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diff --git a/doc/quick-tech-report/rotohsm_tech_report.tex b/doc/quick-tech-report/rotohsm_tech_report.tex new file mode 100644 index 0000000..41938f7 --- /dev/null +++ b/doc/quick-tech-report/rotohsm_tech_report.tex @@ -0,0 +1,459 @@ +\documentclass[12pt,a4paper]{article} +\usepackage[english]{babel} +\usepackage[utf8]{inputenc} +\usepackage[T1]{fontenc} +\usepackage[ + backend=biber, + style=numeric, + natbib=true, + url=false, + doi=true, + eprint=false + ]{biblatex} +\addbibresource{rotohsm.bib} +\usepackage{amssymb,amsmath} +\usepackage{listings} +\usepackage{eurosym} +\usepackage{wasysym} +\usepackage{amsthm} +\usepackage{tabularx} +\usepackage{multirow} +\usepackage{multicol} +\usepackage{tikz} +\usepackage{mathtools} +\DeclarePairedDelimiter{\ceil}{\lceil}{\rceil} +\DeclarePairedDelimiter{\paren}{(}{)} + +\usetikzlibrary{arrows} +\usetikzlibrary{chains} +\usetikzlibrary{backgrounds} +\usetikzlibrary{calc} +\usetikzlibrary{decorations.markings} +\usetikzlibrary{decorations.pathreplacing} +\usetikzlibrary{fit} +\usetikzlibrary{patterns} +\usetikzlibrary{positioning} +\usetikzlibrary{shapes} + +\usepackage[binary-units]{siunitx} +\DeclareSIUnit{\baud}{Bd} +\usepackage{hyperref} +\usepackage{tabularx} +\usepackage{commath} +\usepackage{graphicx,color} +\usepackage{ccicons} +\usepackage{subcaption} +\usepackage{float} +\usepackage{footmisc} +\usepackage{array} +\usepackage[underline=false]{pgf-umlsd} +\usetikzlibrary{calc} +%\usepackage[pdftex]{graphicx,color} +\usepackage{epstopdf} +\usepackage{pdfpages} +\usepackage{minted} % pygmentized source code + +\renewcommand{\floatpagefraction}{.8} +\newcommand{\degree}{\ensuremath{^\circ}} +\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}} + +\usepackage{fancyhdr} +\fancyhf{} +\fancyfoot[C]{\thepage} +\newcommand{\includenotebook}[2]{ + \fancyhead[C]{Included Jupyter notebook: #1} + \includepdf[pages=1, + pagecommand={\thispagestyle{fancy}\section{#1}\label{#2_notebook}} + ]{resources/#2.pdf} + \includepdf[pages=2-, + pagecommand={\thispagestyle{fancy}} + ]{resources/#2.pdf} +} + +\begin{document} + +\title{A High-Security Physical Security Primitive Based On Mechanical Movement} +\author{Jan Götte} +\date{2020-09-15} +\maketitle + +\section{Abstract} +In this paper, we introduce a novel, highly effective countermeasure against physical attacks: Inertial hardware +security modules. Whereas conventional technology can be categorized into systems monitoring a thin boundary (such as +security meshes) and systems monitoring the interior volume (such as the "enclosure PUF" of Tobisch et al.). What all of +these systems have in common is that they try to detect attacks by crafting sensors responding to increasingly minute +manipulations of the monitored medium. Our approach is novel in that we alleviate the sensitivity requirement of a +security mesh by increasing the complexity of any manipulation at all by orders of magnitude by fastly rotating the +security mesh--presenting a moving target to an attacker. Attempts to modify the rotation itself are easily monitored +with commercial MEMS accelerometers and gyroscopes. + +Our approach leads to a HSM that can easily be built from off-the-shelf parts by any university electronics lab, yet is +as secure or more secure than even the best commercial offerings. + +\section{Introduction} +Since the early days of computers, physical security has often been a core component of any computer system's security +architecture. Physical security in fact predates our modern concept of computer security by decades. Long before +passwords, access control lists, role-based authentication and other modern concepts of information security were +developed, information was secured by physically locking away the computers that held it. + +Nowadays, concerns of physical security are mostly limited to certain applications. Credit card processing and medical +data processing are two instances where a combination of smartcards and hardware security modules is used to provide a +higher level of security than what ordinary computers can provide. Meanwhile, in most commercial data processing +applications, the physical security provided by an average datacenter is considered to be appropriate. + +In modern systems, phyiscal security always is tightly interwoven with the system's overall security architecture. +Beyond the level provided by locks and guards, it is generally considered infeasible to physically secure all parts of a +computer. High-level physical security is usually limited to either a single chip or part of a chip such as a secure +element, enclave or smartcards--or it is limited to a small module acting within a very limited scope, as is the case in +commercial HSMs that largely act as cryptographic