
\chapterquote{Meredith Whittaker~\cite{greenbergSignalMoreEncrypted2024}}{
    It’s not for lack of ideas or possibilities. It’s that we actually have to start taking seriously the shifts that
    are going to be required to do this thing—to build tech that rejects surveillance and centralized control—whose
    necessity is now obvious to everyone.
}

\chaptertitle{Introduction}

All Cops Are Bastards, or ACAB is a slogan popular in far left and anarchist circles since the mid-twentieth century
that expresses a rejection of state authority~\cite{constantinouAppliedResearchPolicing2021}. While politically, this
blanket rejection is a fringe viewpoint with no mainstream acceptance, there exists an interesting parallel between this
and modern cryptographic best practice. In modern cryptography, it is generally seen as best practice to have the least
amount of keys possible involved in any computation, and cryptographers have time and time again strongly rejected
attempts by states and other authorities to insert backdoor access mechanisms into cryptographic systems~\cite{
    abelsonRisksKeyRecovery1997,
    abelsonKeysDoormats2015,
    andersonSecurityEngineeringGuide2020,
}.

The aversion of cryptographers against backdoor access shows up everywhere---from cryptographic protocol standards like
TLS, to cryptographic applications like the Signal messenger, not only is backdoor access excluded from the system
design, its possibility is considered a potential vulnerability and measures such as forward secrecy and post-compromise
security are taken to mitigate its impact when it is achieved through other means. In computing, this design aspect
makes cryptographic protocols a unique holdout. In other parts of the stack, explicit or implicit backdoor access is
commonplace, and attempts at preventing it are rare. For instance, network providers are generally required to comply
with so-called \emph{Lawful Interception} orders on particular customers or traffic types, and datacenter operators
commonly provide hardware access to state authorities. The design decisions in cryptographic protocols generally hold,
and the gold standard for backdoor access to modern systems is either exploiting a \emph{zero-day} flaw that is not yet
publically known, or acquiring physical access to the target system.

\section{Research Questions}

In this thesis, we wish to extend the level of protection afforded by cryptographic protocol design down the technology
stack. While cryptographic protocols and modern software from the operating system up make it possible to secure the
software side of the stack to a high level, the hardware side remains poorly protected. There are a variety of hardware
security solutions in the wild, but the majority of them either do not target protection against local, physical attacks
-- such as Trusted Platform Modules (TPMs) -- or are not widely available due to market segmentation or cost -- such as
conventional Hardware Security Modules (HSMs).

We approach this task by solving three research questions that progress from theory to practical deployment.

\begin{enumerate}
    \item Can we achieve physical security without relying on conventional tamper-sensing meshes?
    \item Can we monitor tamper-sensing meshes at a higher detail level than the state of the art of a single, scalar
        measurement?
    \item Can we integrate our findings into a system that provides a useful security guarantee in practice?
\end{enumerate}

To solve our first research question, we propose the Inertial Hardware Security Module (IHSM), a new type of HSM that
extends the high level of protection offered by the modern cryptographic software stack down to the hardware level,
enabling secure computation in insecure places.

To solve our second question, we propose improvements to the state of the art in HSM tamper sensors such as the use of
low-cost, embeddable Time-Domain Reflectometry (TDR) that not only improve the security of IHSMs, but that can even be
applied to conventional HSMs.

Finally, we solve our last research question by showing in two case studies how an end-to-end design of an IHSM-secured
data processing system could look like. Both case studies concern scenarios that IHSMs unlock that were previously
infeasible using conventional HSMs: Datacenter-scale Secure Multiparty Computation (SMPC) and long-range Quantum Key
Distribution (QKD) networks. As part of this effort we provide a solution adapting and improving upon the state of the
art in wireless power transfer to supply a rotating inertial HSM with a clean, stable power supply.

We chose to publish all of our research as open source and unencumbered by patents to enable widespread adoption. IHSMs
can be custom built with only basic manufacturing capabilities at small scale and enable the deployment of secure
computation in insecure places even to small organizations such as university research departments, NGOs and small
businesses.

\section{Cryptographic Principles and Physical Reality}

Cryptographers' aversion to backdoor access derives from a combination of two fundamental computing principles:
Kerckhoffs' principle, and the principle of least authority. Kerckhoffs' principle, named after Dutch military
cryptographer Auguste Kerckhoffs, expresses that the security of a cryptographic system should only depend on the
secrecy of its keys, not on the secrecy of its design. In this way, Kerckhoff's principle states the opposite of the
widespread industry practice of \emph{Security by Obscurity}, which aims to achieve security by making it sufficiently
annoying to cryptoanalyze a system that nobody bothers. Complementary to Kerckhoff's principle is the principle of least
authority, which describes that in a secure system each component should only have access to the smallest set of
capabilities necessary to fulfill its purpose. Applying both to a cryptographic system means that the system's design
should be transparent and not include any hidden components or opaque parts that cannot be inspected, and that the
system's keys should be scoped to place the least amount of trust possible in each participating party.

