  The rapidly expanding hardware-intrinsic security primitives are aimed at
addressing significant security challenges of a massively interconnected world
in the age of information technology. The main idea of such primitives is to
employ instance-specific process-induced variations in electronic hardware as a
source of cryptographic data. Among the emergent technologies, memristive
devices provide unique opportunities for security applications due to the
underlying stochasticity in their operation. Herein, we report a prototype of a
robust, dense, and reconfigurable physical unclonable function primitives based
on the three-dimensional passive metal-oxide memristive crossbar circuits, by
making positive use of process-induced variations in the devices' nonlinear
I-Vs and their analog tuning. We first characterize security metrics for a
basic building block of the security primitives based on a two layer stack with
monolithically integrated 10x10 250-nm half-pitch memristive crossbar circuits.
The experimental results show that the average uniformity and diffusivity,
measured on a random sample of 6,000 64-bit responses, out of ~697,000 total,
is close to ideal 50% with 5% standard deviation for both metrics. The
uniqueness, which was evaluated on a smaller sample by readjusting conductances
of crosspoint devices within the same crossbar, is also close to the ideal 50%
+/- 1%, while the smallest bit error rate, i.e. reciprocal of reliability,
measured over 30-day window under +/-20% power supply variations, was ~ 1.5%
+/- 1%. We then utilize multiple instances of the basic block to demonstrate
physically unclonable functional primitive with 10-bit hidden challenge
generation that encodes more than 10^19 challenge response pairs and has
comparable uniformity, diffusiveness, and bit error rate.
