6. Optical Contactless Probing

In this chapter, we demonstrate that all Intrinsic soft and hard PUF implementations in reconfigurable hardware, regardless of their architecture, are vulnerable to optical contactless probing. Since in a real scenario the implemented soft or hard PUFs inside of FPGAs are controlled PUFs, a non-invasive access to the CRPs of the PUFs is restricted by either physical or algorithmic countermeasures. Hence, most of the reported modeling techniques and semi-invasive techniques, including EM, PEM, and LFI, are ineffective to attack the PUF. In this fashion, the unprocessed challenges can be transferred within the FSBL to the FPGA, which is processed later on the device by non-linear functions and applied to the PUF. The response of the PUF will also be generated and processed inside the device and cannot be observed in a non-invasive way. We show how the attacker can deploy LVI to locate circuitry of interest, such as key registers and ring-oscillators of an RO PUF, by knowing or estimating the frequency of different operations. We further present how LVP enables us to probe volatile and on-die-only data streams on the chip without having any physical contact to the transistors or wires. Furthermore, one can perform LVP to characterize high frequency signals, such as the output of ring-oscillators of an RO PUF. For our practical evaluation, we consider a PoC RO PUF implementation in key generation mode inside the FPGA. We further propose an approach to using PUFs as physical sensors to monitor the integrity of reconfigurable hardware against LVP and LVI attacks.