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chapter-hsms/chapter.tex
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@ -322,7 +322,7 @@ devices we selected for this study.
\surveypic{31}{survey_diag_S31.jpg}\\
\surveypic{32}{survey_diag_S32.jpg}&
\end{tabular}
\caption{External photos of all survey samples}
\caption{External photos of all survey samples.}
\label{fig_hsm_survey_sample_pics}
\end{figure}
@ -336,9 +336,9 @@ networks, almost all payment terminals on the market irrespective of their count
standards. Adding on to PCI's ecosystem impact, its security standards are thought out well and provide a higher level
of security than one might expect from an industry association.
Physical security standards in card payment applications both on the client side -- payment terminals -- and on the
server side -- HSM appliances -- are more stringent than one might expect since the finance industry has been reluctant
to adopt modern cryptography. Not only are modern cryptographic protocols like Secure Multiparty Computation (SMPC) or
Physical security standards in card payment applications both on the client side (payment terminals) and on the server
side (HSM appliances) are more stringent than one might expect since the finance industry has been reluctant to adopt
modern cryptography. Not only are modern cryptographic protocols like Secure Multiparty Computation (SMPC) or
Zero-Knowledge Proofs (ZKPs) not commonly used. Even asymmetric cryptography has only been adopted reluctantly, and
ancient ciphers such as Triple DES are still commonly referenced in industry
standards~\cite{pcisecuritystandardscouncilPaymentCardIndustry2025}. As a result, increased hardware security is necessary to
@ -350,18 +350,20 @@ terminals are cost-sensitive devices, which is reflected in the construction of
\subsubsection{HSM Appliances}
For datacenter applications, HSMs are sold both as add-in cards and as standalone rackmount appliances with a network
interface. In practice, the standalone appliances are just low-end computers in a rackmount enclosure that expose the
API of an internal HSM add-in card to the network. In this survey, we were only able to procure a single such HSM since
these devices are expensive, and even used specimens of older models are usually listed for several hundreds to several
thousands of EUR. The one sample we procured was a 2011 model Utimaco CryptoServer LAN. Our unit was a white-label
variant procured by premium TV encryption technology provider Irdeto, presumably used in Germany to produce
cryptographic key streams for TV signal encryption. We bought the device from a recycling company specialized on
datacenter components. The device was sold with any HDDs removed. The device consisted of an older mainboard for
embedded applications containing an Intel Core 2 Duo-brand processor and 2 GiB of DDR2 RAM, which was connected to the
HSM add-in card through PCI. The device contained a small Lithium backup battery on the add-in card, and another, larger
battery in an enclosure at the front of the device that was connected to the card through a cable. The device did not
contain any obvious case intrusion sensors.
When credit card payments are handled on the web as opposed to in a physical store, HSMs are used in data centers to
handle plaintext payment data such as credit card numbers. Such HSM appliances are usually standalone rackmount devices
and are used across application domains. Depending on the application, these HSMs can be programmed with custom code, or
can be used as coprocessors through an API. In practice, the standalone appliances are just low-end computers in a
rackmount enclosure that expose the API of an internal HSM add-in card to the network. In this survey, we were only able
to procure a single such HSM since these devices are expensive, and even used specimens of older models are usually
listed for several hundreds to several thousands of EUR. The one sample we procured was a 2011 model Utimaco
CryptoServer LAN. Our unit was a white-label variant procured by premium TV encryption technology provider Irdeto,
presumably used in Germany to produce cryptographic key streams for TV signal encryption. We bought the device from a
recycling company specialized on datacenter components. The device was sold with any HDDs removed. The device consisted
of an older mainboard for embedded applications containing an Intel Core 2 Duo-brand processor and 2 GiB of DDR2 RAM,
which was connected to the HSM add-in card through PCI. The device contained a small Lithium backup battery on the
add-in card, and another, larger battery in an enclosure at the front of the device that was connected to the card
through a cable. The device did not contain any obvious case intrusion sensors.
\subsubsection{ATM Encrypting Pin Pads}
@ -386,7 +388,7 @@ transmit the plaintex PIN.
We acquired three different EPPs for analysis: Two designed by Sagem and apparently re-sold as a whitelabel product by
Cryptera and Diebold, respectively, and one made by and branded NCR. All three devices have robust stainless steel front
cases.
cases, and are built in a sandwich construction of several layers of steel sheets and PCBs.