co-processors with built-in key management functions. + +\subsection{Technical approaches to physical security} +The use of chips as secure elements has recently become popular beyond the smartcards of yesteryear. Apple has carried +over a secure enclave IC from their line of phones into their line of laptops in 2016. Likewise, Google has developed +its own security IC for use in phones and laptops. An issue to consider with all such IC-based security solutions is +that they do not provide any cryptographic security. The real-world security of these solutions solely rests on the +assumption that due to their fine structure, ICs are hard to reverse engineer and manipulate. As of now, this property +holds and in the authors' opinion it will likely be a reasonable assumptions for some years to come. However, in its +essence this is a type of security by obscurity: Obscurity here mostly applying to the rarity of tools that are +necessary for practical attacks such as focused ion beam workstations and accompanying sample preparation equipment. An +important observation in this regard is that already, several people are slowly chipping away at this obscurity: A group +at Ruhr University Bochum is working on advanced tooling for netlist reverse engineering, and there are several +companies offering commercial IC reverse engineering services. + +\subsection{Hardware Security Modules} +At larger physical dimensions, hardware security modules (HSMs) provide an effective solution to the problem: In +conformity with Kerckhoff's principle, their creators do not try to hide the structure of the system within. Instead, +the HSM monitors it for any manipulation and wipes all key material when one is detected. The most common commercial +realization of this is what we call a "boundary-monitoring" HSM. This is a device uses a microcontroller monitoring the +conductivity of usually two electrical traces that are folded many times to cover the entire area of a plastic enclosure +part or a plastic foil wrapped around the module. The security problem thus gets transformed into a manufacturing +challenge: How fine can these traces be made--so they are disturbed by even the tiniest of holes for say, a fine needle; +and how sensitive can they be made to perturbations--so they break from even gentle attempts at mechanical, chemical or +other physical manipulation. + +The other type of HSM that so far has garnered mostly academic interest are what we call "volumetric" HSMs. Where a +boundary-monitoring HSM senses disturbations to a thin boundary between its inside and the outside world, a volumetric +HSM monitors its entire interior volume. Approaches that have been proposed so far include monitoring using +electromagnetic radiation % FIXME: citation (paper1 (this chip thing w/ distributed PAs/LNAs), paper2 (RUB) +and ultrasonic sensing. % FIXME: citation +Common to both approaches is that for technical reasons the wavelength of the employed radiation is in the range of +millimeters or larger. This implies that practical attacks acting on a smaller scale of physical size require sensitive +monitoring circuity to be reliably caught. % FIXME maybe talk to a physicist here. +Since they require advanced transceivers and signal processing, these HSMs incur a high implementation cost compared to +one based on a traditional security mesh, while they in turn promise to be easier and less expensive to scale in +physical size. A severe problem with any previous volumetric designs is that their security analysis is very hard. While +multiple designs have been proposed academically, none of these proposals include an analysis of their physical security +properties that goes beyond guesswork. %FIXME verify this. +The obvious reason for this is that to evaluate the volume inside the HSM that is covered by a given transceiver +combination and a given test signal pattern necessarily requires numerically solving the volumetric electromagnetic +field equations inside the HSM, applying a model of transmitter and receiver to the results that takes into account +receiver sensitivity and ADC resolution, transmitter power and receiver saturation effects and then validating that +every point in space (or at least inside a boundary region) is covered. While the guess that attacks are impractical +might still be true this would be based on the fact that the same problem presents itself to an attacker trying to +circumvent these measures--degrading their security to simple obscurity again. + +\subsection{Inertial HSMs: A new approach to physical security} +We are certain that there is still much work to be done and many insights to be gained from further explorations +of the two concepts described above. Trivially, consider a box with mirrored walls that, suspended on thin wires, +contains a smaller