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Abstract: This work presents a novel technique to physically clone a ring oscillator physically unclonable function (RO PDF) onto another distinct FPG A die, using precise, targeted aging. The resulting cloned RO PDF provides a response that is identical to its copied FPGA counterpart, i.e., the FPGA and its clone are indistinguishable from each other. Targeted aging is achieved by: 1) heating the FPGA using bitstream-Iocated short circuits, and 2) enabling/disabling ROs in the same FPGA bitstream. During self heating caused by short-circuits contained in the FPGA bitstream, circuit areas containing oscillating ROs (enabled) degrade more slowly than circuit areas containing non-oscillating ROs (disabled), due to bias temperature instability effects. This targeted aging technique is used to swap the relative frequencies of two ROs that will, in turn, flip the corresponding bit in the PUF response. Two experiments are described. The first experiment uses targeted aging to create an FPGA that exhibits the same PUF response as another FPGA, i.e., a clone of an FPGA PUF onto another FPGA device. The second experiment demonstrates that this aging technique can create an RO PUF with any desired response. 

This work demonstrates a novel method of accelerating FPGA aging by configuring FPGAs to implement thousands of short circuits, resulting in high on-chip currents and temperatures. Patterns of ring oscillators are placed across the chip and are used to characterize the operating frequency of the FPGA fabric. Over the course of several months of running the short circuits on two-thirds of the reconfigurable fabric, with daily characterization of the FPGA 6 performance, we demonstrate a decrease in FPGA frequency of 8.5%. We demonstrate that this aging is induced in a non-uniform manner. The maximum slowdown outside of the shorted regions is 2.1%, or about a fourth of the maximum slowdown that is experienced inside the shorted region. In addition, we demonstrate that the slowdown is linear after the first two weeks of the experiment and is unaffected by a recovery period. Additional experiments involving short circuits are also performed to demonstrate the results of our initial experiments are repeatable. These experiments also use a more fine-grained characterization method that provides further insight into the non-uniformed nature of the aging caused by short circuits.