222 lines
13 KiB
TeX
222 lines
13 KiB
TeX
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\begin{document}
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\title{A High-Security Physical Security Primitive Based On Mechanical Movement}
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\author{Jan Götte}
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\date{2020-09-15}
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\maketitle
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\section{Abstract}
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In this paper, we introduce a novel, highly effective countermeasure against physical attacks: Inertial hardware
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security modules. Whereas conventional technology can be categorized into systems monitoring a thin boundary (such as
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security meshes) and systems monitoring the interior volume (such as the "enclosure PUF" of Tobisch et al.). What all of
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these systems have in common is that they try to detect attacks by crafting sensors responding to increasingly minute
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manipulations of the monitored medium. Our approach is novel in that we alleviate the sensitivity requirement of a
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security mesh by increasing the complexity of any manipulation at all by orders of magnitude by fastly rotating the
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security mesh--presenting a moving target to an attacker. Attempts to modify the rotation itself are easily monitored
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with commercial MEMS accelerometers and gyroscopes.
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Our approach leads to a HSM that can easily be built from off-the-shelf parts by any university electronics lab, yet is
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as secure or more secure than even the best commercial offerings.
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\section{Introduction}
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Since the early days of computers, physical security has often been a core component of any computer system's security
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architecture. Physical security in fact predates our modern concept of computer security by decades. Long before
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passwords, access control lists, role-based authentication and other modern concepts of information security were
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developed, information was secured by physically locking away the computers that held it.
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Nowadays, concerns of physical security are mostly limited to certain applications. Credit card processing and medical
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data processing are two instances where a combination of smartcards and hardware security modules is used to provide a
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higher level of security than what ordinary computers can provide. Meanwhile, in most commercial data processing
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applications, the physical security provided by an average datacenter is considered to be appropriate.
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In modern systems, phyiscal security always is tightly interwoven with the system's overall security architecture.
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Beyond the level provided by locks and guards, it is generally considered infeasible to physically secure all parts of a
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computer. High-level physical security is usually limited to either a single chip or part of a chip such as a secure
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element, enclave or smartcards--or it is limited to a small module acting within a very limited scope, as is the case in
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commercial HSMs that largely act as cryptographic co-processors with built-in key management functions.
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\subsection{Technical approaches to physical security}
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The use of chips as secure elements has recently become popular beyond the smartcards of yesteryear. Apple has carried
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over a secure enclave IC from their line of phones into their line of laptops in 2016. Likewise, Google has developed
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its own security IC for use in phones and laptops. An issue to consider with all such IC-based security solutions is
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that they do not provide any cryptographic security. The real-world security of these solutions solely rests on the
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assumption that due to their fine structure, ICs are hard to reverse engineer and manipulate. As of now, this property
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holds and in the authors' opinion it will likely be a reasonable assumptions for some years to come. However, in its
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essence this is a type of security by obscurity: Obscurity here mostly applying to the rarity of tools that are
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necessary for practical attacks such as focused ion beam workstations and accompanying sample preparation equipment. An
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important observation in this regard is that already, several people are slowly chipping away at this obscurity: A group
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at Ruhr University Bochum is working on advanced tooling for netlist reverse engineering, and there are several
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companies offering commercial IC reverse engineering services.
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\subsection{Hardware Security Modules}
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At larger physical dimensions, hardware security modules (HSMs) provide an effective solution to the problem: In
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conformity with Kerckhoff's principle, their creators do not try to hide the structure of the system within. Instead,
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the HSM monitors it for any manipulation and wipes all key material when one is detected. The most common commercial
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realization of this is what we call a "boundary-monitoring" HSM. This is a device uses a microcontroller monitoring the
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conductivity of usually two electrical traces that are folded many times to cover the entire area of a plastic enclosure
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part or a plastic foil wrapped around the module. The security problem thus gets transformed into a manufacturing
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challenge: How fine can these traces be made--so they are disturbed by even the tiniest of holes for say, a fine needle;
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and how sensitive can they be made to perturbations--so they break from even gentle attempts at mechanical, chemical or
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other physical manipulation.
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The other type of HSM that so far has garnered mostly academic interest are what we call "volumetric" HSMs. Where a
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boundary-monitoring HSM senses disturbations to a thin boundary between its inside and the outside world, a volumetric
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HSM monitors its entire interior volume. Approaches that have been proposed so far include monitoring using
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electromagnetic radiation % FIXME: citation (paper1 (this chip thing w/ distributed PAs/LNAs), paper2 (RUB)
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and ultrasonic sensing. % FIXME: citation
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Common to both approaches is that for technical reasons the wavelength of the employed radiation is in the range of
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millimeters or larger. This implies that practical attacks acting on a smaller scale of physical size require sensitive
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monitoring circuity to be reliably caught. % FIXME maybe talk to a physicist here.
