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diff --git a/doc/quick-tech-report/Makefile b/doc/quick-tech-report/Makefile index a2c5f12..8a4bc75 100644 --- a/doc/quick-tech-report/Makefile +++ b/doc/quick-tech-report/Makefile @@ -8,22 +8,19 @@ SHELL := bash MAKEFLAGS += --warn-undefined-variables MAKEFLAGS += --no-builtin-rules -main_tex ?= rotohsm_tech_report +main_tex ?= rotohsm_paper +brief_tex ?= rotohsm_tech_report VERSION_STRING := $(shell git describe --tags --long --dirty) -all: ${main_tex}.pdf +all: ${main_tex}.pdf ${brief_tex}.pdf %.pdf: %.tex rotohsm.bib version.tex pdflatex -shell-escape $< biber $* pdflatex -shell-escape $< -.PHONY: preview -preview: - pdflatex -shell-escape ${main_tex}.tex - -version.tex: ${main_tex}.tex rotohsm.bib +version.tex: ${main_tex}.tex ${brief_tex}.tex rotohsm.bib echo "${VERSION_STRING}" > $@ resources/%.pdf: $(LAB_PATH)/%.ipynb @@ -33,4 +30,6 @@ resources/%.pdf: $(LAB_PATH)/%.ipynb clean: rm -f ${main_tex}.aux ${main_tex}.bbl ${main_tex}.bcf ${main_tex}.log ${main_tex}.blg rm -f ${main_tex}.out ${main_tex}.run.xml texput.log + rm -f ${brief_tex}.aux ${brief_tex}.bbl ${brief_tex}.bcf ${brief_tex}.log ${brief_tex}.blg + rm -f ${brief_tex}.out ${brief_tex}.run.xml texput.log diff --git a/doc/quick-tech-report/rotohsm_paper.pdf b/doc/quick-tech-report/rotohsm_paper.pdf Binary files differnew file mode 100644 index 0000000..b988dd4 --- /dev/null +++ b/doc/quick-tech-report/rotohsm_paper.pdf diff --git a/doc/quick-tech-report/rotohsm_paper.tex b/doc/quick-tech-report/rotohsm_paper.tex new file mode 100644 index 0000000..6b9d287 --- /dev/null +++ b/doc/quick-tech-report/rotohsm_paper.tex @@ -0,0 +1,609 @@ +\documentclass[10pt,journal,a4paper]{IEEEtran} +\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} +\DeclareSIUnit{\year}{a} +\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{Can't Touch This: Inerial HSMs Thwart Advanced Physical Attacks} +\author{Jan Götte} +\date{2020-12-20} +\maketitle + +\section*{Abstract} + +In this paper, we introduce a novel countermeasure against physical attacks: Inertial hardware security modules. +Conventional systems have in common that they try to detect attacks by crafting sensors responding to increasingly +minute manipulations of the monitored security boundary or volume. Our approach is novel in that we reduce the +sensitivity requirement of security meshes and other sensors and increase the complexity of any manipulations by +rotating the security mesh or sensor at high speed---thereby presenting a moving target to an attacker. Attempts to stop +the rotation 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 offers a level of security that is +comparable to commercial HSMs. By building prototype hardware we have demonstrated solutions to the concept's +engineering challenges. + +\section{Introduction} + +While information security technology has matured a great deal in the last half century, physical security has barely +changed. Given the right skills, physical access to a computer still often means full compromise. The physical +security of modern server hardware hinges on what lock you put on the room it is in. + +Currently, servers and other computers are rarely physically secured as a whole. Servers sometimes have a simple lid +switch and are put in locked ``cages'' inside guarded facilities. This usually provides a good compromise between +physical security and ease of maintenance. To handle highly sensitive data in applications such as banking or public key +infrastructure, general-purpose and low-security servers are augmented with dedicated, physically secure cryptographic +co-processors such as trusted platform modules (TPMs) or hardware security modules (HSMs). Using a limited amount of +trust in components such as the CPU, the larger system's security can then be reduced to that of its physically secured +TPM~\cite{newman2020,frazelle2019,johnson2018}. + +Like smartcards, TPMs rely on a modern IC being hard to tamper with. Shrinking things to the nanoscopic level to secure +them against tampering is a good engineering solution for some years to come. However, in essence this is a type of +security by obscurity: Obscurity here referring to the rarity of the equipment necessary to attack modern +ICs~\cite{albartus2020,anderson2020}. + +HSMs rely on a fragile foil with much larger-scale conductive traces being hard to remove intact. While we are certain +that there still are many insights to be gained in both technologies, we wish to introduce a novel