
(PUF) based on ultrafast nonlinear optical interactions in a chaotic silicon
microcavity that is both unclonable and impossible to emulate. This device
provides remarkable improvements to total information content (raw
cryptographic material), information density, and key generation rates over
existing optical scattering PUFs and is also more easily integrated with both
CMOS electronics and telecommunications hardware. Our device exploits the
natural nonlinear optical behavior of silicon to neutralize commonly used
attacks against PUFs and vastly enhance device complexity. We confirm this
phenomenon with thorough experimental results on prototype devices and present
a detailed estimate of their total information content. Our compact,
micron-scale approach represents an entirely new generation of ultrafast and
high information density photonic PUF devices that can be directly incorporated
into integrated circuits to ensure authenticity and provide secure physical
storage of private key material.