box that has cameras looking outward in all directions at the mirrored walls. Given that the defender +can control lighting conditions inside this kaleidoscopic box in this application modern cameras can be considered +equivalent to or better than the human eye. Thus, a successful physical attack on this system would likely an +"invisibility cloaks"--and the system would remain secure as long as no such thing exists. This example is a useful +point of reference. To be viable, a HSM technology must be either smaller or more sensitive than such a setup. + +The candidate we wish to introduce in this paper uses a novel approach to side-step the issues of both the concepts +introduced in the previous section and provides radically better security against physical attacks--both in theory and +in practice. + +Our core observation is that given any less expensive but more coarse HSM technology, we can make it radically more +difficult to attack by introducing fast mechanical motion. As a trivial example, consider a HSM as it is used in +ecommerce applications for credit card payments. Focusing on its main defense for simplicity, its physical security is +limited by the structure size of the mesh that is likely used in its shell. If an attacker can tap the mesh's electrical +traces and bridge across the mesh in a way the HSM cannot detect (e.g. by making sure the bridge has the same electrical +impedance as the mesh traces have e.g. by comparing against another device of the same type), they have circumvented the +device's protections. Any such attack would likely involve some fine drill bits, needles, wires, glue, perhaps solder or +even lasers. + +Now consider the same HSM, but this time mounted on a large flywheel. In this scenario the HSM uses the same +protections as before, but is now additionally equipped with an accelerometer that it uses to verify that it is in fact +rotating at a very high speed. How would an attacker approach this HSM? They would have to either slow down the rotation +(which would quickly be sensed by the accelerometer) or they would have to attack the moving HSM--the HSM literally +becomes a moving target. While rotating the entire attack workbench might be possible for slow speeds, rotating frames +of reference quickly become inhospitable to human life and at some point the technical means to rotate a CNC attack +robot probably weighing several kilograms become inconvenient as well. Contact-less EM or optical attacks are more +limited in the first place, and can effectively be shielded. + +\subsection{Contributions} +This work contains the following contributions: +\begin{enumerate} + \item Presentation of the \emph{Inertial HSM} concept, allowing cost-effective prototype and small-scale production + of highly secure HSMs. + \item Discussion of possible boundary sensing modes in the intertial HSM model. + \item Exploration of the design space of inertial HSMs. + % FIXME \item Presentation of a prototype inertial HSM. + % FIXME \item Measurement of the prototype HSM's susceptibility to various types of attack. +\end{enumerate} + +\section{Related work} +% summaries of research papers on HSMs. +% I have not found any actual prior art on anything involving mechanical motion beyond ultrasound. +In chapter 18 of the forthcoming 3rd edition of his seminal book on "Security Engineering"\cite{anderson2020}, Ross +Anderson gives a background on physical security in general and on HSMs in particular. As an example he cites the IBM +4758 HSM whose details are laid out in depth in \cite{smith1998}. This HSM is an example of an industry-standard +construction. Though it is now a bit dated, the construction techniques of the physical security mechanisms have not +changed much in the last two decades. Apart from some auxiliary temperature and radiation sensors to guard against +attacks on the built-in SRAM memory the module's main security barrier uses the traditional construction of a flexible +mesh wrapped around the module's core. In \cite{smith1998}, the authors claim the module monitors this mesh for +short circuits, open circuits and conductivity. The fundamental approach to tamper detection and construction is similar +to other commercial offerings\cite{obermaier2018}. + +In \cite{immler2019}, Immler et al. describe a HSM based on precise capacitance measurements of a mesh. In contrast to +traditional meshes, the mesh they use consists of a large number of individual traces (more than 32 in their example). +Their concept promises a very high degree of protection. The main disadvantages of their concept are a limitation in +both covered area and component height, as well as the high cost of the advanced analog circuitry required for +monitoring. A core component of their design is that they propose its use as a PUF to allow for protection even when +powered off, similar to a smart card--but the design is not limited to this use. + +In \cite{tobisch2020}, Tobisch et al.