\subsubsection{Other miscellaneous devices}
@ -448,7 +450,7 @@ cutting and prying, and applying heat from a heat gun as necessary to soften pol
% overlapping the previous row
\rule{0pt}{25mm}
\end{tabular}
\caption{Internal overview photos of the survey samples}
\caption{Internal overview photos of the survey samples.}
\label{fig_hsm_survey_sample_internal_pics}
\end{figure}
@ -464,12 +466,12 @@ We found meshes constructed from rigid PCBs as well as a number of Flexible Prin
tamper sensing meshes constructed from PCBs sometimes used parts of an existing PCB, and sometimes additional PCBs only
containing a mesh were added. Sometimes, multiple rigid PCB meshes were assembled in a house of cards fashion to enclose
part of a device. For flexible meshes, with the exception of the Utimaco HSM appliance's HSM card that used an
off-the-shelf Gore tamper sensing mesh foil were all clearly manufactured either entirely or mostly in standard
off-the-shelf Gore tamper sensing mesh foil, all were clearly manufactured either entirely or mostly in standard
processes. We found silkscreened silver ink and silkscreened carbon ink-based foils similar to those used for membrane
keyboards, as well as conventional photolithographically etched copper/polyimide Flexible Printed Circuits (FPCs).
Overall, etched PCBs showed better resolution compared to silkscreen-printed meshes. Feature size for both rigid and
flexible etched PCB meshes was generally in the order of \qtyrange{100}{200}{\micro\meter}, while feature size for
printed foil meshes was coarser at between \qtyrange{500}{3000}{\micro\meter}.
screen printed foil meshes was coarser at between \qtyrange{500}{3000}{\micro\meter}.
\subsubsection{Mesh layout.}
@ -504,7 +506,8 @@ printed foil meshes was coarser at between \qtyrange{500}{3000}{\micro\meter}.
A key goal in tamper sensing mesh design is to avoid any gaps in coverage. In single-layer meshes, gaps between adjacent
mesh traces cannot be avoided, and provide an easy approach for an attack. In multi-layer meshes, these structure
size-dependent gaps can be mitigated in multiple ways as shown in Figure~\ref{hsm_fig_mesh_layout}.
size-dependent gaps can be mitigated in multiple ways as shown in Figure~\ref{hsm_fig_mesh_layout}. In the following
paragraphs, we will address several common structural features that we observed across samples.
\paragraph{Offset patterns.} In a two-sided foil mesh, most of the gaps between adjacent traces can be covered by simply
offsetting the pattern by one structure size in both axes between the foil's top and bottom layers as shown in
@ -522,7 +525,7 @@ foil used in an Utimaco HSM. This mesh consists of two foil layers bonded to eac
both sides with a sparse pattern of thin serpentine traces with the patterns on both layers being orthogonal to each
other. Both patterns are oriented at a \qty{45}{\degree} angle relative to the sides of the rectangular enclosed volume.
The inner foil is only patterned on one side, and contains a thicker serpentine trace laid out in a zigzag pattern. The
two foil layers are aligned such that no gaps remain between the layers.
two foil layers are aligned such that no gaps remain between the layers.\todo{sample number here and below (ingenico)}
\paragraph{Using layer spacing.} Figure~\ref{hsm_fig_mesh_layout_epp} shows how an ATM Encrypting Pin Pad (EPP)
implemented the mesh on its keypad. Off-the-shelf metal snap dome contacts were used on the surface of a conventional
@ -600,14 +603,16 @@ inks have high resistance, and can be used to create embedded resistors. The cir
Figure~\ref{hsm_fig_materials_silver_ink} contains a tamper sensing mesh on a lower layer, and a keypad matrix with
carbon contacts on its surface.
Figure~\ref{hsm_fig_materials_gold_lds} shows part of a mesh and a contact created using Laser Direct Structuring and
electroless gold plating. Where in electroplating electrical current is used to deposit metal atoms on a surface, in
electroless plating a series of chemical reactions is used. Electroplating requires all traces to be electrically
connected to form a single electrode, while electroless plating can be used on the finished circuit. In
Figure~\ref{hsm_fig_materials_gold_lds}, it is visible how the trace was created using three parallel passes by the
laser. The micrograph also shows the rather coarse edge structure created by LDS, which is caused by the rough surface
left after pulsed laser ablation. The uneven, thin layer of metallization created by LDS results in mechanically fragile
contacts. They must be contacted using a soft material, usually an elastomeric connector.
