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+---
+title: "Hardware Security Module Basics"
+date: 2019-05-17T15:29:20+08:00
+---
+
+Hardware Security Modules and Security Research and Cryptography
+================================================================
+
+On May 17 2019 I gave a short presentation on the fundamentals of hardware security modules at the weekly seminar of
+Prof. Mori's security research working group at Waseda University. The motivation for this was that outside of low-level
+hardware security people and people working in the financial industry HSMs are not thought about that often. In
+particular most network or systems security people would not consider them an option. Also it could turn out to be
+really interesting to think about what could be done with an HSM in conjunction with modern cryptography (instead of
+just plain old RSA-OAEP and AES-CBC).
+
+`Click here to download a PDF with the slides for this talk. <mori_semi_hsm_talk_web.pdf>`__
+
+Ideas for research in HSMs
+==========================
+
+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
+I'll find the time to properly research this topic, I thought it would be great to write down some rought outlines first
+for future reference.
+
+The Problem with current HSM tech
+---------------------------------
+
+Currently, HSMs are only used in certain specific niche applications such as certificate authority key management and
+financial transaction data handling. One key reason for this is that HSMs currently don't provide the affordances that
+would be needed for them to be adopted more widely by the cryptographic and security engineering community. As far as I
+can tell, the two core missing affordances are:
+
+1. To be more widely adopted, HSMs must become less expensive. Currently, they go for several tens of thousands of Euro,
+   which puts them outside most budgets.
+2. To be more widely adopted, HSMs must provide the standardized programming interfaces familiar to cryptographic
+   developers. Currently, every HSM vendor has their own custom cryptographic API and a developer will have to train on
+   one specific vendor's tooling. Furthermore, any documentation of these internals is kept secret behind NDAs. This
+   constitutes a high barrier to entry, decreasing adoption in particular with young developers accustomed to
+   open-source ecosystems.
+
+Attacking cost of implementation
+--------------------------------
+
+The first issue can be addressed by simply creating a viable low-cost alternative. There is no fundamental technical
+reason for the high cost of HSMs. This cost is instead due to manufacturers trying to recoup their expenses for R&D as
+well as certification from the small volumes HSMs are sold in.
+
+Compared to system integration and certification the pure R&D cost of HSM defense mechanisms themselves is not too high
+in an academic context it should be feasible to develop a sort of HSM blueprint that can then be cheaply produced by
+anyone in need.  Since the application areas outlined here are far from the core business areas of the clients of
+established HSM vendors this would most likely not be a realistic threat to any established vendor's business and a
+co-existence of both should not pose any problems in the short term.
+
+Benefits of an academic HSM standard
+------------------------------------
+
+Tackling the high cost of current HSM hardware with an open-source HSM blueprint would yield
+several academic advantages beyond cost reduction.
+
+1. An open-source blueprint could serve as an academic reference design to evaluate and compare other HSM designs
+   against. For instance this would not only allow quantifying the effectiveness of academic security measures but also
+   allow an evaluation of commercial HSMs.
+2. An open-source blueprint could stimulate academic research in this academically very quiet albeit commercially
+   important area. This research would ultimately benefit everyone employing HSMs by raising security standards in the
+   field. Since HSMs are never solely relied upon for overal system security both defensive and offensive security
+   research would yield these benefits.
+3. An open-source blueprint would encourage new people to get into the field and both apply HSMs to practical problems
+   as well as improve HSMs themselves. Currently, this is highly discouraged due to the strictly proprietary nature of
+   all available systems.
+4. Finally, developing an open-source HSM blueprint might yield new findings in adjacent academic areas due to the
+   hightly multi-disciplinary nature of security research in general and HSM design in particular.
+
+Scope of an academic HSM standard
+---------------------------------
+
+An academic HSM blueprint would need to be flexible so that researchers can adapt it to their particular problem. A
+modular architecture would lend itself to this flexibility. Fundamentally, there would be three components to this
+architecture. First, a **base** containing infrastructure such as the surveillance microcontroller, power supplies,
+power supply filtering and hardware DPA countermeasures, and possibly a standardized mechanical and electrical
+interface.
+
+Next to the base, a system integrator would put their *payload*. The nature of this payload is intentionally kept
+unspecified, and it might be anything from a cryptographic microcontroller to a small embedded system such as a
+raspberry pi single board computer. Keeping the *payload* open like this achieves two benefits: It gives the HSM
+blueprint's user *their* familiar tooling and the hardware *they* need, allowing fast adoption. Someone well-versed in
+e.g. Javascript could literally implement their cryptography in Javascript, run it on an off-the-shelf raspberry pi and
+just apply the HSM blueprint around it. In addition, keeping the *payload* open reduces the scope of what needs to be
+implemented. Building a general SDK on top of something like a bare ARM SoC such as a TI OMAP or a Freescale/NXP IMX
+would be a considerable additional burden, on top of the actual HSM design. Keeping the *payload* open allows research
+to concentrate on the actual point, the HSM design.