\ describe a construction technique for a hardware security module that is based +around commodity Wifi hardware inside a conductive enclosure. In their design, an RF transmitter transmits a reference +signal into the RF cavity formed by the conductive enclosure. One or more receivers listen for the signal's reflections +and use them to characterize the RF cavity w.r.t.\ phase and frequency response. Their fundamental assumption is that +the RF behavior of the cavity is inscrutable from the outside, and that even a small disturbance anywhere within the +volume of the cavity will cause a significant change in its RF response. The core idea in \cite{tobisch2020} is to use +commodity Wifi hardware to reduce the cost of the HSM's sensing circuitry. The resulting system is likely both much +cheaper and capable of protecting a much larger security envelope than e.g. the design from \cite{immler2019}, at the +cost of worse and less predictable security guarantees. + +While \cite{tobisch2020} approach the sensing frontend cost as their only optimization target, the prior work of Kreft +and Adi\cite{kreft2012} considers sensing quality. Their target is an HSM that envelopes a volume barely larger than a +single chip. They theorize how an array of distributed RF transceivers can measure the physical properties of a potting +compound that has been loaded with RF-reflective grains. In their concept, the RF response characterized by these +transceivers is shaped by the precise three-dimensional distribution of RF-reflective grains within the potting +compound. + +\subsection{Comparison to prior research} + +Our concept is truly novel in that neither academic literature, nor patent databases contain any mention of mechanical +motion being used as part of a hardware security module. Most academic research concentrates on the issue of creating +new, more sensitive security barriers for HSMs while commercial vendors concentrate on means to cheaply manufacture +these security barriers. Our concept instead focuses on the issue of taking any existing, cheap low-performance security +barrier and transforming it into a marginally more expensive but very high-performance one. The closes to a mechanical +HSM that we were able to find during our research is an 1988 patent\cite{rahman1988} that describes an mechanism to +detect tampering along a communication cable by enclosing the cable inside a conduit filled with pressurized gas. + +\section{Intertial HSM construction and operation} +\subsection{Using motion for tamper detection} +Mechanical motion has been proposed as a means of making things harder to see with the human eye\cite{haines2006} but we +seem to be the first to use it in tamper detection. Let us think about how one would go about increasing the security of +a primitive tamper detection sensor. + +\begin{enumerate} + \item We need the sensor's motion to be fairly fast. If any point of the sensor moves slow enough for a human to + follow, it becomes a weak spot. + \item We need the sensor's motion to be periodic to keep it within a reasonable space. Otherwise we could just load + our HSM on an airplane and assume that airplanes are hard to stop non-destructively mid-flight. + \item We need the sensor's motion to be very predictable so that we can detect an attacker trying to stop it. +\end{enumerate} + +From this, we can make a few observations. + +\begin{enumerate} + \item Linear motion is likely to be a poor choice since it requires a large amount of space, and it is comparatively + easy to follow something moving linearly. + \item Oscillatory motion such as linear vibration or a pendulum motion might be a good candidate, but for the + instant at its apex when the vibration reverses direction the object is stationary, which is a weak spot. + \item Rotation is a very good choice. Not only does it not require much space to execute, but also if the axis of + rotation is within the HSM itself, an attacker trying to follow the motion would have to rotate around the same + axis. Since their tangential linear velocity would rise linearly with the radius from the axis of rotation, an + assumption on tolerable centrifugal force allows one to limit the approximate maximum size and mass of an + attacker. For an HSM measuring at most a few tens of centimeters across, it is easy to build something that + rotates too fast for a human to be able to follow it. The axis of rotation is a weak spot, but this can be + alleviated by placing additional internal sensors around it and locating all sensitive parts of the sensing + circuit radially away from it. +\end{enumerate} + +Another important observation is that we do not have