Figure~\ref{hsm_fig_materials_gold_lds} shows part of a mesh and a contact created using Laser Direct Structuring, a
technique combining selective activation of a plastic surface using a scanning laser and electroless gold plating. Where
in electroplating electrical current is used to deposit metal atoms on a surface, in electroless plating a series of
chemical reactions is used. Electroplating requires all traces to be electrically connected to form a single electrode,
while electroless plating can be used on the finished circuit. Laser Direct Structuring allows patterning complex
surfaces with fine structures made from metal deposited in a thin layer. In Figure~\ref{hsm_fig_materials_gold_lds}, it
is visible how the trace was created using three parallel passes by the laser. The micrograph also shows the rather
coarse edge structure created by LDS, which is caused by the rough surface left after pulsed laser ablation. The uneven,
thin layer of metallization created by LDS results in mechanically fragile contacts that must be contacted using a soft
material, usually an elastomeric connector.
\subsubsection{Connection methods}
@ -615,37 +620,37 @@ contacts. They must be contacted using a soft material, usually an elastomeric c
\centering
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_castellated_edge.jpg}
\caption{}
\caption{Direct soldering}
\label{hsm_fig_connector_castellations}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_stacking.jpg}
\caption{}
\caption{Elastomeric connector landing pattern as well as stacking board-to-board connector}
\label{hsm_fig_connector_stack}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_zif_fpc_2.jpg}
\caption{}
\caption{Landing pads for tactile contact domes as well as FPC connector}
\label{hsm_fig_connector_fpc}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_elastomeric.jpg}
\caption{}
\caption{Direct soldering of an FPC and an elastomeric connector}
\label{hsm_fig_connector_elastomeric}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_rf_gasket.jpg}
\caption{}
\caption{Soft, conductive EM shielding gaskets used as connectors}
\label{hsm_fig_connector_gasket}
\end{subfigure}
\quad
\begin{subfigure}[t]{0.3\textwidth}
\centering\includegraphics[width=\linewidth]{connector_metal_dome.jpg}
\caption{}
\caption{Tactile dome}
\label{hsm_fig_connector_dome}
\end{subfigure}
\caption[Mesh connecting methods]{Connecting methods used between tamper sensing mesh assemblies and their base PCBs}
@ -658,6 +663,7 @@ a PCB using either a Land Grid Array (LGA) technique where pads on both PCBs are
\emph{castellated} edges, where pads on the base PCB are soldered sideways to holes on the top PCB that have been milled
in half as shown in Figure~\ref{hsm_fig_connector_castellations}. FPCs can also be soldered by draggin a solder blob
across the contact as shown in Figure~\ref{hsm_fig_connector_elastomeric}, but this technique is only suitable for hand
soldering. Hand soldering increases unit cost over mechanized soldering techniques such as wave soldering or reflow
soldering.
FPCs are suitable for use with standard Zero Insertion Force (ZIF) FPC connectors as shown in
@ -738,24 +744,24 @@ connection while guaranteeing adjacent spheres never touch each other.
\label{hsm_fig_3d_struct}
\end{figure}
In practice, meshes are almost always manufactured in planar processes first, and then transformed into a
three-dimensional shape. Figure~\ref{hsm_fig_3d_struct}
\subref{hsm_fig_3d_struct_folded_overlap}-\subref{hsm_fig_3d_struct_house_of_cards} show the construction styles we saw
among our samples that shape a planar mesh into a three-dimensional structure.
Figure~\ref{hsm_fig_3d_struct_folded_overlap} and Figure~\ref{hsm_fig_3d_struct_folded_no_overlap} have meshes produced
as flexible printed circuits, in Figure~\ref{hsm_fig_3d_struct_folded_overlap} using a standard photolithographic
copper/polyimide FPC process usually used for flexible PCBs, and in Figure~\ref{hsm_fig_3d_struct_folded_overlap} using
a standard silver ink screenprinting process. The choice in Figure~\ref{hsm_fig_3d_struct_folded_no_overlap} not to
overlap the mesh in the corner is likely caused by manufacturing considerations, since it mig~ht be difficult to ensure
proper folding of a small foil tab with adhesive pre-applied.