approach to sidestep +the manufacturing issues of both and provide radically better security against physical attacks. Our core observation +is that any cheap but coarse HSM technology can be made much more difficult to attack by moving it very quickly. + +For example, consider an HSM as it is used in online credit card payment processing. Its physical security level is set +by the structure size of its security mesh. An attack on its mesh might involve fine drill bits, needles, wires, glue, +solder and lasers~\cite{drimer2008}. Now consider the same HSM mounted on a large flywheel. In addition to its usual +defenses the HSM is now equipped with an accelerometer that it uses to verify that it is spinning at high speed. How +would an attacker approach this HSM? They would have to either slow down the rotation---which triggers the +accelerometer---or they would have to attack the HSM in motion. The HSM literally becomes a moving target. At slow +speeds, rotating the entire attack workbench might be possible but rotating frames of reference quickly become +inhospitable to human life (see Appendix~\ref{sec_minimum_angular_velocity}). Since non-contact electromagnetic or +optical attacks are more limited in the first place and can be shielded, we have effectively forced the attacker to use +an attack robot. + +This work contains the following contributions: +\begin{enumerate} + \item We present the \emph{Inertial HSM} concept. Inertial HSMs enable cost-effective small-scale production of + highly secure HSMs. + \item We discuss possible boundary sensing modes for inertial HSMs. + \item We explore the design space of our inertial HSM concept. + \item We present our work on a prototype inertial HSM. + % FIXME \item Measurement of the prototype HSM's susceptibility to various types of attack. +\end{enumerate} + +In Section~\ref{sec_related_work}, we will give an overview of the state of the art in the physical security of HSMs. On +this basis, in Section~\ref{sec_ihsm_construction} we will elaborate the principles of our inertial HSM approach. We +will analyze its weaknesses in Section~\ref{sec_attacks}. Based on these results we have built a prototype system that +we will illustrate in Section~\ref{sec_proto}. We conclude this paper with a general evaluation of our design in +Section~\ref{sec_conclusion}. + +\section{Related work} +\label{sec_related_work} +% summaries of research papers on HSMs. I have not found any actual prior art on anything involving mechanical motion +% beyond ultrasound. + +In this section, we will briefly explore the history of HSMs and the state of academic research on active tamper +detection. + +HSMs are an old technology tracing back decades in their electronic realization. Today's common approach of monitoring +meandering electrical traces on a fragile foil that is wrapped around the HSM essentially transforms the security +problem into the challenge to manufacture very fine electrical traces on a flexible foil~\cite{isaacs2013, immler2019, +anderson2020}. There has been some research on monitoring the HSM's inside using e.g.\ electromagnetic +radiation~\cite{tobisch2020, kreft2012} or ultrasound~\cite{vrijaldenhoven2004} but none of this research +has found widespread adoption. + +In~\cite{anderson2020}, Anderson gives a comprehensive overview on physical security. An example they cite is 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 its turn of the century design is now a bit dated, the construction techniques of the physical +security mechanisms have not evolved 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 state 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,drimer2008,anderson2020,isaacs2013}. + +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 30 in their example). +Their concept promises a very high degree of protection. The main disadvantages of their concept are a limitation in +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. Where~\cite{tobisch2020} use electromagnetic radiation, +Vrijaldenhoven in~\cite{vrijaldenhoven2004} uses ultrasound waves travelling on a surface acoustic wave (SAW) device to +a similar end. + +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. + +We are the the first to propose a mechanically moving HSM security barrier as part of a hardware security module. Most +academic research concentrates on the issue of creating new, more sensitive security barriers for HSMs~\cite{immler2019} +while commercial vendors concentrate on means to cheaply manufacture and certify these security +barriers~\cite{drimer2008}. 