+
+The final and most important component would be a set of *security measures* that can be combined with the base to
+form the final HSM. Each of these *security measures* would entail a detailed specification of its design, manufacture
+and security properties. These *security measures* could be simple things like tamper switches or potting, but could
+also be complex things like security meshes.
+
+Given these three components -- *base*, *payload* and *security measures* as detailed specifications any engineer should
+be able to design and manufacture a HSM customized to their needs. Unifying these three components within the HSM
+blueprint would be a set of reference designs. Each reference design would implement a particular parametrization of the
+three architectural components with a physical hole cut out where the payload would go..  These reference designs would
+for one serve to guide any implementer on the customization and integration of their own derivation from the blueprint.
+In addition it would serve as an extremely simple, low-cost point of entry into the ecosystem. A curious researcher
+could simply replicate the reference design and put their existing payload inside.  Practically this might mean e.g. a
+researcher having PCBs produced according to the design files for a reference design for a mesh-based HSM, producing
+their own mesh, physically glueing a raspberry pi SBC into the middle of it, and potting the resulting system. Given the
+ease of prototype PCB fabrication today this would realistically allow evaluation of HSM technologies on a budget that
+is orders of magnitude less than the cost of current HSMs.
+
+Research ideas for tamper detection mechanisms
+==============================================
+
+The core component of an HSM blueprint would be a suite of tamper detection mechanisms. Following are a few ideas on how
+to improve on the current state of the art of membrane tamper switches plus temperature sensors plus PCB and printed
+security meshes plus potting.
+
+DIY or small lab mesh production
+--------------------------------
+**Analog sensing** meshes are a proven technology where instead of just monitoring for continuity and shorts, analog
+parameters of the mesh traces such as inductance and mutual capacitance are monitored. In 2019, `Immler et al. published
+a paper <https://tches.iacr.org/index.php/TCHES/article/view/7334>`__ where took this principle and turned it all the
+way up. They directly derived a cryptographic secret from the analog properties of their HSM's security mesh in an
+attempt to built a `Physically Unclonable Function, or PUF
+<https://en.wikipedia.org/wiki/Physical_unclonable_function>`__. The idea with PUFs is that they reproduce some entropy
+that comes from random tolerances of their production process. The same PUF will always yield (approximately) the same
+key, but since you cannot control these random production variations, in practice the resulting PUF cannot be cloned.
+Note however, that its secrets can of course be copied if you find a way to read them out.
+
+As Immler et al. demonstrated in their paper, you don't need any secret sauce to create an analog mesh sensing circuit.
+All you need are a bunch of (admittedly, expensive) off-the-shelf analog ICs. The interesting bit here is that by
+applying more advanced analog sensing, weaknesses of an otherwise coarse mesh desing could maybe be alleviated. That is,
+instead of monitoring a very fine mesh for continuity, you could instead closely monitor inductance and capacitance of a
+more coarse mesh. This trade-off between sensing circuit complexity (resp. cost) and mesh production capabilities may
+allow someone with a poorly equipped lab to still make a decent HSM. The question is, how do you produce a "decent" mesh
+given only basic tools? Here are some ideas.
+
+**3D metal patterning techniques** refers to any technique for producing thin, patterned metal structures on a
+three-dimensional plastic substrate. The basic process would consist of 3D-printing the polymer substrate, depositing a
+thin metal layer on top and then patterning this metal layer. A good starting point here would be the recent work of
+`Ben Kraznow`_ on this exact thing.
+
+**Copper filament methods** would be any method embedding copper wire from a spool in some resin or other matrix. This
+could mean either of a systematic approach of carefully winding or folding the copper wire into patterns or a
+non-systematic approach of simply stuffing a large tangle of copper wire into a small space. The main challenge with the
+former would be to find a non-tedious way of production. The main challenge with the latter would be to find process
+parameters that guarantee complete coverage of the HSM without holes or other areas of lower sensitivity to intrusions.
+Both approaches would require careful consideration of the overall design including the polymer resin supporting
+structure to ensure sensitivity against attacks since copper wire is mechanically much stronger than the micrometre-thin
+metal coatings used in patterning techniques.
+
+Envelope measurement
+--------------------
+
+Finally, I think there is another set of currently under-utilized tamper-detection methods that would be very
+interesting to explore. I am not aware of an academic term for these, so I am just going to dub them *envelope
+measurement* here.
