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<h1>Hardware Security Module Basics</h1>
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<ul class="breadcrumbs">
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<li><a href="/">jaseg.de</a></li>
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<li><a href="/blog/">Blog</a></li><li><a href="/blog/hsm-basics/">Hardware Security Module Basics</a></li>
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<strong>2019-05-17</strong>
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<div class="document">
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<div class="section" id="hardware-security-modules-and-security-research-and-cryptography">
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<h2>Hardware Security Modules and Security Research and Cryptography</h2>
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<p>On May 17 2019 I gave a short presentation on the fundamentals of hardware security modules at the weekly seminar of
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Prof. Mori's security research working group at Waseda University. The motivation for this was that outside of low-level
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hardware security people and people working in the financial industry HSMs are not thought about that often. In
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particular most network or systems security people would not consider them an option. Also it could turn out to be
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really interesting to think about what could be done with an HSM in conjunction with modern cryptography (instead of
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just plain old RSA-OAEP and AES-CBC).</p>
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<p><a class="reference external" href="mori_semi_hsm_talk_web.pdf">Click here to download a PDF with the slides for this talk.</a></p>
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</div>
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<div class="section" id="ideas-for-research-in-hsms">
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<h2>Ideas for research in HSMs</h2>
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<p>Preparing for this talk brought me back to some research ideas I've been working on for a while now. Since I'm not sure
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I'll find the time to properly research this topic, I thought it would be great to write down some rought outlines first
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for future reference.</p>
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<div class="section" id="the-problem-with-current-hsm-tech">
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<h3>The Problem with current HSM tech</h3>
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<p>Currently, HSMs are only used in certain specific niche applications such as certificate authority key management and
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financial transaction data handling. One key reason for this is that HSMs currently don't provide the affordances that
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would be needed for them to be adopted more widely by the cryptographic and security engineering community. As far as I
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can tell, the two core missing affordances are:</p>
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<ol class="arabic simple">
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<li>To be more widely adopted, HSMs must become less expensive. Currently, they go for several tens of thousands of Euro,
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which puts them outside most budgets.</li>
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<li>To be more widely adopted, HSMs must provide the standardized programming interfaces familiar to cryptographic
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developers. Currently, every HSM vendor has their own custom cryptographic API and a developer will have to train on
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one specific vendor's tooling. Furthermore, any documentation of these internals is kept secret behind NDAs. This
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constitutes a high barrier to entry, decreasing adoption in particular with young developers accustomed to
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open-source ecosystems.</li>
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</ol>
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</div>
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<div class="section" id="attacking-cost-of-implementation">
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<h3>Attacking cost of implementation</h3>
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<p>The first issue can be addressed by simply creating a viable low-cost alternative. There is no fundamental technical
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reason for the high cost of HSMs. This cost is instead due to manufacturers trying to recoup their expenses for R&D as
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well as certification from the small volumes HSMs are sold in.</p>
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<p>Compared to system integration and certification the pure R&D cost of HSM defense mechanisms themselves is not too high
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in an academic context it should be feasible to develop a sort of HSM blueprint that can then be cheaply produced by
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anyone in need. Since the application areas outlined here are far from the core business areas of the clients of
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established HSM vendors this would most likely not be a realistic threat to any established vendor's business and a
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co-existence of both should not pose any problems in the short term.</p>
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</div>
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<div class="section" id="benefits-of-an-academic-hsm-standard">
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<h3>Benefits of an academic HSM standard</h3>
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<p>Tackling the high cost of current HSM hardware with an open-source HSM blueprint would yield
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several academic advantages beyond cost reduction.</p>
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<ol class="arabic simple">
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<li>An open-source blueprint could serve as an academic reference design to evaluate and compare other HSM designs
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against. For instance this would not only allow quantifying the effectiveness of academic security measures but also
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allow an evaluation of commercial HSMs.</li>
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<li>An open-source blueprint could stimulate academic research in this academically very quiet albeit commercially
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important area. This research would ultimately benefit everyone employing HSMs by raising security standards in the
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field. Since HSMs are never solely relied upon for overal system security both defensive and offensive security
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research would yield these benefits.</li>
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<li>An open-source blueprint would encourage new people to get into the field and both apply HSMs to practical problems