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 closest 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{Inertial HSM construction and operation} +\label{sec_ihsm_construction} + +Mechanical motion has been proposed as a means of making things harder to see with the human eye~\cite{haines2006} and is +routinely used in military applications to make things harder to hit~\cite{terdiman2013} but we seem to be the first to +use it in tamper detection. If we consider different ways of moving an HSM to make it harder to tamper with, we find +that making it spin has several advantages. + +First, the HSM has to move fairly fast. If any point of the HSM's tamper sensing mehs moves slow enough for a human to +follow, it becomes a weak spot. E.g.\ in a linear pendulum motion, the pendulum becomes stationary at its apex. Second, +a spinning HSM is compact compared to alternatives like an HSM on wheels. Finally, rotation leads to predictable +accelerometer measurements. A beneficial side-effect of spinning the HSM is that 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. Their tangential linear +velocity would rise linearly with the radius from the axis of rotation, which allows us to limit the approximate maximum +size and mass of an attacker using an assumption on tolerable centrifugal force (see Appendix +\ref{sec_minimum_angular_velocity}). In this consideration the axis of rotation is a weak spot, but that can be +mitigated using multiple nested layers of protection. + +\begin{figure} + \center + \includegraphics{concept_vis_one_axis.pdf} + \caption{Concept of a simple spinning inertial HSM. 1 - Shaft. 2 - Security mesh. 3 - Payload. 4 - + Accelerometer. 5 - Shaft penetrating security mesh.} + \label{fig_schema_one_axis} +\end{figure} + +In a rotating reference frame, centrifugal force is proportional to the square of angular velocity and proportional to +distance from the axis of rotation. We can exploit this fact to create a sensor that detects any disturbance of the +rotation by placing a linear accelerometer at some distance from the axis of rotation. During constant rotation, both +acceleration tangential to the rotation and along the axis of rotation will be zero. Centrifugal acceleration will be +constant. + +Large centrifugal acceleration at high speeds poses the engineering challenge of preventing the whole thing from flying +apart, but it also creates an obstacle to any attacker trying to manipulate the sensor. We do not need to move the +entire contents of the HSM. It suffices if we move the tamper detection barrier around a stationary payload. This +reduces the moment of inertia of the moving part and it means we can use cables for payload power and data. + +From our back-of-the-envelope calculation in Appendix \ref{sec_minimum_angular_velocity} we conclude that even at +moderate speeds above $\SI{500}{rpm}$, an attack would have to be carried out using a robot. + +In Appendix \ref{sec_degrees_of_freedom} we consider sensor configurations and we conclude that one three-axis +accelerometer each in the rotor and in the stator are a good baseline configuration. In general, the system will be more +sensitive to attacks if we over-determine the system of equations describing its motion by using more sensors than +necessary. + +\subsection{Mechanical layout} + +Thinking about the concrete construction of our mechanical HSM, the first challenge is mounting both mesh and payload on +a single shaft. The simplest way we found to mount a stationary payload inside of a spinning security mesh is a hollow +shaft. The payload can be mounted on a fixed rod threaded through this hollow shaft along with wires for power and +data. The shaft is a weak spot of the system, but this weak spot can be alleviated through either careful construction +or a second layer of rotating meshes with a different axis of rotation. Configurations that do not use a hollow-shaft +motor are possible, but may require additional bearings to keep the stator from vibrating. + +The next design choice we have to make is the physical structure of the security mesh. The spinning mesh must be +designed to cover the entire surface of the payload, but compared to a traditional HSM it suffices if it sweeps over +every part of the payload once per rotation. This means we can design longitudinal gaps into the mesh that allow outside +air to flow through to the payload. In traditional boundary-sensing HSMs, cooling of the payload processor