From 69daf158fe83e49d420e97fc5bbf91f32798585a Mon Sep 17 00:00:00 2001 From: jaseg Date: Mon, 15 Mar 2021 11:25:49 +0100 Subject: Repo re-org: rename paper dir --- doc/quick-tech-report/.gitignore | 10 - doc/quick-tech-report/Makefile | 35 - doc/quick-tech-report/circuits.ipynb | 1115 ------- doc/quick-tech-report/concept_vis_one_axis.pdf | Bin 6623 -> 0 bytes doc/quick-tech-report/concept_vis_one_axis.svg | 344 --- .../goette_inertial_hsms_v1_5_eprint.pdf | Bin 112344 -> 0 bytes doc/quick-tech-report/ir_tx_schema.pdf | Bin 4112 -> 0 bytes doc/quick-tech-report/ir_tx_schema.svg | 340 -- doc/quick-tech-report/mesh_gen_viz.pdf | Bin 6880 -> 0 bytes doc/quick-tech-report/mesh_gen_viz.svg | 1547 ---------- doc/quick-tech-report/mesh_scan_crop.jpg | Bin 400578 -> 0 bytes doc/quick-tech-report/photolink_schematic.pdf | Bin 15968 -> 0 bytes doc/quick-tech-report/photolink_schematic.pro | 43 - doc/quick-tech-report/photolink_schematic.sch | 486 --- doc/quick-tech-report/photolink_schematic.svg | 3246 -------------------- doc/quick-tech-report/proto_3d_design.jpg | Bin 63447 -> 0 bytes .../prototype_early_comms_small.jpg | Bin 518517 -> 0 bytes doc/quick-tech-report/rotohsm.bib | 200 -- doc/quick-tech-report/rotohsm_paper.pdf | Bin 1190616 -> 0 bytes doc/quick-tech-report/rotohsm_paper.tex | 609 ---- doc/quick-tech-report/rotohsm_tech_report.pdf | Bin 111459 -> 0 bytes doc/quick-tech-report/rotohsm_tech_report.tex | 300 -- 22 files changed, 8275 deletions(-) delete mode 100644 doc/quick-tech-report/.gitignore delete mode 100644 doc/quick-tech-report/Makefile delete mode 100644 doc/quick-tech-report/circuits.ipynb delete mode 100644 doc/quick-tech-report/concept_vis_one_axis.pdf delete mode 100644 doc/quick-tech-report/concept_vis_one_axis.svg delete mode 100755 doc/quick-tech-report/goette_inertial_hsms_v1_5_eprint.pdf delete mode 100644 doc/quick-tech-report/ir_tx_schema.pdf delete mode 100644 doc/quick-tech-report/ir_tx_schema.svg delete mode 100644 doc/quick-tech-report/mesh_gen_viz.pdf delete mode 100644 doc/quick-tech-report/mesh_gen_viz.svg delete mode 100644 doc/quick-tech-report/mesh_scan_crop.jpg delete mode 100644 doc/quick-tech-report/photolink_schematic.pdf delete mode 100644 doc/quick-tech-report/photolink_schematic.pro delete mode 100644 doc/quick-tech-report/photolink_schematic.sch delete mode 100644 doc/quick-tech-report/photolink_schematic.svg delete mode 100644 doc/quick-tech-report/proto_3d_design.jpg delete mode 100644 doc/quick-tech-report/prototype_early_comms_small.jpg delete mode 100644 doc/quick-tech-report/rotohsm.bib delete mode 100644 doc/quick-tech-report/rotohsm_paper.pdf delete mode 100644 doc/quick-tech-report/rotohsm_paper.tex delete mode 100644 doc/quick-tech-report/rotohsm_tech_report.pdf delete mode 100644 doc/quick-tech-report/rotohsm_tech_report.tex (limited to 'doc/quick-tech-report') diff --git a/doc/quick-tech-report/.gitignore b/doc/quick-tech-report/.gitignore deleted file mode 100644 index c49262e..0000000 --- a/doc/quick-tech-report/.gitignore +++ /dev/null @@ -1,10 +0,0 @@ -*.out -*.bbl -*.aux -*.toc -*.blg -*.bcf -*.log -*.run.xml - -version.tex diff --git a/doc/quick-tech-report/Makefile b/doc/quick-tech-report/Makefile deleted file mode 100644 index 8a4bc75..0000000 --- a/doc/quick-tech-report/Makefile +++ /dev/null @@ -1,35 +0,0 @@ - -LAB_PATH ?= ../lab-windows - -SHELL := bash -.ONESHELL: -.SHELLFLAGS := -eu -o pipefail -c -.DELETE_ON_ERROR: -MAKEFLAGS += --warn-undefined-variables -MAKEFLAGS += --no-builtin-rules - -main_tex ?= rotohsm_paper -brief_tex ?= rotohsm_tech_report - -VERSION_STRING := $(shell git describe --tags --long --dirty) - -all: ${main_tex}.pdf ${brief_tex}.pdf - -%.pdf: %.tex rotohsm.bib version.tex - pdflatex -shell-escape $< - biber $* - pdflatex -shell-escape $< - -version.tex: ${main_tex}.tex ${brief_tex}.tex rotohsm.bib - echo "${VERSION_STRING}" > $@ - -resources/%.pdf: $(LAB_PATH)/%.ipynb - jupyter-nbconvert --to=pdf --output-dir=resources --output=$* --LatexExporter.template_file=resources/nbexport.tplx $^ - -.PHONY: clean -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/circuits.ipynb b/doc/quick-tech-report/circuits.ipynb deleted file mode 100644 index 2e78b30..0000000 --- a/doc/quick-tech-report/circuits.ipynb +++ /dev/null @@ -1,1115 +0,0 @@ -{ - "cells": [ - { - "cell_type": "code", - "execution_count": 1, - "metadata": {}, - "outputs": [], - "source": [ - "import schemdraw\n", - "from schemdraw import elements as elm" - ] - }, - { - "cell_type": "code", - "execution_count": 123, - "metadata": {}, - "outputs": [ - { - "data": { - "image/png": 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"source": [ - "class DiodeOptocoupler(schemdraw.elements.compound.ElementCompound):\n", - " def __init__(self, *args, **kwargs):\n", - " unit = 1.5\n", - " super().