phd-thesis/hsm-terminology-notes.tex

\chapter*{A Note on Hardware Security Module Terminology}
\adjustmtc
\addcontentsline{toc}{chapter}{A Note on Hardware Security Module Terminology}

In this thesis, we use the term \emph{Hardware Security Module (HSM)} to refer to a security device that has the
following three properties.

\begin{enumerate}
\item A HSM targets the prevention of any conceivable physical attack. In particular, this includes intrusion attempts
        such as careful drilling or cutting into the device from any direction.
\item A HSM includes tamper sensors that when triggered result in an active tamper response, usually deleting all
        cryptographic secrets and rendering the device inoperable.
\item A HSM's tamper sensing and response subsystem is continuously powered from a backup power supply, usually a
        battery. Loss of power triggers the tamper response.
\end{enumerate}

This use of the term \emph{HSM} aligns with common usage of the term both in the academic literature and in everyday
conversation. Particularly the requirement of active tamper detection and response is crucial to distinguish a HSM from
simpler devices such as TPMs, smart cards or secure enclaves in SoCs. Note that our use of the term HSM is slightly
different from its use in government standards, from its use in the PCI SSC (Payment Card Industry Security Standards
Council) standards, and from its industry use.

In industry, the term HSM is often used for solutions that are only logically segregated and that do not include any
particular defense against hardware attacks. Our conjecture is that this is a consequence of the standardization
landscape, where for applications outside of card payment processing the US FIPS
140-22~\cite{usnationalinstituteofstandardsandtechnologySecurityRequirementsCryptographic2002} standard was central to
the industry. Despite encompassing both devices that include active tamper detection and response, FIPS 140-2 did not
draw a distinction in its terminology between the two classes.

\section{Use in government standards}

Under the still widely used US national standard FIPS 140 in in its 2002 version
2~\cite{usnationalinstituteofstandardsandtechnologySecurityRequirementsCryptographic2002}, a HSM would be called a
\emph{Multiple-Chip Cryptographic Module} that conforms to the standard's \emph{Security Level} 4 out of 4. Interesting
to note are that only level 4 requires any active tamper detection and response, so devices compliant only up to levels
3 and below do not align with our HSM definition. Futher of note is that according to the standard, a single-chip
solution does not require any tamper detection and response either to meet the standard's security level 4, which is in
misalignment with our definition. The standard's 2019 updated version FIPS
140-3~\cite{usnationalinstituteofstandardsandtechnologySecurityRequirementsCryptographic2019} defers to the
international standards ISO/IEC 19790 and 24759.

ISO/IEC 19790~\cite{ISOIEC19790} and ISO/IEC 24759~\cite{ISOIEC24759} call what we call a HSM a \emph{Hardware
Cryptographic Module} corresponding with the standards \emph{Security Level 4}. However, these standards only require
active tamper detection and response when cryptographic secrets are transmitted in plaintext between chips.

\section{Use in card payment processing (PCI SSC) standards}

The Payment Card Industry Security Standards Council (PCI SSC) is an association of credit card network operators that
defines standards for all layers of card payment processing, from card payment terminals in stores to the handling of
payment data in online shop backend systems.

PCI SSC terminology aligns with our definition and with common everyday use of the term HSM. In PCI SSC terminology, a
HSM is a crytographic device that has active tamper detecion and response circuitry. However, PCI SSC terminology
differs from our use of the term HSM in one nuance: In PCI SSC terminology, a HSM is specifically a datacenter device
used for backend processing of payment data. The general class of ``hardware devices performing some security function
with or without particular physical security requirements'' that ISO/IEC 19790 and other standards call a \emph{Hardware
Cryptographic Module}, in PCI SSC terminology is termed \emph{Secure Cryptographic Device (SCD)} in more recent standard
versions, which was updated from the previous term \emph{Tamper-Resistant Security Module (TRSM)}. Other than HSMs, PCI
SSC includes smartcards and card payment terminals in this category. Card payment terminals, referred to as
\emph{Pin-Entry Device (PED)} in PCI SSC standards, have to include a surprising amount of active tamper detection and
response functionality including partial coverage of areas like their main cryptographic processor and smart card reader
by battery-backed tamper-sensing meshes. Under our definition, these devices can be classified as a type of HSM. 

\section*{Tamper-Sensing Meshes}
\addcontentsline{toc}{subsection}{Tamper-Sensing Meshes}

In this thesis, we use the terms \emph{Tamper-Sensing Mesh} and \emph{Security Mesh} synonymous. We use both terms to
refer to any electrical circuit whose path is laid out to cover a surface with the intent of detecting attempts at
drilling, cutting or otherwise manipulating this surface. While the term \emph{Security Mesh} is more concise, it is
less clear to people unfamiliar with the matter. It is also polysemous, and depending on context can also refer to woven
or stamped metal meshes used as fences or as screens in front of windows to prevent break-ins. As a result, it is harder
to use in online searches, and when using Large Language Models (LLMs), it frequently leads to amusing hallucinations.

% FIXME note leo: Das ganze wirkt wie ein guter baustein für eine Einleitung. Für einen Terminologie übersicht ist es
% ansonsten auch eigentlich zu lang.
% Splitte das vielleicht auf, ein paar mehr details in den Abstract um die HSM definition etwas zu präzisieren, den rest
% in die Intro?