  Embedded systems play a crucial role in fueling the growth of the
Internet-of-Things (IoT) in application domains such as healthcare, home
automation, transportation, etc. However, their increasingly network-connected
nature, coupled with their ability to access potentially sensitive/confidential
information, has given rise to many security and privacy concerns. An
additional challenge is the growing number of counterfeit components in these
devices, resulting in serious reliability and financial implications.
Physically Unclonable Functions (PUFs) are a promising security primitive to
help address these concerns. Memory-based PUFs are particularly attractive as
they require minimal or no additional hardware for their operation. However,
current memory-based PUFs utilize only a single memory technology for
constructing the PUF, which has several disadvantages including making them
vulnerable to security attacks. In this paper, we propose the design of a new
memory-based combination PUF that intelligently combines two memory
technologies, SRAM and DRAM, to overcome these shortcomings. The proposed
combination PUF exhibits high entropy, supports a large number of
challenge-response pairs, and is intrinsically reconfigurable. We have
implemented the proposed combination PUF using a Terasic TR4-230 FPGA board and
several off-the-shelf SRAMs and DRAMs. Experimental results demonstrate
substantial improvements over current memory-based PUFs including the ability
to resist various attacks. Extensive authentication tests across a wide
temperature range (20 - 60 deg. Celsius) and accelerated aging (12 months)
demonstrate the robustness of the proposed design, which achieves a 100%
true-positive rate and 0% false-positive rate for authentication across these
parameter ranges.