__init__(*args, unit=unit, **kwargs)\n", - "\n", - " box = kwargs.get('box', True)\n", - " boxfill = kwargs.get('boxfill', False)\n", - " bpad = kwargs.get('boxpad', .2)\n", - " label1, label2 = kwargs.get('label1'), kwargs.get('label2')\n", - " rev1, rev2 = kwargs.get('reverse1', False), kwargs.get('reverse2', False)\n", - "\n", - " D1 = self.add(elm.Diode(d='down', reverse=rev1))\n", - " D2 = self.add(elm.Diode(d='down', reverse=rev2, at=[2, 0]))\n", - " if label1:\n", - " self.segments.append(schemdraw.segments.SegmentText(D1.start + (0, 0.5), label1))\n", - " if label2:\n", - " self.segments.append(schemdraw.segments.SegmentText(D2.start + (0, 0.5), label2))\n", - " \n", - " self.add(elm.Arrow('r', at=[.6, -unit/2 + .2], l=.4, headwidth=.15, headlength=.4))\n", - " self.add(elm.Arrow('r', at=[.6, -unit/2 - .2], l=.4, headwidth=.15, headlength=.4))\n", - "\n", - " bbox = self.get_bbox()\n", - " if box:\n", - " self.add(elm.Rect(\n", - " 'r', at=[0, 0],\n", - " corner1=[bbox.xmin-bpad, bbox.ymin-bpad],\n", - " corner2=[bbox.xmax+bpad, bbox.ymax+bpad],\n", - " fill=boxfill, zorder=0))\n", - "\n", - " A = self.add(elm.Line('r', at=D2.start, l=bpad*2))\n", - " B = self.add(elm.Line('r', at=D2.end, l=bpad*2))\n", - " C = self.add(elm.Line('l', at=D1.start, tox=bbox.xmin-bpad))\n", - " D = self.add(elm.Line('l', at=D1.end, tox=bbox.xmin-bpad))\n", - " self.anchors['anode1'] = C.end\n", - " self.anchors['cathode1'] = D.end\n", - " self.anchors['anode2'] = B.end\n", - " self.anchors['cathode2'] = A.end\n", - "DiodeOptocoupler(box=False, reverse2=True, label2='D2')" - ] - }, - { - "cell_type": "code", - "execution_count": 177, - "metadata": {}, - "outputs": [ - { - "data": { - "image/png": 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label='R1'))\n", - "coupler = d.add(DiodeOptocoupler(d='right', box=False, label1='D1', label2='D2', anchor='anode1', reverse2=True))\n", - "d.here = coupler.cathode1\n", - "Q1 = d.add(elm.BjtNpn(d='right', anchor='collector', label='Q1'))\n", - "d.add(elm.Line(xy=Q1.emitter, d='down', l=d.unit*0.25))\n", - "d.add(elm.Line(d='left', tox=V1.start))\n", - "d.add(elm.Line(d='up', toy=V1.start))\n", - "d.add(elm.Resistor(xy=Q1.base, d='left', label='R2'))\n", - "d.add(elm.Dot(open=True, lftlabel='TX in'))\n", - "\n", - "d.add(elm.Line(xy=coupler.cathode2, d='up', toy=V1.end + d.unit*0.5))\n", - "vbus = d.add(elm.Line(d='right', l=d.unit*5))\n", - "\n", - "d.add(elm.Line(xy=coupler.anode2, d='right', l=d.unit*0.5))\n", - "j1 = d.add(elm.Dot())\n", - "d.add(elm.Line(l=d.unit*0.5))\n", - "amp1 = d.add(elm.Opamp(d='right', anchor='in1'))\n", - "\n", - "d.add(elm.Line(xy=j1.xy, d='up', l=d.unit))\n", - "j2 = d.add(elm.Dot())\n", - "\n", - "d.add(elm.Resistor(label='R3', d='right'))\n", - 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- Rotor - - - - - - - - - Stator - - - - - - - - - - - IR link - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - diff --git a/doc/quick-tech-report/proto_3d_design.jpg b/doc/quick-tech-report/proto_3d_design.jpg deleted file mode 100644 index f527828..0000000 Binary files a/doc/quick-tech-report/proto_3d_design.jpg and /dev/null differ diff --git a/doc/quick-tech-report/prototype_early_comms_small.jpg b/doc/quick-tech-report/prototype_early_comms_small.jpg deleted file mode 100644 index 506da48..0000000 Binary files a/doc/quick-tech-report/prototype_early_comms_small.jpg and /dev/null differ diff --git a/doc/quick-tech-report/rotohsm.bib b/doc/quick-tech-report/rotohsm.bib deleted file mode 100644 index 1092c3a..0000000 --- a/doc/quick-tech-report/rotohsm.bib +++ /dev/null @@ -1,200 +0,0 @@ -% Encoding: UTF-8 -@comment{x-kbibtex-encoding=utf-8} - -@Book{anderson2020, - author = {Ross Anderson}, - date = {2020-09-16}, - title = {Security Engineering}, - isbn = {978-1-119-64281-7}, -} - -@techreport{smith1998, - author = {Sean Smith and Steve Weingart}, - date = {1998-02-19}, - institution = {IBM T.J. 