to move the entire contents of the HSM. It suffices if we can +somehow move the tamper detection barrier around these contents while keeping the contents stationary. This reduces the +inertial mass of the moving part and eases data communication and power supply of the payload. + +In a rotating reference frame, at any point the centrifugal force is proportional to the square of the angular frequency +and linearly proportional to the distance from the axis of rotation. We can exploit this fact to create a sensor that +detects any disturbance of the rotation by simply placing a linear accelerometer at some distance to the axis of +rotation. During constant rotation, the linear acceleration tangential to the rotation will be zero. The centrifugal +force is orthogonal to this, and will be constant as long as the angular velocity remains constant (assuming a fixed +axis of rotation). At high angular velocities, considerable forces can be created this way. This poses the engineering +challenge of preventing the whole thing from flying apart, but also creates an obstacle to any attacker trying to +manipulate the sensor. + +\subsection{Payload mounting mechanisms} +The simplest way to mount a stationary payload in a rotating security mesh is to drive the rotor through a +hollow axis. This allows the payload to be mounted on a fixed rod threaded through the hollow axis, along with wires for +power and data. + +\subsection{Rotating mesh power supply} +There are several options to transfer power to the rotor from its stationary frame. + +\begin{enumerate} + \item Slip ring contacts are a poor candidate as they are limited in their maximum speed and lifetime, and as + precision mechanical components are expensive. + \item Inductive power transfer as used in inductive charging systems can be used without modification. + \item A second brushless motor on the axis of rotation can be used as a generator, with its axis connected to the + fixed frame and its stator mounted and connected to the rotor. + \item A bright LED along with some small solar cells may be a practical approach for small amounts of energy. + \item For a very low-power security mesh, a battery specified to last for the lifetime of the device may be + practical. +\end{enumerate} + +\subsection{Rotating mesh data communication} +As we discussed above, while slip rings are the obvious choice to couple electrical signals through a rotating joint, +they are likely to be too expensive and have too short a life span for our application. Since the only information that +needs to pass between payload and rotor are the occassional status report and a high-frequency heartbeat signal that +acts as the alarm trigger, a simple optocoupler close to the axis of rotation is a good solution. + +\section{Future work} +With this paper, we intend to spread the word on our idea. Thus, below we include a selection of the open questions we +are currently working on. If you wish to tackle some of these, please feel free to contact the authors. + +\subsection{Other modes of movement} +Though we decided to use rotation as an easy-to-implement yet secure option, other modes of movement bear promise as +well. Particularly for less high-security applications without strict space constraints, a variant based on a pendulum +motion may be worth investigating as it would simplify the mechanical construction. Power and data transfer to the +moving part could simply be done with very flexible cables. + +\subsection{Multiple axes of rotation} +One option to alleviate the weak spot a rotating mesh has at its axis of rotation, a system with two or more axes of +rotation could be used. A single mesh would still suffice in this case, but when evaluating accelerometer readings, the +braking detection algorithm would have to superimpose both. + +\subsection{Means of power transmission} +Power transmission from payload to rotor is another point worth investigating. It may be possible to use some statically +mounted permanent magnets with a coil integrated into the rotor's PCB as a low-power generator. While likely +inefficient, this setup would be low-cost and would still suffice for the meager power requirements of the rotor's +monitoring circuitry. + +\subsection{Payload cooling} +An issue with existing HSM concepts is that the mesh has to fully envelope the payload, and thus traditional air cooling +or heat pipes cannot be used. Existing systems rely on heat conduction through the mesh alone for cooling, severly +limiting the maximum power dissipation of the payload. In our rotating HSM concept, the rotating mesh can have radial +gaps in the mesh without impeding its function. This allows air to pass through the mesh during rotation, and a future +evolution of the concept could even integrate a fan into the rotating component. This