International Association
for Cryptologic Research

FPGAs have been used in the cloud since several years, as accelerators for various workloads such as machine learning, database processes and security tasks. As for other cloud services, a highly desired feature is virtualization in which multiple tenants can share a single FPGA to increase utilization and by that efficiency. By solely using standard FPGA logic in the untrusted tenant, on-chip logic sensors allow remote power analysis side-channel and covert channel attacks on the victim tenant. However, such sensors are implemented by unusual circuit constructions, such as ring oscillators, delay lines, or unusual interconnect configuration, which might be easily detected by bitstream and/or netlist checking. In this paper, we show that such structural checking methods are not universal solutions as the attacks can make use of any normal circuits, which mean they are ``benign-looking'' to any checking method. We indeed demonstrate that -- without any additional and suspicious implementation constraints -- standard circuits intended for legitimate tasks can be misused as a sensor thereby monitoring instantaneous power consumption, and hence conducting key-recovery attacks. This extremely stealthy attack is a threat that can originate from the application layers, i.e. through various high-level synthesis approaches.