is a serious +issue since any air duct or heat pipe would have to penetrate the HSM's security boundary. This problem can only be +solved with complex and costly siphon-style constructions, so in commercial systems heat conduction is used +exclusively~\cite{isaacs2013}. This limits the maximum power dissipation of the payload and thus its processing power. +Our setup allows direct air cooling of regular heatsinks. This greatly increases the maximum possible power dissipation +of the payload and unlocks much more powerful processing capabilities. In an evolution of our design, the spinning mesh +could even be designed to *be* a cooling fan. + +\subsection{Spinning mesh power and data transmission} + +On the electrical side, the idea of a security mesh spinning at more than $\SI{500}{rpm}$ leaves us with a few +implementation challenges. Since the spinning mesh must be monitored for breaks or short circuits continuously, we need +both a power supply for the spinning monitoring circuit and a data link to the stator. + +We found that a bright lamp shining at a rotating solar panel is a good starting point. In contrast to e.g.\ slip +rings, this setup is mechanically durable at high speeds and it also provides reasonable output power (see Appendix +\ref{sec_energy_calculations} for some calculations on power consumption). A battery may not provide a useful lifetime +without power-optimization. Likewise, an energy harvesting setup may not provide enough current to supply peak demand. + +Since the monitoring circuit uses little current, power transfer efficiency is not important. On the other hand, cost +may be a concern in a production device. Here it may prove worthwhile to replace the solar cell setup with an extra +winding on the rotor of the BLDC motor driving the spinning mesh. This rotor is likely to be a custom part, so adding +an extra winding is unlikely to increase cost significantly. More traditional inductive power transfer may also be an +option if it can be integrated into the mechanical design. + +Besides power, the data link between spinning mesh and payload is critical to the HSM's design. This link is used to +transmit the occassional status report along with a low-latency alarm trigger (``heartbeat'') signal from mesh to payload. +As we will elaborate in Section~\ref{sec_proto} a simple infrared optical link turned out to be a good solution for this +purpose. + +\section{Attacks} +\label{sec_attacks} + +After outlining the basic mechanical design of an inertial HSM above, in this section we will detail possible ways to +attack it. Fundamentally, attacks on an inertial HSM are the same as those on a traditional HSM since the tamper +detection mesh is the same. Only, in the inertial HSM any attack on the mesh has to be carried out while the mesh is +rotating, which for most types of attack will require some kind of CNC attack robot moving in sync with it. + +\subsection{Attacks on the mesh} + +There are two locations where one can attack a tamper-detection mesh. On one hand, 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~\cite{dexter2015}. 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. We +consider this contact infeasible to be performed on an object spinning at high speed without a complex setup that +rotates along with the object or that involves ion beams, electron beams or liquids. Thus, we consider them to be +practically infeasible outside of a well-funded, special-purpose laboratory. + +\subsection{Attacks on the rotation sensor} + +Instead of attacking the mesh in motion, an attacker may also try to first stop the rotor. To succeed, they would need +to fool the rotor's MEMS accelerometer. An electronic attack on the sensor or the monitoring microcontroller would be no +easier than directly bridging the mesh traces. + +MEMS accelerometers usually use a cantilever design, where a proof mass moves a cantilever whose precise position can be +measured electronically. A topic of recent academic interest have been acoustic attacks tampering with these +mechanics~\cite{trippel2017}. In the authors' estimate these attacks are too hard to control to be practically useful +against an inertial HSM. + +A possible way to attack the accelerometer inside an inertial HSM may 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, locking them in place. To mitigate this type of attack the accelerometer should be mounted in a +shielded place inside the security envelope. Further, this attack can only work if the rate of rotation and thus the +expected accelerometer readings are constant. If the rate of rotation is set to vary over time this type of attack is +quickly detected. In Appendix \ref{sec_degrees_of_freedom} we outline the constraints on sensor placement. + +\subsection{Attacks on the alarm circuitry} + +Besides trying to deactivate the tamper detection mesh, an electronic attack could also target the alarm circuitry +inside the stationary payload, or the communication link between rotor and payload. The link can be secured using a +cryptographically secured protocol like one would use for wireless radio links 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. +Like in conventional HSMs it has to be built to either tolerate or detect environmental attacks such as ones using +temperature, ionizing radiation, lasers, supply voltage variations, ultrasound or other vibration and gases or liquids. +Conventionally, incoming power rails are filtered thoroughly to prevent electrical attacks and other types of attacks +are prevented by sensors that thrigger an alarm. + +In an inertial HSM, the mesh monitoring circuit's tamper alarm is transmitted from rotor to stator through a wireless +link. Since an attacker may wirelessly spoof this link, it must be cryptographically secured. It also must be +bidirectional to allow the alarm signal receiver to verify link latency: If it were unidirectional, an attacker could +act as a Man-in-the-Middle and replay the mesh's authenticated ``no alarm'' signal at slightly below real-time speed +(say at $\SI{99}{\percent}$ speed). The receiver would not be able to distinguish between this attack and ordinary +deviations in the transmitter's local clock frequency. Thus, after some time the attacker can simply stop the rotor and +break the mesh while replaying the leftover recorded ``no alarm'' signal. Given the frequency stability of commercial +crystals, this would yield the attacker several seconds of undisturbed attack time per hour of recording time. + +\subsection{Fast and violent attacks} + +A variation of the above attacks on the alarm circuitry is to simply destroy the part of the HSM that erases data in +response to tampering before it can finish its job. This attack could use a tool such as a large hammer or a gun. +Mitigations for this type of attack include potting the payload inside a mechanically robust enclosure. Additionally, +the integrity of the entire alarm signalling chain can be checked continuously using a cryptographic heartbeat protocol. +A simple active-high or active-low alarm signal as it is used in traditional HSMs cannot be considered fail-safe in this +scenario as such an attack may well short-circuit or break PCB traces. + +\section{Prototype implementation} +\label{sec_proto} + +After elaborating the design principles of inertial HSMs and researching potential attack vectors we have validated +these theoretical studies by implementing a prototype rotary HSM. The main engineering challenges we solved in our +prototype are: + +\begin{enumerate} + \item Fundamental mechanical design suitable for rapid prototyping that can withstand a rotation of $\SI{500}{rpm}$. + \item Automatic generation of security mesh PCB layouts for quick adaption to new form factors. + \item Non-contact power transmission from stator to rotor. + \item Non-contact bidirectional data communication between stator and rotor. +\end{enumerate} + +\subsection{Mechanical design} + +We sized our prototype to have space for up to two full-size Raspberry Pi boards. Each one of these boards is already +more powerful than an ordinary HSM, but they are small enough to simplify our prototype's design. For low-cost +prototyping we designed our prototype to use printed circuit boards as its main structural material. The interlocking +parts were designed in FreeCAD as shown in Figure \ref{proto_3d_design}. The mechanical designs were exported to KiCAD +for electrical design before being sent to a commercial PCB manufacturer. Rotor and stator are built from interlocking, +soldered PCBs. The components are mounted to a $\SI{6}{\milli\meter}$ brass tube using FDM 3D printed flanges. The rotor +is driven by a small hobby quadcopter motor. + +Security is provided by a PCB security mesh enveloping the entire system and extending to within a few millimeters of +the shaft. For security it is not necessary to cover the entire circumference of the module with mesh, so we opted to +use only three narrow longitudinal struts to save weight. + +To mount the entire HSM, we chose to use ``2020'' modular aluminium profile. + +\begin{figure} + \center + \includegraphics[height=7cm]{proto_3d_design.jpg} + \caption{The 3D CAD design of the prototype.