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as well as improve HSMs themselves. Currently, this is highly discouraged due to the strictly proprietary nature of
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all available systems.</li>
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<li>Finally, developing an open-source HSM blueprint might yield new findings in adjacent academic areas due to the
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hightly multi-disciplinary nature of security research in general and HSM design in particular.</li>
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</ol>
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</div>
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<div class="section" id="scope-of-an-academic-hsm-standard">
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<h3>Scope of an academic HSM standard</h3>
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<p>An academic HSM blueprint would need to be flexible so that researchers can adapt it to their particular problem. A
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modular architecture would lend itself to this flexibility. Fundamentally, there would be three components to this
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architecture. First, a <strong>base</strong> containing infrastructure such as the surveillance microcontroller, power supplies,
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power supply filtering and hardware DPA countermeasures, and possibly a standardized mechanical and electrical
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interface.</p>
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<p>Next to the base, a system integrator would put their <em>payload</em>. The nature of this payload is intentionally kept
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unspecified, and it might be anything from a cryptographic microcontroller to a small embedded system such as a
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raspberry pi single board computer. Keeping the <em>payload</em> open like this achieves two benefits: It gives the HSM
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blueprint's user <em>their</em> familiar tooling and the hardware <em>they</em> need, allowing fast adoption. Someone well-versed in
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e.g. Javascript could literally implement their cryptography in Javascript, run it on an off-the-shelf raspberry pi and
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just apply the HSM blueprint around it. In addition, keeping the <em>payload</em> open reduces the scope of what needs to be
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implemented. Building a general SDK on top of something like a bare ARM SoC such as a TI OMAP or a Freescale/NXP IMX
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would be a considerable additional burden, on top of the actual HSM design. Keeping the <em>payload</em> open allows research
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to concentrate on the actual point, the HSM design.</p>
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<p>The final and most important component would be a set of <em>security measures</em> that can be combined with the base to
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form the final HSM. Each of these <em>security measures</em> would entail a detailed specification of its design, manufacture
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and security properties. These <em>security measures</em> could be simple things like tamper switches or potting, but could
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also be complex things like security meshes.</p>
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<p>Given these three components -- <em>base</em>, <em>payload</em> and <em>security measures</em> as detailed specifications any engineer should
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be able to design and manufacture a HSM customized to their needs. Unifying these three components within the HSM
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blueprint would be a set of reference designs. Each reference design would implement a particular parametrization of the
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three architectural components with a physical hole cut out where the payload would go.. These reference designs would
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for one serve to guide any implementer on the customization and integration of their own derivation from the blueprint.
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In addition it would serve as an extremely simple, low-cost point of entry into the ecosystem. A curious researcher
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could simply replicate the reference design and put their existing payload inside. Practically this might mean e.g. a
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researcher having PCBs produced according to the design files for a reference design for a mesh-based HSM, producing
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their own mesh, physically glueing a raspberry pi SBC into the middle of it, and potting the resulting system. Given the
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ease of prototype PCB fabrication today this would realistically allow evaluation of HSM technologies on a budget that
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is orders of magnitude less than the cost of current HSMs.</p>
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</div>
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</div>
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<div class="section" id="research-ideas-for-tamper-detection-mechanisms">
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<h2>Research ideas for tamper detection mechanisms</h2>
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<p>The core component of an HSM blueprint would be a suite of tamper detection mechanisms. Following are a few ideas on how
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to improve on the current state of the art of membrane tamper switches plus temperature sensors plus PCB and printed
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security meshes plus potting.</p>
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<div class="section" id="diy-or-small-lab-mesh-production">
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<h3>DIY or small lab mesh production</h3>
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<p><strong>Analog sensing</strong> meshes are a proven technology where instead of just monitoring for continuity and shorts, analog
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parameters of the mesh traces such as inductance and mutual capacitance are monitored. In 2019, <a class="reference external" href="https://tches.iacr.org/index.php/TCHES/article/view/7334">Immler et al. published
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a paper</a> where took this principle and turned it all the
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way up. They directly derived a cryptographic secret from the analog properties of their HSM's security mesh in an
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attempt to built a <a class="reference external" href="https://en.wikipedia.org/wiki/Physical_unclonable_function">Physically Unclonable Function, or PUF</a>. The idea with PUFs is that they reproduce some entropy
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that comes from random tolerances of their production process. The same PUF will always yield (approximately) the same
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key, but since you cannot control these random production variations, in practice the resulting PUF cannot be cloned.
