Among all proposed Physical Unclonable Functions (PUFs), those based on Ring Oscillators (ROs) are a popular solution for ASICs as well as for FPGAs. However, compared to other PUF architectures, oscillators emit electromagnetic (EM) signals over a relatively long run time, which directly reveal their unique frequencies. Previous work by Merli et al. exploited this fact by global EM measurements and proposed a countermeasure for their attack. In this paper, we first demonstrate that it is feasible to measure and locate the EM emission of a single tiny RO consisting of only three inverters, implemented within a single configurable logic block of a Xilinx Spartan-3A. Second, we present a localized EM attack for standard and protected RO PUFs. We practically investigate the proposed side-channel attack on a protected FPGA RO PUF implementation. We show that RO PUFs are prone to localized EM attacks and propose two countermeasures, namely, randomization of RO measurement lo gic and interleaved placement.