~
Figure~\ref{hsm_fig_3d_struct_vacuum_form} shows a sample of a flexible circuit manufactured in a screenprinted
silver-ink process thermoformed into a three-dimensional shape~\cite{weidnerHardwareschutzFormHalbschalen2007}. The
flexible circuit mesh is first produced in a standard planar printing process. After printing and curing, the resulting
foil is then heated to soften it, and forced into a three-dimensional shape using a mold. Depending on the process, one
or two molds, and vacuum or pressured air can be used to shape the foil. The process requires a screenprinted flexible
circuit, and would not work with copper/polyimide flexible PCBs since their copper layer is too thick to plastically
deform without tearing, and because polyimide is not sufficiently thermoplastic at low temperatures.
While practical meshes are almost always manufactured in planar processes first, their applications usually require at
least partially covering a three-dimensional volume. In our survey, we saw a number of methods being used to create
three-dimensional structures from planar meshes. Figure~\ref{hsm_fig_3d_struct}
\subref{hsm_fig_3d_struct_folded_overlap}-\subref{hsm_fig_3d_struct_house_of_cards} show the major construction styles
we saw among our samples. Figure~\ref{hsm_fig_3d_struct_folded_overlap} and
Figure~\ref{hsm_fig_3d_struct_folded_no_overlap} have meshes produced as flexible printed circuits, in
Figure~\ref{hsm_fig_3d_struct_folded_overlap} using a standard photolithographic copper/polyimide FPC process usually
used for flexible PCBs, and in Figure~\ref{hsm_fig_3d_struct_folded_overlap} using a standard silver ink screenprinting
process. The choice in Figure~\ref{hsm_fig_3d_struct_folded_no_overlap} not to overlap the mesh in the corner is likely
caused by manufacturing considerations, since it might be difficult to ensure proper folding of a small foil tab with
adhesive pre-applied. Figure~\ref{hsm_fig_3d_struct_vacuum_form} shows a sample of a flexible circuit manufactured in a
screenprinted silver-ink process thermoformed into a three-dimensional
shape~\cite{weidnerHardwareschutzFormHalbschalen2007}. The flexible circuit mesh is first produced in a standard planar
printing process. After printing and curing, the resulting foil is then heated to soften it, and forced into a
three-dimensional shape using a mold. Depending on the process, one or two molds, and vacuum or pressured air can be
used to shape the foil. The process requires a screenprinted flexible circuit, and would not work with copper/polyimide
flexible PCBs since their copper layer is too thick to plastically deform without tearing, and because polyimide is not
sufficiently thermoplastic at low temperatures.
Thermoforming is a cheap industry standard process, but applied to flexible circuits it has some limitations. First,
only 2.5-dimensional structures can be created since the starting product is always a planar sheet. Second, the sheet
@ -822,10 +828,10 @@ which would be a flaw in a more standard HSM application.
Besides the house of cards construction style shown in Figure~\ref{hsm_fig_3d_struct_house_of_cards} where PCBs are
hand-assembled into a 3D shape, rigid PCBs are also often soldered planar on top of other PCBs to serve as meshes.
Figure~\ref{hsm_fig_3d_sandwich} shows examples of such sandwich-style constructions.
Figure~\ref{hsm_fig_3d_sandwich_obstacle} and Figure~\ref{hsm_fig_3d_sandwich_via_fence} show a popular construction
Figure~\ref{hsm_fig_3d_sandwich_obstacle} and Figure~\ref{hsm_fig_3d_sandwich_via_fence} show a widely used construction
technique where a small mesh PCB coupon is soldered using a Land Grid Array (LGA)-technique on top of a larger base PCB
containing circuitry. The goal in this technique is to project a small part of the mesh into the space above the base
PCB. While this does not prvevent targeted drilling, as the small coupon is easy to avoid, it does prevent an attacker
PCB. While this does not prevent targeted drilling as the small coupon is easy to avoid, it does prevent an attacker
from sawing or laser-cutting into the side of the device parallel to the base PCB. In the implementation shown in
Figure~\ref{hsm_fig_3d_sandwich_obstacle}, the coupon simply contains a small mesh embedded in an inner layer.