} + \label{proto_3d_design} +\end{figure} + +\subsection{PCB security mesh generation} + +The security mesh covers a total of five interlocking PCBs. A sixth PCB contains the monitoring circuit and connects to +these mesh PCBs. To allow us to quickly iterate our design without manually re-routing several large security meshes +for every mechanical chage we wrote a plugin for the KiCAD EDA suite that automatically generates parametrized security +meshes. When KiCAD is used in conjunction with FreeCAD through FreeCAD's KiCAD StepUp plugin, this ends up in an +efficient toolchain from mechanical CAD design to security mesh PCB gerber files. The mesh generation plugin can be +found at its website\footnote{\url{https://blog.jaseg.de/posts/kicad-mesh-plugin/}}. The meshes it produces have a +practical level of security in our application. + +The mesh generation process starts by overlaying a grid on the target area. It then produces a randomized tree covering +this grid. The individual mesh traces are then traced along a depth-first search through this tree. A visualization of +the steps is shown in Figure \ref{mesh_gen_viz}. A sample of the production results from our prototype is shown in +Figure \ref{mesh_gen_sample}. + +\begin{figure} + \center + \includegraphics[width=9cm]{mesh_gen_viz.pdf} + \caption{Overview of the automatic security mesh generation process. 1 - the blob is the example target area. 2 - A + grid is overlayed. 3 - Grid cells outside of the target area are removed. 4 - A random tree covering the remaining + cells is generated. 5 - The mesh traces are traced along a depth-first walk of the tree. 6 - Result.} + \label{mesh_gen_viz} +\end{figure} + +\begin{figure} + \center + \includegraphics[width=6cm]{mesh_scan_crop.jpg} + \caption{A section of the security mesh PCB we produced with our toolchain for the prototype HSM.} + \label{mesh_gen_sample} +\end{figure} + +\subsection{Data transmission through rotating joint} + +With the mesh done, the next engineering challenge was the mesh monitoring data link between rotor and stator. As a +baseline solution, we settled on a $\SI{115}{\kilo\baud}$ UART signal sent through a simple bidirectional infrared link. +In the transmitter, the UART TX line on-off modulates a $\SI{920}{\nano\meter}$ IR LED through a common-emitter driver +transistor. In the receiver, an IR PIN photodiode reverse-biased to $\frac{1}{2}V_\text{CC}$ is connected to a +reasonably wideband transimpedance amplifier (TIA) with a $\SI{100}{\kilo\ohm}$ transimpedance. As shown in Figure +\ref{photolink_schematic}, the output of this TIA is fed through another $G=100$ amplifier whose output is then squared +up by a comparator. We used an \texttt{MCP6494} quad CMOS op-amp. At a specified $\SI{2}{\milli\ampere}$ current +consumption it is within our rotor's power budget, and its Gain Bandwidth Product of $\SI{7.5}{\mega\hertz}$ yields a +useful transimpedance in the photodiode-facing TIA stage. + +To reduce the requirements on power transmission to the rotor, we have tried to reduce power consumption of the +rotor-side receiver/transmitter pair trading off stator-side power consumption. One part of this is that we use +a wide-angle photodiode and IR LED on the stator, but use narrow-angle components on the rotor. The two rx/tx pairs are +arranged next to the motor on opposite sides. By placing the narrow-angle rotor rx/tx components on the outside as +shown in Figure \ref{ir_tx_schema}, the motor shields both IR links from crosstalk. The rotor transmitter LED is +driven at $\SI{1}{\milli\ampere}$ while the stator transmitter LED is driven at $\SI{20}{\milli\ampere}$. + +\begin{figure} + \center + \includegraphics{ir_tx_schema.pdf} + \caption{Schema of our bidirectional IR communication link between rotor and stator, view along axis of rotation. 1 + - Rotor base PCB. 2 - Stator IR link PCB. 3 - Motor. 4 - receiver PIN photodiode. 5 - transmitter IR LED.} + \label{ir_tx_schema} +\end{figure} + +\begin{figure} + \center + \includegraphics[width=9cm]{photolink_schematic.pdf} + \caption{Schematic of the IR communication link. Component values are only examples. In particular C2 depends highly + on the photodiode used and stray capacitances due to the component layout.