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Note however, that its secrets can of course be copied if you find a way to read them out.</p>
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<p>As Immler et al. demonstrated in their paper, you don't need any secret sauce to create an analog mesh sensing circuit.
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All you need are a bunch of (admittedly, expensive) off-the-shelf analog ICs. The interesting bit here is that by
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applying more advanced analog sensing, weaknesses of an otherwise coarse mesh desing could maybe be alleviated. That is,
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instead of monitoring a very fine mesh for continuity, you could instead closely monitor inductance and capacitance of a
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more coarse mesh. This trade-off between sensing circuit complexity (resp. cost) and mesh production capabilities may
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allow someone with a poorly equipped lab to still make a decent HSM. The question is, how do you produce a "decent" mesh
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given only basic tools? Here are some ideas.</p>
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<p><strong>3D metal patterning techniques</strong> refers to any technique for producing thin, patterned metal structures on a
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three-dimensional plastic substrate. The basic process would consist of 3D-printing the polymer substrate, depositing a
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thin metal layer on top and then patterning this metal layer. A good starting point here would be the recent work of
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<a class="reference external" href="https://www.youtube.com/watch?v=Z228xymQYho">Ben Kraznow</a> on this exact thing.</p>
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<p><strong>Copper filament methods</strong> would be any method embedding copper wire from a spool in some resin or other matrix. This
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could mean either of a systematic approach of carefully winding or folding the copper wire into patterns or a
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non-systematic approach of simply stuffing a large tangle of copper wire into a small space. The main challenge with the
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former would be to find a non-tedious way of production. The main challenge with the latter would be to find process
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parameters that guarantee complete coverage of the HSM without holes or other areas of lower sensitivity to intrusions.
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Both approaches would require careful consideration of the overall design including the polymer resin supporting
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structure to ensure sensitivity against attacks since copper wire is mechanically much stronger than the micrometre-thin
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metal coatings used in patterning techniques.</p>
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</div>
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<div class="section" id="envelope-measurement">
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<h3>Envelope measurement</h3>
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<p>Finally, I think there is another set of currently under-utilized tamper-detection methods that would be very
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interesting to explore. I am not aware of an academic term for these, so I am just going to dub them <em>envelope
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measurement</em> here.</p>
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<p>The fundamental apporach of a mesh is to build a physical security envelope (the mesh) that physically detects when it
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is disturbed (open or short circuits). This approach works well but has the disadvantage that these meshes are rather
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complex to manufacture since effectively every part of them is acting as a sensing element. A conceptually more complex
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but in practice potentially simpler approach might be to split the functions of security envelope and sensing element.
