age. But so, when none of the registers are leak-
ing, the second source of leakage is the combinational
logic. This part of the circuit is less predictable, as:
• before tapeout, it is mapped, most probably re-
structured, and potentially even simplified, and
• upon execution, it features races which might dif-
fer on the PVTA (Process, Voltage, Temperature,
and Transistor Aging) environmental conditions.
There is therefore the need for tools and methods to
evaluate masked netlists. Obviously, preventing is
better than curing. Therefore, in this paper, we study
pre-silicon techniques. The central question is to de-
termine which pre-silicon technique is reasonable.
2 STATE OF THE ART
Predicting whether an implementation is vulnerable
to side-channel attacks when still at pre-silicon stage
has been addressed by many works. Software sim-
ulators can be beneficial in this respect, since soft-
ware leakage is complex. There are multiple points
of interest that can leak, hence checking those mitiga-
tions in a program which embeds countermeasures is
not trivial. Therefore, multiple simulators have been
designed to extract the power leakage. They can be
dedicated tools, or can consist in a post-processing of
execution traces dumped by a COTS tool.
Program Inferred Power Analysis Simulator (PIN-
PAS) is the first exclusive software for analyzing
side-channel power written in JAVA which is pri-
mary designed for testing smart-cards (Hartog et al.,
2003). This tool is able to simulate the execution
of a program (in assembly code) in its virtual envi-
ronment to subsequently analyze its estimated power.
Kirschbaum et al. (Kirschbaum, 2007) designed a
simulator based on the Cadence NCSim to generate
an accurate leakage tracing. The designed tool ex-
tracts power traces based on the gate level netlist,
cell information, and gates propagation time to count
the transitions. However, it requires a lot of infor-
mation of hardware implementation of devices which
is not always accessible. Debande et al. (Debande
et al., 2012) designed a profiled simulator based on
the stochastic models and the successive values of de-
vice’s registers. This tool is able to extract leakage
traces close to the real measurement. This simulator
does not need any specific information of the device,
however, profiling step should be performed for ev-
ery new device. SILK was the first fully open-source
simulator for side-channel analysis which is unveil in
(Veshchikov, 2014). The flexibility of SILK makes
users capable to use different model such as Ham-
ming Weight (HW), Hamming Distance (HD), and
even user-defined leakage models.
Most of simulations leverage a toggle count leak-
age model (namely, the Hamming Distance between
signals at each trace time sample). Indeed, it has
been acknowledged for long in the field of embedded
systems security, for instance, in the seminal paper
about Correlation Power Analyses (Brier et al., 2004,
§2). In this paper, the model focuses on flip-flops.
But it has also been extended to combinational logic
with fair correlation, e.g., to estimate glitching activ-
ity (Liu et al., 2011) (in unprotected circuits tough).
Compared to the aforementioned tools, SPICE
1
simulations are more realistic. They can be consid-
ered as the most accurate possible simulations, and
are seen as references (Li et al., 2005). Notice that
the Synopsys VCS and HSPICE netlists we consider
are the same, except that Synopsys VCS reports only
an “impulse” for each gate when toggles, whereas
HSPICE produces a complete waveform (illustrations
will be given in Fig. 3 and Fig. 4).
To fill the gap, in this paper, we revisit the use of
toggle count vs SPICE simulations, applied to masked
netlists. More precisely, we address the relevance of
deploying toggle count in evaluating pre-silicon secu-
rity level:
• If there is a significant correlation between sensi-
tive variables and the toggle count, then there is
obviously a vulnerability, hence the netlist must
be patched. Notice that, in the past, the compro-
mise in the netlists has been identified based on
toggle count analyses. For example, the flaw in
(non-glitch resistant) masking of AND gates has
been detected by SPICE in (Mangard et al., 2005,
§4) and subsequently qualified based on a toggle
count in (Mangard et al., 2006, §2.2.). Recall
that “glitches” are races between signals, which
are likely to generate transient activity observable
through side-channels.
• However, assuming that the toggle count analysis
does not reveal signification biases (in terms of
correlation with the underlying sensitive variable
value), can it be claimed that the netlist is secure?
Actually, the absence of correlation can also arise
from the fact that the “toggle count may not be a
valid abstraction” in some cases. To the best of
our knowledge, this question has never been for-
mulated as such and consequently has never been
thoroughly studied. In this paper, we carry out
a meticulous analysis and conclude that, though
SPICE is always suitable a pre-silicon simula-
1
For the sake of illustration, we use HSPICE from Syn-
opsys Inc. as an electrical simulation tool for SPICE netlists.
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