} + \label{photolink_schematic} +\end{figure} + +\subsection{Power transmission through rotating joint} +Besides the data link, the other electrical interface we need between rotor and stator is for power transmission. We +power Since this prototype serves only demonstration purposes, we chose to use the simplest possible method of power +transmission: solar cells. We mounted six series-connected solar cells in three commercially available modules on the +circular PCB at the end of our cylindrical rotor. The solar cells direclty feed the rotor's logic supply with buffering +by a large $\SI{33}{\micro\farad}$ ceramic capacitor. With six cells in series, they provide around $\SI{3.0}{\volt}$ at +several tens of $\si{\milli\ampere}$ given sufficient illumination. + +For simplicity and weight reduction, at this point we chose to forego large buffer capacitors on the rotor. This means +variations in solar cell illumination directly couple into the microcontroller's supply rail. Initially, we experimented +with regular residential LED light bulbs, but those turned out to have too much flicker and lead to our microcontroller +frequently rebooting. Trials using an incandecent light produced a stable supply, but the large amount of infrared light +emitted by the incandecent light bulb severely disturbed our near-infrared communication link. As a consequence of +this, we settled on a small LED light intended for use as a studio light that provdided us with almost flicker-free +light at lower frequencies, leading to a sufficiently stable microcontroller VCC rail without any disturbance to the IR +link. + +\subsection{Evaluation} + +After building our prototype inertial HSM according to the design decisions we outlined above, we performed a series of +experiments to validate the critical components of the design. + +During these experiments, our prototype performed as intended. Both power and data transmission through the rotating +joint were working reliably. Figure \ref{prototype_early_comms} shows our prototype performing reliably at maximum speed +for the first time. Our improvised IR link is open in both directions for about $\SI{60}{\degree}$ of the rotation, +which allows us to reliably transfer several tens of bytes in each direction during the receivers' fly-by even at high +speed of rotation. As a result of our prototype experiments, we consider a larger-scale implementation of the inertial +HSM concept practical. + +\begin{figure} + \center + \includegraphics[width=8cm]{prototype_early_comms_small.jpg} + \caption{The protoype when we first achieved reliable power transfer and bidirectional communication between stator + and rotor. In the picture, the prototype was communicating reliably up to the maximum $\approx\SI{1500}{rpm}$ that + we could get out of its hobby quadcopter parts.} + \label{prototype_early_comms} +\end{figure} + +\section{Conclusion} + +\label{sec_conclusion} To conclude, in this paper we introduced inertial hardware security modules (iHSMs), a +novel concept for the construction of highly secure hardware security modules from inexpensive, commonly available +parts. We elaborated the engineering considerations underlying a practical implementation of this concept. We +implemented a prototype demonstrating practical solutions to the significant engineering challenges of this concept. We +analyzed the concept for its security properties and highlighted its ability to significantly strengthen otherwise weak +tamper detection barriers. + +Inertial HSMs offer a high level of security beyond what traditional techniques can offer. They allow the construction +of devices secure against a wide range of practical attacks at prototype quantities and without specialized tools. We +hope that this simple construction will stimulate academic research into secure hardware. + +\printbibliography[heading=bibintoc] +\appendix +\subsection{Spinning mesh energy calculations} +\label{sec_energy_calculations} +Assume that the spinning mesh sensor should send its tamper status to the static monitoring circuit at least once every +$T_\text{tx} = \SI{10}{\milli\second}$. At $\SI{100}{\kilo\baud}$ a transmission of a one-byte message in standard UART +framing would take $\SI{100}{\micro\second}$ and yield an $\SI{1}{\percent}$ duty cycle. If we assume an optical or RF +transmitter that requires $\SI{10}{\milli\ampere}$ of active current, this yields an average operating current of +$\SI{100}{\micro\ampere}$. Reserving another $\SI{100}{\micro\ampere}$ for the monitoring circuit itself we arrive at an +energy consumption of $\SI{1.7}{\ampere\hour\per\year}$. + +\subsubsection{Battery