This paper presents a study of two types of on-chip FPGA voltage sensors based on ring oscillators (ROs) and time-to-digital converter (TDCs), respectively. It has previously been shown that these sensors are often used to extract side-channel information from FPGAs without physical access. The performance of the sensors is evaluated in the presence of circuits that deliberately waste power, resulting in localized voltage drops. The effects of FPGA power supply features and sensor sensitivity in detecting voltage drops in an FPGA power distribution network (PDN) are evaluated for Xilinx Artix-7, Zynq 7000, and Zynq UltraScale+ FPGAs. We show that both sensor types are able to detect supply voltage drops, and that their measurements are consistent with each other. Our findings show that TDC-based sensors are more sensitive and can detect voltage drops that are shorter in duration, while RO sensors are easier to implement because calibration is not required. Furthermore, we present a new time-interleaved TDC design that sweeps the sensor phase. The new sensor generates data that can reconstruct voltage transients on the order of tens of picoseconds.
To lower cost and increase the utilization of Cloud FPGAs, researchers have recently been exploring the concept of multi-tenant FPGAs, where multiple independent users simultaneously share the same FPGA. Despite its benefits, multitenancy opens up the possibility of malicious users co-locating on the same FPGA as a victim user, and extracting sensitive information. This issue becomes especially serious when the user is running a machine learning algorithm that is processing sensitive or private information. To demonstrate the dangers, this paper presents the first remote, power-based side-channel attack on a deep neural network accelerator running in a variety of Xilinx FPGAs and also on Cloud FPGAs using Amazon Web Services (AWS) F1 instances. This work in particular shows how to remotely obtain voltage estimates as a deep neural network inference circuit executes, and how the information can be used to recover the inputs to the neural network. The attack is demonstrated with a binarized convolutional neural network used to recognize handwriting images from the MNIST handwritten digit database. With the use of precise time-to-digital converters for remote voltage estimation, the MNIST inputs can be successfully recovered with a maximum normalized crosscorrelation of 84% between the input image and the recovered image on local FPGA boards and 77% on AWS F1 instances. The attack requires no physical access nor modifications to the FPGA hardware.
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