Let's take a basic videoconferencing system as an example. In our example system's deployment, users log on to a central
conference server, which receives and distributes the users' video streams. Allowing backdoor access to the video
streams to some third party like a datacenter operator or a state would violate Kerckhoffs' principle since it would
have to be hidden from the systems' participants, who would therefore not have a complete view of the systems' deployed
architecture. The principle of least authority would also be violated since in almost all cases, such a backdoor access
system would not see legitimate use. As a result, it would possess capabilities that almost never would be essential to
the proper function of the videoconference system.

In their design, almost all modern software -- especially open source -- cleanly applies these principles. However, the
practical reality after deployment almost always deviates from them. While backdoors are vanishingly rare in modern
open-source software, practical depoloyments usually are vulnerable to physical attacks. Computer hardware generally is
not designed with a local attacker with advanced physical attack capabilities in mind since no mitigation can fully
prevent them---such attacks usually can only be detected, or at best slowed down. As a result, commonplace attacks
against modern software often involve taking over the hardware at some point in the chain. Even End-to-End-Encrypted
(E2EE) communication systems can be compromised if one of the encrypted channel's endpoints can be physically
compromised. Corresponding \emph{digital forensics} capabilities are commonplace among state actors, and are available
as a turnkey solution on the market.

\section{Inertial HSMs}

In this thesis, we propose Inertial HSMs to fill this gap in the protection of systems that are not critical enough to
warrant the expensive existing solutions such as conventional HSMs, while still handling highly sensitive data. In a
system with a secure software stack, the role of a HSM is to secure the hardware part of the stack. The basic approach
of a HSM is to combine a secure software stack with a fast self-destruct mechanism and tamper sensors. The self-destruct
mechanism can be hardware or software that quickly and securely destroys all cryptographic secrets, thereby rendering
the device worthless to an attacker. The tamper sensors are tasked with detecting any physical attack an attacker could
mount on the device. Common classes of such sensors include environmental sensors such as temperature or radiation
sensors that detect attempts at causing controllable faults in the HSM by heating, cooling or irradiating it. Building
on the basic protection offered by such sensors, \emph{tamper-sensing meshes} are often employed. These \emph{meshes}
are flexible foils containing circuit traces that are attached to the HSM's enclosure to detect attempts at penetrating
the shell of the device with probes. Tamper-sensing meshes usually are the primary line of defense against most physical
attacks. They are very effective at mitigating a large variety of physical attacks, but they are difficult to construct
securely as they usually require bespoke manufacturing processes. As a result, they are currently only used in niche
applications, and even there not every realization is equally secure.

Inertial HSMs are a new design approach that utilizes mechanical motion to create secure tamper-sensing meshes from
simple components. IHSMs solve the issue of creating an impenetrable tamper-sensing envelope by replacing the bespoke
tamper-sensing mesh foil with a set of simple, rigid meshes made from commodity Printed Circuit Boards (PCBs) that are
rotating at high speed. In motion, these simple PCB tamper-sensing meshes are as secure as the much more sophisticated
bespoke foils used in conventional HSMs, yet they are simpler and less expensive to manufacture. To verify that the mesh
is rotating correctly, an accelerometer is placed on the rotating mesh, and its centrifugal force reading is used to
validate itk path of motion.

IHSMs enable the protection of much larger payloads compared to conventional mesh designs, and they can support larger
power dissipation. This and their low cost enables the implementation of high-level hardware security in applications
that previously would not have been possible to secure.

Inertial HSMs are the first fully open source HSM with advanced tamper sensing features. Across application domains,
Inertial HSMs can be applied to gain resistance to physical attacks in scenarios where conventional HSMs were not used
because of cost, computing power or implementation effort. Where conventional HSMs come as fully integrated devices that
only expose limited APIs to their users, Inertial HSMs at their core are just an enclosure that the user can put
whatever hardware they need into. Since the simpler tamper-sensing mesh construction of IHSMs scales to larger payload
volumes, entire servers can be protected---something that is impossible with conventional HSMs. Since the mesh in an
IHSM is constantly moving, unlike a mesh in a convetional HSM, it does not have to entirely cover the payload. Instead,
it can have gaps that allow for air flow between outside and inside, enabling active cooling of the IHSM's payload. This
cooling capability sharply increases computing power by increasing feasible payload power dissipation by
two orders of magnitude.

\section{Conclusion}

Looking at the practice of applied hardware security, we observe that despite ample availability of commercial solutions
promising easy hardware security, clearly there is still a lack of solutions that provide the adaptability necessary for
some real use cases at low enough cost. By publishing the tamper-sensing technology we developed during the making of
this thesis as open source hardware designs, we wish to provide this missing building block to provide high-level
hardware security in real-world applications. Our hardware designs can be adapted to a devices ranging from Single-Board
Computers (SBCs) to servers, they are compatible with non-computing applications like Quantum Key Distribution (QKD) and
their design approaches can even be integrated into existing HSM designs to provide better security at little additional
cost.

