Evaluation Metric. We utilize key rank (a.k.a. guessing
entropy ) as the metric to measure the attack performance of
side-channel attacks. Specifically, given a power trace
t′i, classifier F outputs a HW score vector
(si[0],...,si[8]), where
si[j] is the score for Hamming weight j. Next, an
attacker obtains a key score vector
(ri[k1*],...,ri[k256*])
by calculating
Key rank is denoted as w, where w ∈ [0,255], if correct key k′ is ranked as the (w+1)-th key among all the possible keys k1*, ..., k256* based on the aggregated key scores. A key rank of 0 over N′ traces suggests that an attacker can recover key k′ with N′ traces. We use KRC to indicate the number of test traces that an attacker needs for key rank to converge to 0. If key rank converges to 0 with a lower number of traces, it indicates that an attack is more effective.