greatly increases the maximum +possible power dissipation of the payload, allowing for much more powerful processing. + +\subsection{Other sensing modes} +Since the security requirement the primary tamper-detection barrier needs to measure up to are much more lenient in the +rotating HSM concept than in traditional HSMs, other coarse sensing modes besides low-tech meshes may be attractive. One +possibility that would also eliminate the need of any active circuitry on the rotor would be to print the inside of the +rotor with a pattern, then have a linear array of reflective optical sensors located close to the rotor along a +longitudinal line. These sensors would observe the printed pattern passing by at high speed, and could compare their +measurements against a model of the rotor. Tampering by drilling holes or slots would show up as adding an offset to +part or all of the pattern. Likewise, the speed of rotation can be deducted directly from a sequence of measurements. + +\subsection{Longevity} +A core issue with a mechanical HSM is component longevity. Save for dust and debris clogging up the system's mechanics +the primary failure point are the bearings. A good partner for further development or even commercialization might be a +manufacturer of industrial ducted fans as they are used e.g.\ in servers for cooling. Small industrial fans usually use +BLDC motors and bearings specially optimized for longevity. + +\subsection{Transportation of an active device} +A rotating mass responds to torque not co-linear with its axis of rotation with a gyroscopic precession force. In +practice, this means that moving a device containing a spun-up rotating HSM on its inside might induce significant +forces on both the HSM (posing the danger of false alarms) and on the carrier of the device (potentially making handling +challenging). This effect would have to be taken into account in a real-world deployment, especially if the finished +device is to be shipped by post or courier services after spin-up. + +\subsection{Hardware prototype} +We are currently working on a hardware prototype that demonstrates the fundamental components of our concept. The +prototype will be based on a security mesh made with a commercial printed circuit board manufacturing process. In our +prototype we intend to use two commercially available hollow-shaft brushless DC (BLDC) motors originally intended for +quadcopter-mounted camera gimbals, one for driving and one for power transfer. The prototype will have a usable internal +volume sufficient to house a small form factor PC ($\approx\SI{2}{\liter}$). + +\section{Attacks} +\subsection{Attacks on the mesh} +There are two locations where one can attack a tamper-detection mesh. Either, the mesh itself can be tampered with. This +includes bridging its traces to allow for a hole to be cut. The other option is to tamper with the monitoring circuit +itself, to prevent a damaged mesh from triggering an alarm and causing the HSM to erase its contents. Attacks in both +locations are electronic attacks, i.e. they require electrical contact to parts of the circuit. Traditionally, this +contact is made by soldering, or by placing a probe such as a thin needle. Any kind of electrical contact that does not +involve an electron or ion beam or a liquid requires mechanical contact. We consider none of these forms feasible to be +performed on an object rotating at high speed without a complex setup that rotates along with the object. Thus, we +consider them to be practically infeasible outside of a well-funded, special-purpose laboratory. + +\subsection{Attacks on the alarm circuitry} +An electronic attack could also target the alarm circuitry inside the stationary payload, or the communication link +between rotor and payload. The link can easily be proofed by using a cryptographically secured protocol along with a +high-frequency heartbeat message. The alarm circuitry has to be designed such that it is entirely contained within the +HSM's security envelope and has to tolerate environmental attacks such as through temperature, ionizing radiation, +lasers, supply voltage variations, ultrasound or other vibration and gases or liquids. The easiest way to proof an alarm +system against these is to employ adequate filtering of the incoming power supply and use sensors for the others, +triggering an alarm in case extraordinary environmental variations are detected. + +\subsection{Fast and violent attacks} +A variation of the above attacks on the alarm circuitry would be an attack that attempts to simply destroy this +circuitry before the alarm can be acted upon. This type of attack might involve things such as a large hammer, or a gun. +Mitigations for this type of attack include putting the entire payload and monitoring circuit in a mechanically