Figure~\ref{hsm_fig_3d_sandwich_via_fence} shows a different technique, where the mesh inside the coupon is not
@ -855,7 +861,7 @@ via fence layers, at the bottom of the PCB is one more layer containing the pads
\begin{subfigure}[t]{0.45\textwidth}
\centering
\includegraphics[width=\linewidth]{mesh_contact_joint.pdf}
\caption{CT section cut with part of a mesh layer and the riveted metal mesh contacts visible.}
\caption{CT section cut with part of a mesh layer and the crimped metal mesh contacts visible.}
\label{hsm_fig_ingenico_potted_ct_cut}
\end{subfigure}
\quad
@ -877,14 +883,20 @@ via fence layers, at the bottom of the PCB is one more layer containing the pads
\end{figure}
% FIXME put the CT people in the acknowledgements! Also the microwave people!
To evaluate CT imaging as an attack method, we performed CT imaging of the potted HSM module of an Ingenico payment
terminal. Figure~\ref{hsm_fig_ingenico_potted} shows the module we analyzed and two images exported from the resulting
CT scan data. Figure~\ref{hsm_fig_ingenico_potted_ct_cut} shows a horizontal cut across part of the module. In this cut,
we can clearly identify a mesh layer with multiple traces, four solid metal contacts riveted to the mesh foil, and two
unused contact pads and mesh traces in the lower part of the picture. An attacker would be able to use this information
to target the metal contacts with a tool like a needle probe. From the CT scan we were able to measure that the mesh of
the device has a pitch of \qty{1.0}{\milli\meter}. Thus, even inserting a thin needle probe right through one of the
mesh's traces should be possible without breaking the trace.
Hardware manufacturers implementing security meshes often attempt to keep the meshes' layouts hidden as a way of
security by obscurity. In practice, this can take the form of opaque potting compounds (cf.
Figure~\ref{hsm_fig_ingenico_potted_seated}), opaque cover layers (cf. Figure~\ref{hsm_fig_materials_gold_lds}), and
burying the mesh beneath other features such as PCB ground planes (cf. Figure~\ref{hsm_fig_3d_sandwich_lid}).
\todo{Pictures/refs of opaque materials, mention sample numbers}
To circumvent such attempts, an obvious attack vector is to use radiographical imaging techniques such as X-ray or CT
imaging. To evaluate CT imaging as an attack method, we experimentally imaged the potted HSM module of an Ingenico
payment terminal using an industrial CT. Figure~\ref{hsm_fig_ingenico_potted} shows the module we analyzed and two
images exported from the resulting CT scan data. Figure~\ref{hsm_fig_ingenico_potted_ct_cut} shows a horizontal cut
across part of the module. In this cut, we can clearly identify a mesh layer with multiple traces, four solid metal
contacts crimped to the mesh foil, and two unused contact pads and mesh traces in the lower part of the picture. An
attacker would be able to use this information to target the metal contacts with a tool like a needle probe. From the CT
scan we were able to measure that the mesh of the device has a pitch of \qty{1.0}{\milli\meter}. Thus, even inserting a
thin needle probe right through one of the mesh's traces should be possible without breaking the trace.
Figure~\ref{hsm_fig_ingenico_potted_ct_3d} shows a 3D reconstruction of the mesh's conductor layout. While the
reconstruction is slightly noisy due to the limited scan time available, it contains ample detail to reconstruct the
@ -904,8 +916,8 @@ Concluding both our patent research and our experimental survey, we find that ta
commonplace technology throughout the past 150 years. While mesh manufacturing technology has experienced some
advancements from historical wire-wound meshes to modern meshes always being constructed in printed circuit processes,
mesh monitoring approaches have received surprisingly little attention through the centuries and even in recent,
state-of-the-art systems, a simple comparator monitoring a mesh arranged in a wheatstone bridge configuration is still
considered sufficient by manufacturers.
state-of-the-art systems, a simple comparator monitoring a mesh arranged in a bridge configuration is still considered
sufficient by manufacturers.