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Since they require advanced transceivers and signal processing, these HSMs incur a high implementation cost compared to
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one based on a traditional security mesh, while they in turn promise to be easier and less expensive to scale in
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physical size. A severe problem with any previous volumetric designs is that their security analysis is very hard. While
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multiple designs have been proposed academically, none of these proposals include an analysis of their physical security
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properties that goes beyond guesswork. %FIXME verify this.
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The obvious reason for this is that to evaluate the volume inside the HSM that is covered by a given transceiver
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combination and a given test signal pattern necessarily requires numerically solving the volumetric electromagnetic
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field equations inside the HSM, applying a model of transmitter and receiver to the results that takes into account
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receiver sensitivity and ADC resolution, transmitter power and receiver saturation effects and then validating that
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every point in space (or at least inside a boundary region) is covered. While the guess that attacks are impractical
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might still be true this would be based on the fact that the same problem presents itself to an attacker trying to
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circumvent these measures--degrading their security to simple obscurity again.
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\subsection{A new approach to physical security}
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We are certain that there is still much work to be done and many insights to be gained from further explorations
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of the two concepts described above. Trivially, consider a box with mirrored walls that, suspended on thin wires,
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contains a smaller box that has cameras looking outward in all directions at the mirrored walls. Given that the defender
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can control lighting conditions inside this kaleidoscopic box in this application modern cameras can be considered
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equivalent to or better than the human eye. Thus, a successful physical attack on this system would likely an
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"invisibility cloaks"--and the system would remain secure as long as no such thing exists. This example is a useful
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point of reference. To be viable, a HSM technology must be either smaller or more sensitive than such a setup.
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The candidate we wish to introduce in this paper uses a novel approach to side-step the issues of both the concepts
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introduced in the previous section and provides radically better security against physical attacks--both in theory and
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in practice.
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Our core observation is that given any less expensive but more coarse HSM technology, we can make it radically more
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difficult to attack by introducing fast mechanical motion. As a trivial example, consider a HSM as it is used in
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ecommerce applications for credit card payments. Focusing on its main defense for simplicity, its physical security is
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limited by the structure size of the mesh that is likely used in its shell. If an attacker can tap the mesh's electrical
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traces and bridge across the mesh in a way the HSM cannot detect (e.g. by making sure the bridge has the same electrical
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impedance as the mesh traces have e.g. by comparing against another device of the same type), they have circumvented the
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device's protections. Any such attack would likely involve some fine drill bits, needles, wires, glue, perhaps solder or
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even lasers.
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Now consider the same HSM, but this time mounted on a large flywheel. In this scenario the HSM uses the same
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protections as before, but is now additionally equipped with an accelerometer that it uses to verify that it is in fact
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rotating at a very high speed. How would an attacker approach this HSM? They would have to either slow down the rotation
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(which would quickly be sensed by the accelerometer) or they would have to attack the moving HSM--the HSM literally
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becomes a moving target. While rotating the entire attack workbench might be possible for slow speeds, rotating frames
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of reference quickly become inhospitable to human life and at some point the technical means to rotate a CNC attack
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robot probably weighing several kilograms become inconvenient as well. Contact-less EM or optical attacks are more
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limited in the first place, and can effectively be shielded.
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\section{Related work}
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% summaries of research papers on HSMs.
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% I have not found any actual prior art on anything involving mechanical motion beyond ultrasound.
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\section{The physics of hardware security}
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% approaching the issue from measurable quantities
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\section{Intertial HSMs}
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\section{Hardware prototype}
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\section{Future work}
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\subsection{Other modes of movement}
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\subsection{Multiple axes of rotation}
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\subsection{Means of power transmission}
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\subsection{Other sensing modes}
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\subsection{Longeivity}
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\section{Conclusion}
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\printbibliography[heading=bibintoc]
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\appendix
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\section{License}
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{\center{
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\begin{minipage}[t][10cm][b]{\textwidth}
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\center{\ccbysa}
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\center{This work is licensed under a Creative-Commons ``Attribution-ShareAlike 4.0 International'' license. The
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full text of the license can be found at:}
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\center{\url{https://creativecommons.org/licenses/by-sa/4.0/}}
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\center{For alternative licensing options, source files, questions or comments please contact the authors.}
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\center{This is version \texttt{\input{version.tex}\unskip} generated on \today. The git repository can be found at:}
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\center{\url{https://git.jaseg.de/rotohsm.git}}
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\end{minipage}
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}}
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\end{document}
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