+
+The fundamental apporach of a mesh is to build a physical security envelope (the mesh) that physically detects when it
+is disturbed (open or short circuits). This approach works well but has the disadvantage that these meshes are rather
+complex to manufacture since effectively every part of them is acting as a sensing element. A conceptually more complex
+but in practice potentially simpler approach might be to split the functions of security envelope and sensing element.
+This would mean that in place of the mesh, some form of passive element such as metal foil forms the security envelope
+which is then checked for tampering using a very sensitive sensor inside. This remote-sensing approach might simplify
+the manufacture of the envelope itself and thus yield a design that is more easily customized. Following are a few ideas
+on how to approach this envelope measurement problem.
+
+**Ultrasonic** If the HSM is potted, a few ultrasonic transducers could be added inside the potting. With several
+transducers, any one could be used to transmit ultrasound while the others measure complex phase and energy of the
+signal they receive. The circuitry for this could be made fairly simple if using a static transmit frequency  or a low
+chirp rate by using a homodyne receiver built around a comparator fed into some timers. This approach would likely
+detect any mechanical attack and would also rule out chemical attacks involving liquids (though starting from which
+amount of liquid depends on receiver sensitivity). The main disadvantages might be high power consumption and cost and
+size of the ultrasonic transducers. Traditional cheap transducers made for air as a transmission medium are fairly large
+and might not adequately couple into potting compound. If somehow one could convince a standard small piezo element to
+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
+suceptibility to ambient noise, though this could easily be reduced at the expense of space and heat dissipation
+capacity by adding sound dampening on the outside. A likely attack vector against this approach might be using a laser
+cutter to drill a hole through the potting compound, then inserting probes carefully chosen to not couple too much
+to the potting compound ultrasonically.
+
+**Light** In either an unpotted HSM or one potted with a transparent (at some wavelengths) potting compound one could
+embed LEDs and photodiodes in a similar setup to the ultrasonic setup described above. In contrast to the ultrasound,
+the LEDs would literally have to light up the HSM's interior and shadows might be an issue since the HSM is likely some
+flat rectangular shape. A possible solution to this would be to coat both the embedded payload and the lid with some
+highly reflective paint such as some glossy silver paint or simple white paint. The basic approach might be as simple as
+simply turning on several LEDs distributed throughout the HSM in turn and measuring amplitude at several photodetectors,
+or as complex as doing a LIDAR-like phase measurement sweeping through a range of frequencies to determine not only
+absorption but also phase/distance characteristics between emitter LED and detector photodiode. Using some high-gain TIA
+along with a homodyne detector (lock-in amplifier) and changing emitter intensity, very precise measurements of both
+absorption and phase might be possible, as might be measurements through almost opaque, diffuse potting compounds such
+as a grey epoxide resin. The main disadvantages of this method would likely be the need to thoroughly light-proof the
+entire HSM (likely by wrapping it in metal foil) and the potentially high cost of transmitter and receiver circuitry
+(nice TIAs aren't cheap). To be effective against attacks using e.g. very fine drills and probes the system would likely
+have to be very sensitive.
+
+**Radar** Finally, one could turn to standard radar techniques to fingerprint the inside of the HSM. The goal here would
+be fingerprinting instead of mapping since only changes need to be detected. In this approach one could use homodyne
+detection to improve sensitivity and reduce receiver complexity, and sweep frequencies similar to an FMCW radar (but
+probably without exploiting the self-demodulation effect). Besides high cost, this approach has two disadvantages.
+First, such a system would likely not go beyond 24GHz or maybe 40GHz due to component availability issues. Even at 40GHz
+the wavelength in the potting compound would be in the order of magnitude of several millimeters. Fine intrusions using
+some tool chosen to not interact too much with the EM field inside the HSM such as a heated ceramic needle or simply a
+laser cutter might not be detectable using this approach. In any case, this system would certainly not be able to detect
+small holes piercing the HSM enclosure. The HSM enclosure would have to be made into an RF shield, likely by using some
+metal foil in it.
+
+Overall in the author's opinion these three techniques are most promising in order *Light*, *Ultrasonic*, *Radar*. Light
+would prbably provide the best sensitivity at expense of some cost. Ultrasonic might be used in conjunction with light
+to cover some additional angles since it is potentially very low-cost. Radar seems hard to engineer into a solution that
+works reliably and also would likely be at least an order of magnitude more expensive than the other two technologies
+while not providing better sensitivity.
+
+.. _`Ben Kraznow`: https://www.youtube.com/watch?v=Z228xymQYho
+.. _affordances: https://en.wikipedia.org/wiki/Affordance
+