% FIXME todo above: show wheatstone bridge schematic
\subsection{Mesh construction techniques}
@ -914,14 +926,16 @@ We found that in almost all cases, practical tamper sensing meshes are construct
processes. In some card payment terminals, we found meshes that used slightly customized standard processes and e.g.
integrated a mesh layer produced in a carbon printing process into a membrane keypad, but customizations were minimal.
We only found one mesh manufactured in a bespoke process in the datacenter HSM appliance we examined, and that bespoke
process turns out to be a turnkey solution used by at least two HSM vendors.
process turns out to be a turnkey solution used by at least two HSM vendors. Underscoring stagnating development in the
field, this particular mesh manufacturing process seems to have seen only minimal changes since the first patents
covering it were published in the late 1990ies.\todo{source}
\subsection{Mesh monitoring circuits}
We observed that in general, academic research leads before patent literature, which is ahead of actual implementations
in the field. Practical monitoring circuitry seems basic. Particularly the datacenter HSM appliance we examined showed a
contrast between a mesh manufactured in a bespoke process combined with a unsophisticated, discrete monitoring circuit
based around a number of voltage comparators.
contrast between a mesh manufactured in a bespoke process combined with an unsophisticated, discrete monitoring circuit
based around a number of voltage comparators.\todo{refer sample number}
\subsection{Computed Tomography Imaging}
@ -975,14 +989,14 @@ large-area photodiode coupled to a scintillator crystal converting X-ray photons
The widespread use of inexpensive but low-security commodity processes shows that in practical applications, cost is
often prioritized over security. The IHSM approach naturally complements such a system that uses a low-security mesh
material, increasing its security without the use of a more advanced mesh material. The beneficial construction
material and increases its security without needing a more advanced mesh material. The beneficial construction
techniques that we identified above such as the use of multiple, spaced layers and low-contrast trace materials
complement IHSM technology naturally. The three-dimensional layout of a mesh becomes easier in an IHSM implementation
since features like corners between mesh panels or gaps between mesh layers are often naturally protected by the mesh's
since features like corners between mesh panels or gaps between mesh layers in most layouts are protected by the mesh's
motion. An unintended advantage that results in IHSM implementations over conventional meshes is that they would provide
a level of intrinsic resistance to X-ray and CT imaging. In contrast to optical cameras in the visible spectrum, X-ray
image sensors need integration times in the hundreds of milliseconds or longer, which makes them unsuitable to image a
quickly moving targets.
quickly moving target.
\section{Conclusion}
@ -1001,13 +1015,17 @@ can even be forcibly separated from some potting compounds without destroying th
The weakest systems we found completely omitted a tamper sensing mesh. Ironically, all of these systems were devices
marketed as hardware security modules. Given the inexpensive nature of tamper sensing meshes and the high price point of
such devices, we suspect market segmentation as a driving force behind their manufacturers' decision to omit tamper
sensing meshes. We conclude from this observation that the term ``HSM'' does not imply state-of-the-art physical tamper
sensing.
sensing meshes despite their low cost. The primary security standard that is most often cited for the certification of
HSMs is the US government's FIPS-140\todo{cite}, now in its third version. A peculiarity of this standard is that it
only requires active tamper sensing meshes in the highest of the four security levels it defeies. Overall, we can
conclude that the term ``HSM'' does not imply state-of-the-art physical tamper sensing.
From an academic point of view, the core finding of our survey is that tamper sensing meshes manufactured in a number of
commercial manufacturing processes would yield acceptable surrogates for real devices found in the wild. With the
exception of a single device that used a particularly fine structure size in the \qty{100}{\micro\meter} range, none of
the devices we examined utilized particularly non-obvious construction techniques.
From an academic point of view, the core finding of our survey is that for academic research on mesh manufacturing,
monitoring or attacks on meshese, realistic tamper sensing mesh samples can easily be created. A number of commercial
manufacturing processes would yield acceptable standins for real devices found in the wild. With the exception of a
single device that used a particularly fine structure size in the \qty{100}{\micro\meter} range approaching the limit of
inexpensive PCB manufacturing processes, none of the devices we examined utilized particularly non-obvious construction
techniques.
Form an engineering point of view, we observe that across application domains, tamper sensing meshes often use basic
construction techniques. Implementing such a system that matches the security of other systems seen in the wild should