\chapterquote{Phillip Rogaway~\cite{rogawayMoralCharacterCryptographic2015}}{
    Cryptography rearranges power: it configures who can do what, from what. This makes cryptography an inherently
    political tool, and it confers on the field an intrinsically moral dimension.}
\chapter{Conclusion}

In this thesis, we provided an examination of the field of Hardware Security Modules both from an academic perspective
and with regards to their practical implementation. We answered our first research question introduced in
Chapter~\ref{chapter-intro} on the current state of the art in Chapters~\ref{chapter-epa} and \ref{chapter-survey},
providing a comprehensive view of practical implementations. Chapter~\ref{chapter-epa} motivates our research using the
German national digital health record system as an example that demonstrates the difficulties in achieving practical
hardware security. Besides some minor cryptographic oddities, our analysis reveals at least one essential specification
mistake that negates the hardware security of the system by unnecessarily introducing a poorly protected HSM. In
Chapter~\ref{chapter-survey}, we answer our second research question in a detailed survey of a wide range of devices
that utilize tamper-sensing meshes, distilling a set of criteria for the design of secure tamper-sensing meshes. In
Chapter~\ref{chapter-ihsm}, we propose Inertial Hardware Security Modules (IHSMs), a new approach to physical security
that combines conventional tamper-sensing meshes with physical movement. IHSMs enable bootstrapping a highly secure
system from low-security, off-the-shelf parts, thereby solving our third research question on achieving physical
security without bespoke components. We support the construction of concretely secure IHSMs by providing deep analyses
of two key engineering challenges in IHSM construction, mesh monitoring and power transfer. Solving our fourth research
question on mesh monitoring fidelity, we propose a low-cost TDR-based mesh monitoring system that exceeds the
capabilities of previous systems from academic or from patent literature. Our system is capable of monitoring large
meshes while simultaneously providing detailed results. Our TDR-based mesh monitoring system is of independent interest,
since it can also be integrated into traditional HSM designs. Solving our fifth research question on ripple reduction
for rotating Wireless Power Transfer for IHSMs, we propose a new, generalized design for high-frequency PCB inductors
with low parasitic capacitance. Beyond our IHSM application, our design provides better bandwidth and lower parasitic
capacitance compared to the state of the art without increasing implementation cost. We conclude this thesis with two
chapters elaborating on two new use cases that are made possible by IHSM technology due to its ability to protect large
payloads that have high power consumption. Together, these results answer our sixth and final research question.

The research presented in this thesis is aimed at advancing both academic research and applied engineering in hardware
security. We believe that by publishing our research including its artifacts under open source licenses, we provide the
basis for future research in tamper-sensing technology, a field that remains under-served in today's academic landscape.

Recent history has shown that state-level adversaries are a mounting threat to civil rights organizations, human rights
lawyers, members of minorities, and many others. While western democracies used to be considered safe havens of human
rights, today human rights are under attack both from within and from the outside in countries across the globe.
Publishing IHSM technology as open source, we hope to provide one building block for new computing systems accessible to
all that are resilient and secure in the face of growing adversity.

\section*{Outlook}

With the research contributions we presented in this thesis, we open up a new field of hardware security research
centered on Inertial HSMs and improvements to conventional tamper sensing meshes. Below, we will list some research
directions that we consider worthwhile for future investigation.

\begin{itemize}
\item Improving the resolution of the sampling mesh monitoring approach we presented in
    Chapter~\ref{chapter_sampling_mesh_mon}. Possible improvements include increasing pulse risetime through a discrete
    transistor amplifier circuit, as well as evaluating an FPGA as a replacement for the microcontroller to take
    advantage of the improved delay primitives offered by many FPGA families.
\item Characterizing the PUF-like effects we observed in Chapter~\ref{chapter_sampling_mesh_mon} in mesh coupons using
    our sampling mesh monitoring approach.
\item Integrating IHSM technology with a HSM firmware implementation into a small form factor to create a portable IHSM.
    A small form factor introduces new challenges besides the mere integration of the necessary circuitry and placement
    of the mesh. For instance, wireless power and data transfer would need to be integrated with the device without
    disrupting mesh monitoring. An on-axis solution would likely require magnetic shielding materials and possible
    non-magnetic ceramic bearings. Furthermore, integrating a sufficiently small motor and optimizing the design for
    long bearing life is challenging at the high rotation speed necessary at a small overall diameter. Finally, at high
    speeds, precisely balancing the whole assembly to avoid vibrations that could lead to early mechanical failure is
    difficult.
\item Tackling motor control algorithms for IHSMs and developing tamper sensors based on counter-electromotive force as
    a defense-in-depth measure.
\item Integrating the IHSM hardware concept with software research on secure enclave and cryptographic coprocessors.
\item Exploring IHSM applications beyond what we outlined in this thesis. For instance, one application of recent
    interests would be physically securing GPUs used for AI training. The background for such work could be either
    export control motivations, or a concern for security and privacy of user input, training data, or even trained
    weights.
\end{itemize}

We will proceed with future research into IHSM applications. We have published our results up to this point as open
source hardware and software, and we intend to build on these publications.