robust +enclosure and potting them, and linking all components of the alarm chain in such a way cryptographically and on a +protocol level that the destruction of any of its parts leads to the secrets being destroyed before an attack would be +able to probe them. An implication of this is that the electrical realization of the alarm signal up to its eventual +destination cannot be a simple active-high or active-low line, since neither can be considered fail-safe in this +scenario. + +\subsection{Attacks on the rotation sensor} +An attacker trying to stop the rotor to tamper with the mesh may first try to deceive the rotation monitoring circuit +such that it misses the rotor being stopped. In a realization based on a commercial MEMS accelerometer, this attack +could take two forms: An electronic attack on the MEMS sensor, the monitoring microcontroller or the link in between, +and a physical attack on the MEMS sensor itself. The former would be no easier than an electronic attack that attempts +to bridge the mesh traces at the monitoring microcontroller. Thus, we consider it not to be practically feasible outside +of a laboratory built especially for this purpose. + +There are several options for the latter attack. A recent paper %FIXME +has shown that accelerometers respond to certain ultrasonic stimuli with bogus measurements. Since this primitive does +not, however, yield accurate control over these bogus measurements, we deem it to be impractical for our scenario. +Another possible attack scenario would be to somehow stop the rotating motion while subjecting the HSM to an external +linear motion. Given the low error margins in the measurements of commercial accelerometers we consider this attack +infeasible. A last type of attack might be to try to physically tamper with the accelerometer's sensing mechanism. MEMS +accelerometers usually use a simple cantilever design, where a proof mass moves a cantilever whose precise position can +be measured electronically. A possible way to attack such a device might be to first decapsulate it using laser ablation +synchronized with the device's rotation. Then, a fast-setting glue such as a cyanoacrylate could be deposited on the +moving MEMS parts in either liquid or gaseous form, locking them in place after hardening. This attack would require +direct access to the accelerometer from the outside and can be prevented by mounting the accelerometer inside the +security envelope. This attack only works if the rate of rotation is constant and is trivially detectable if the rate of +rotation is set to change on a schedule. + +\section{Conclusion} +In this paper, we have presented inertial hardware security modules, a novel concept for the construction of highly +secure hardware security modules from inexpensive, commonly available parts. We have elaborated the engineering +considerations underlying a practical implementation of this concept. We have analyzed the concept for its security +properties and highlighted its ability to significantly strengthen otherwise weak tamper detection barriers. We have +laid out some ideas for future research on the concept, and we will continue our own research on the topic. + +\printbibliography[heading=bibintoc] +\appendix +\section{Patents and licensing} +During devlopment, we performed several hours of research on prior art for the inertial HSM concept. Yet, we could not +find any mentions of similar concepts either in academic literature or in patents. Thus, we deem ourselves to be the +inventors of this idea and we are fairly sure it is not covered by any patents or other restrictions at this point in +time. + +Since the concept is primarily attractive for small-scale production and since cheaper mass-production alternatives are +already commercially available, we have decided against applying for a patent and we wish to make it available to the +general public without any restrictions on its use. This paper itself is licensed CC-BY-SA (see below). As for the +inertial HSM concept, we invite you to use it as you wish and to base your own work on our publications without any fees +or commercial restrictions. Where possible, we ask you to cite this paper and attribute the inertial HSM concept to its +authors. + +\center{ + \center{\ccbysa} + + \center{This work is licensed under a Creative-Commons ``Attribution-ShareAlike 4.0 International'' license. The + full text of the license can be found at:} + + \center{\url{https://creativecommons.org/licenses/by-sa/4.0/}} + + \center{For alternative licensing options, source files, questions or comments please contact the authors.} + + \center{This is version \texttt{\input{version.tex}\unskip} generated on \today. The git repository can be found at:} + + \center{\url{https://git.jaseg.de/rotohsm.git}} +} +\end{document} |