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This would mean that in place of the mesh, some form of passive element such as metal foil forms the security envelope
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which is then checked for tampering using a very sensitive sensor inside. This remote-sensing approach might simplify
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the manufacture of the envelope itself and thus yield a design that is more easily customized. Following are a few ideas
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on how to approach this envelope measurement problem.</p>
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<p><strong>Ultrasonic</strong> If the HSM is potted, a few ultrasonic transducers could be added inside the potting. With several
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transducers, any one could be used to transmit ultrasound while the others measure complex phase and energy of the
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signal they receive. The circuitry for this could be made fairly simple if using a static transmit frequency or a low
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chirp rate by using a homodyne receiver built around a comparator fed into some timers. This approach would likely
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detect any mechanical attack and would also rule out chemical attacks involving liquids (though starting from which
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amount of liquid depends on receiver sensitivity). The main disadvantages might be high power consumption and cost and
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size of the ultrasonic transducers. Traditional cheap transducers made for air as a transmission medium are fairly large
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and might not adequately couple into potting compound. If somehow one could convince a standard small piezo element to
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do the same job that would be great as far as cost and size are concerned. A concern in some fringe use cases might be
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suceptibility to ambient noise, though this could easily be reduced at the expense of space and heat dissipation
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capacity by adding sound dampening on the outside. A likely attack vector against this approach might be using a laser
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cutter to drill a hole through the potting compound, then inserting probes carefully chosen to not couple too much
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to the potting compound ultrasonically.</p>
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<p><strong>Light</strong> In either an unpotted HSM or one potted with a transparent (at some wavelengths) potting compound one could
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embed LEDs and photodiodes in a similar setup to the ultrasonic setup described above. In contrast to the ultrasound,
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the LEDs would literally have to light up the HSM's interior and shadows might be an issue since the HSM is likely some
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flat rectangular shape. A possible solution to this would be to coat both the embedded payload and the lid with some
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highly reflective paint such as some glossy silver paint or simple white paint. The basic approach might be as simple as
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simply turning on several LEDs distributed throughout the HSM in turn and measuring amplitude at several photodetectors,
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or as complex as doing a LIDAR-like phase measurement sweeping through a range of frequencies to determine not only
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absorption but also phase/distance characteristics between emitter LED and detector photodiode. Using some high-gain TIA
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along with a homodyne detector (lock-in amplifier) and changing emitter intensity, very precise measurements of both
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absorption and phase might be possible, as might be measurements through almost opaque, diffuse potting compounds such
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as a grey epoxide resin. The main disadvantages of this method would likely be the need to thoroughly light-proof the
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entire HSM (likely by wrapping it in metal foil) and the potentially high cost of transmitter and receiver circuitry
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(nice TIAs aren't cheap). To be effective against attacks using e.g. very fine drills and probes the system would likely
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have to be very sensitive.</p>
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<p><strong>Radar</strong> Finally, one could turn to standard radar techniques to fingerprint the inside of the HSM. The goal here would
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be fingerprinting instead of mapping since only changes need to be detected. In this approach one could use homodyne
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detection to improve sensitivity and reduce receiver complexity, and sweep frequencies similar to an FMCW radar (but
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probably without exploiting the self-demodulation effect). Besides high cost, this approach has two disadvantages.
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First, such a system would likely not go beyond 24GHz or maybe 40GHz due to component availability issues. Even at 40GHz
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the wavelength in the potting compound would be in the order of magnitude of several millimeters. Fine intrusions using
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some tool chosen to not interact too much with the EM field inside the HSM such as a heated ceramic needle or simply a
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laser cutter might not be detectable using this approach. In any case, this system would certainly not be able to detect
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small holes piercing the HSM enclosure. The HSM enclosure would have to be made into an RF shield, likely by using some
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metal foil in it.</p>
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<p>Overall in the author's opinion these three techniques are most promising in order <em>Light</em>, <em>Ultrasonic</em>, <em>Radar</em>. Light
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would prbably provide the best sensitivity at expense of some cost. Ultrasonic might be used in conjunction with light
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to cover some additional angles since it is potentially very low-cost. Radar seems hard to engineer into a solution that
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works reliably and also would likely be at least an order of magnitude more expensive than the other two technologies
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while not providing better sensitivity.</p>
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