
\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}
\label{chapter-intro}

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 a 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,
    rogawayMoralCharacterCryptographic2015,
}.

While at a glance it might sound like a fringe position held by people from the Cypherpunk and Hacker movements~\cite{
    andersonCypherpunkEthicsRadical2022,
    hughesCypherpunksManifesto,
    jarvisCryptoWarsFight2020,
    marlinspikeWeShouldAll2013},
it enjoys support far beyond those circles and throughout mainstream academic cryptography. From cryptographic protocol
standards like TLS, to cryptographic applications like the Signal messenger, backdoor access is not only excluded from
the system design, its possibility is considered a vulnerability.
% Measures such as forward secrecy and post-compromise security are taken to mitigate its impact. 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.

In this thesis, we aim 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 used in practice, 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).

While anarchists, Cypherpunks and Hackers often reject backdoor access out of political conviction alone,
Cryptographers' aversion to backdoor access derives from a combination of two fundamental computing principles:
Kerckhoffs' principle, and the principle of least authority. Kerckhoffs' principle\footnote{
    \textcite{petitcolasKerckhoffsPrinciplesCryptographie} contains a high-quality OCR'ed copy of the original source,
    as well as a translation of the cited part from French. The original source is
    \textcite{kerckhoffsCryptographieMilitaire1883}.
}, 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 costly to cryptoanalyze a system that the attempt becomes unattractive. The reliance of
contemporary hardware security measures such as the majority of Physically Unclonable Functions (PUFs) on chip-scale
integration as their main barrier against manipulation is an instance where Kerckhoffs' principle is violated.

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. Existing HSMs are an example of a violation of the principle of least authority
since they elevate the HSM manufacturer to a single point of failure. Since the tamper sensing mesh foils used in
conventional HSMs are made in proprietary, bespoke processes, they cannot be manufactured independently.

\section{Research Questions and Contributions}

Based on the current state of the field of hardware security, we deduce three overarching research questions for this
thesis 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 create the support components necessary to integrate a system that provides a practical security
        guarantee?
\end{enumerate}

% FIXME: quote from anderson: Security economics remains a big soft spot, with security chips being in many
% ways a market for lemons. A banker buying HSMs probably won’t be aware of
% the huge gap between FIPS level 3 and level 4, and understand that level 3 can
% sometimes be defeated with a Swiss army knife. The buying incentive there is
% compliance, and where real security clashes with operations it’s not surprising
% to see weaker standards designed to make compliance easier. API security is
% too hard, and the difference between HSMs’ internal and external APIs makes
% it too confusing. The near-abdication of FIPS in favour of ISO 19790 and vari-
% ous protection profiles touted under the Common Criteria will confuse things
% further, as will the UK’s move away from the Criteria. Confusion marketing
% and liability games appear set to continue. But does this matter?
% First, most of the HSM business is moving to the cloud, with Azure and AWS
% each having of the order of 2,000 HSMs, and Google playing catchup. Instead of
% having a few thousand banks each running a few, or a few dozen, HSMs we’ll
% have three companies running a few thousand. As the prices are driven down,
% the HSM vendor engineers’ expertise will be lost; and as the cloud service
% providers guard their datacentres, HSMs are likely to be replaced by crypto
% chips.

To answer 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 answer 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 answer 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}

%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 deployments 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 Hardware Security Modules}

In this thesis, we propose Inertial Hardware Security Modules (IHSMs) to fill the gap of protecting systems that handle
highly sensitive data but that cannot use conventional HSMs for cost or performance reasons. 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 tamper sensors connected to a fast self-destruct mechanism. 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. 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. 

IHSMs 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 its path of motion.

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

IHSMs are the first fully open source HSM with advanced tamper sensing features. Across application domains, IHSMs 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, IHSMs at their core are just an enclosure that the user can put whatever hardware they need
into, adapting the tamper response to their application's needs. 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 conventional 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 aim to provide this missing building block to provide high-level
hardware security in real-world applications. Our hardware designs can be adapted to 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.

