Hardware masking is a well-known countermeasure against Side-Channel Attacks (SCA). Like many other countermeasures, the side-channel resistance of masked circuits is susceptible to low-level circuit effects. However, no detailed analysis is available that explains how, and to what extent, these low-level circuit effects are causing side-channel leakage. Our first contribution is a unified and consistent analysis to explain how glitches and inter-wire capacitance cause side-channel leakage on masked hardware. Our second contribution is to show that inter-wire capacitance and glitches are causing side-channel leakage of comparable magnitude according to HSPICE simulations. Our third contribution is to confirm our analysis with a successful DPA-attack on a 90nm CMOS FPGA implementation of a glitch-free masked AES S-Box. According to existing literature, this circuit would be side-channel resistant, while according to our analysis and measurement, it shows side-channel leakage. Our conclusion is that circuit-level effects, not only glitches, present a practical concern for masking schemes.
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