power} +\label{sec_energy_calculations_battery} +The annual energy consumption we calculated above is about equivalent to the capacity of a single CR123A +lithium primary cell. Using several such cells or optimizing power consumption would thus easily yield several years of +battery life. + +\subsubsection{LED and solar cell} +\label{sec_energy_calculations_led} +Let us assume an LED with a light output of $\SI{1}{W}$ illuminating a small solar cell. Let us pessimistically assume a +$\SI{5}{\percent}$ conversion efficiency in the solar cell. Let us assume that when the rotor is at its optimal +rotational angle, $\SI{20}{\percent}$ of the LED's light output couple into the solar cell. Let us assume that we loose +another $\SI{90}{\percent}$ of light output on average during one rotation when the rotor is in motion. This results in +an energy output from the solar cell of $\SI{1}{\milli\watt}$. Assuming a $\SI{3.3}{\volt}$ supply this yields +$\SI{300}{\micro\ampere}$ for our monitoring circuit. This is enough even with some conversion losses in the step-up +converter boosing the solar cell's $\SI{0.6}{\volt}$ working voltage to the monitoring circuit's supply voltage. + +\subsection{Minimum angular velocity: Rotating human attacker} +\label{sec_minimum_angular_velocity} + +An attacker might try to rotate along with the HSM to attack the security mesh without triggering the accelerometer. Let +us pessimistically assume that the attacker has the axis of rotation running through their center of mass. The +attacker's body is probably at least $\SI{200}{\milli\meter}$ wide along its shortest axis, resulting in a minimum +radius from axis of rotation to surface of about $\SI{100}{\milli\meter}$. We choose $\SI{250}{\meter\per\second^2}$ as +an arbitrary acceleration well past the range tolerable by humans according to Wikipedia. Centrifugal acceleration is +$a=\omega^2 r$. In our example this results in a minimum angular velocity of $\omega_\text{min} = \sqrt{\frac{a}{r}} = +\sqrt{\frac{\SI{250}{\meter\per\second^2}}{\SI{100}{\milli\meter}}} \approx 8\cdot 2\pi\frac{1}{\si{\second}} \approx 500 +\text{rpm}$. + +\subsection{Fooling the accelerometer} +\label{sec_degrees_of_freedom} + +Let us consider a general inertial HSM with one or more sensors that is attacked by an attacker. In this scenario, it is +reasonable to assume that the rotating parts of the HSM are rigidly coupled to one another and will stay that way: For +the attacker to decouple parts of the HSM (e.g. to remove one of its accelerometers from the PCB), the attacker would +already have to circumvent the rotor's security mesh. + +Assuming the HSM is stationary, a sensor on the rotating part will experience two significant accelerations: +\begin{enumerate} + \item Gravity $g = 9.8\frac{m}{s^2}$ + \item Centrifugal force $a_C=\omega^2 r$, in the order of $\SI{1000}{\meter\per\second^2}$ or $100 g$ at + $r=\SI{100}{\milli\meter}$ and $\SI{1000}{rpm}$ +\end{enumerate} + +Due to the vast differences in both radius and angular velocity, we can neglegt any influence of the earth's rotation on +our system. + +In normal operation, the HSM is stationary ($\mathbf v=0$) and the HSM's motor is tuned to exactly counter-balance +friction so the rotor's angular velocity remains constant. As a rigid body, the rotor's motion is fully defined by its +rotation and translation. In total, this makes for six degrees of freedom. The three degrees of freedom of linear +translation we can measure directly with an accelerometer in the stationary part on the inside of the HSM. This +accelerometer could detect any rapid acceleration of the HSM's rotor. To measure rotation, we could mount a +gyroscope on the rotor to detect deceleration. The issue with this is that like other MEMS acceleration sensors, +commercial MEMS gyroscopes are vulnerable to drift and an attacker could slowly decelerate the rotor without being +detected. + +A linear accelerometer mounted on the rotor however is able to catch even this attack. Subtracting gravity, it could +determine both magnitude and direction of the centrifugal force, which is proportional to the square of angular velocity +and not its derivative. + +In summary, a single three-axis accelerometer on the rotor combined with a three-axis accelerometer in the stator would +be a good baseline configuration. + +\subsection{Patents and licensing} +During development, 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 are likely 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} |