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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 (iHSMs). -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 yet. - -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. Although 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. Besides 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. - -To the best of our knowledge, 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 certify and cheaply manufacture -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 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 a 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 mesh 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 easily 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, after -subtracting gravity 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 \emph{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 an estimation of 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 motor 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} diff --git a/doc/quick-tech-report/rotohsm_tech_report.pdf b/doc/quick-tech-report/rotohsm_tech_report.pdf deleted file mode 100644 index d87bc8e..0000000 Binary files a/doc/quick-tech-report/rotohsm_tech_report.pdf and /dev/null differ diff --git a/doc/quick-tech-report/rotohsm_tech_report.tex b/doc/quick-tech-report/rotohsm_tech_report.tex deleted file mode 100644 index e9d571f..0000000 --- a/doc/quick-tech-report/rotohsm_tech_report.tex +++ /dev/null @@ -1,300 +0,0 @@ -\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{Tech Report: Inerial HSMs Thwart Advanced Physical Attacks} -\author{\IEEEauthorblockN{ - Jan Sebastian Götte\IEEEauthorrefmark{1}\IEEEauthorrefmark{2} \and - Björn Scheuermann\IEEEauthorrefmark{1}\IEEEauthorrefmark{2} - }\\ - \IEEEauthorblockA{ - \IEEEauthorrefmark{1}Alexander von Humboldt Institut für Internet und Gesellschaft (HIIG)\\ - \IEEEauthorrefmark{2}Humboldt-Universität zu Berlin\\ - \texttt{\textbf{\small goette@jaseg.de}}, \texttt{\textbf{\small scheuermann@informatik.hu-berlin.de}} - } -} -\date{2021-01-05} -\maketitle - -\section*{Abstract} - -In this tech report, we introduce a novel countermeasure against physical attacks: Inertial hardware security modules -(iHSMs). 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. - -This tech report is the abridged version of our forthcoming paper. - -\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. 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. - -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 -conclude this paper with a general evaluation of our concept 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 yet. - -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. Although 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. Besides 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}. - -To the best of our knowledge, 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 certify and cheaply manufacture -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 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 a 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 mesh 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 easily 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. 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, after -subtracting gravity 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. Even at -moderate speeds above $\SI{500}{rpm}$, an attack would have to be carried out using a robot. - -\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 \emph{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 think 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. 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 motor 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. - -\begin{figure} - \center - \includegraphics{ir_tx_schema.pdf} - \caption{Example of a bidirectional IR communication link between rotor and stator, view along axis of rotation. 1 - - Rotor base plate. 2 - Stator base plate. 3 - Motor. 4 - receiver PIN photodiode. 5 - transmitter IR LED.} - \label{ir_tx_schema} -\end{figure} - -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. -A simple infrared optical link as shown in Figure~\ref{ir_tx_schema} may be a good solution for this purpose. - -\section{Conclusion} - -\label{sec_conclusion} To conclude, in this tech report 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. - -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{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. Once the full paper has been - published, this project's git repository will be available at:} - - \center{\url{https://git.jaseg.de/rotohsm.git}} -} -\end{document} -- cgit