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+title: "{{ replace .Name "-" " " | title }}"
+date: {{ .Date }}
+draft: true
+---
+

config.toml

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+baseURL = "http://jaseg.de/"
+languageCode = "en-us"
+title = "Home"
+copyright = "Jan Sebastian Götte"
+theme = "conspiracy"
+enableRobotsTXT = true
+
+[outputs]
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+taxonomy = ['html', 'rss']
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+url = "/"
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+[[menu.main]]
+name = "Blog"
+url = "/blog/"
+weight = 2
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+[[menu.main]]
+name = "Projects"
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+weight = 3
+
+[[menu.main]]
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+weight = 4
+
+[[params.profile_links]]
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+weight = 2
+
+[[params.profile_links]]
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+url = "https://gitlab.com/neinseg"
+weight = 3
+
+[[params.profile_links]]
+name = "Mastodon"
+url = "https://chaos.social/@jaseg"
+weight = 4
+
+[[params.footer_links]]
+name = "About"
+url = "/about/"
+weight = 1
+
+[[params.footer_links]]
+name = "Imprint"
+url = "/imprint/"
+weight = 2
+
+[[params.homepage_categories]]
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+key = "blog"
+weight = 2
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content/_index.rst

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+---
+title: jaseg.de
+---
+
+Hi there, and welcome to my personal website.
+
+I'm jaseg, and I write about my projects here. You can find long-form articles in the blog, and links to my open-source
+projects on the projects page. On the top right of this page, there are links to my git repositories and social media
+pages. If you want to learn more about me, head over to the about page.

content/about/index.rst

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+---
+title: "About jaseg"
+---
+
+About 
+-----
+
+Hej, I'm Jan, or jaseg. At the moment I'm doing a PhD (Dr.-Ing.) at TU Darmstadt in Computer Science, specializing on
+Hardware Security. This is my personal website where I publish things that I find interesting.
+
+I self-host my code at `git.jaseg.de <https://git.jaseg.de/>`__, but I am also on `github <https://github.com/jaseg>`__
+and on `gitlab <https://gitlab.com/neinseg>`__. I use github for issue tracking for some of my projects such as
+`gerbolyze <https://github.com/jaseg/gerbolyze>`__ and `python-mpv <https://github.com/jaseg/python-mpv>`__. I maintain
+the `python-mpv <https://pypi.org/project/python-mpv/>`__ and `gerbolyze <https://pypi.org/project/gerbolyze/>`__ python
+packages on PyPI. Release tags on these two repositories are signed with the release signing key found `on github
+<https://github.com/jaseg.gpg>`__ and below.
+
+I am not on any social network, but feel free to write me an email at `hello@jaseg.de
+<mailto:hello@jaseg.de?subject=About\ page\ on\ blog.jaseg.de>`__.
+
+I do not use application-level email encryption such as S/MIME or PGP. If you need a higher level of secrecy than
+regular old email provides, please ask around for my signal contact or email me a file encrypted using `age
+<https://github.com/FiloSottile/age>`__ with one of the SSH keys listed `on my github
+<https://github.com/jaseg.keys>`__.  You can find both PGP and other SSH keys that I have used in the past on the
+internet, but please consider these keys revoked, and do not use them to encrypt anything you send me.
+
+Python package release signing key
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+I use this GPG key (key ID ``ED7A208EEEC76F2D``) to sign git release tags of both `gerbolyze <https://github.com/jaseg/gerbolyze>`__ and `python-mpv
+<https://github.com/jaseg/python-mpv>`__:
+    
+.. code::
+
+    -----BEGIN PGP PUBLIC KEY BLOCK-----
+    mDMEXom49xYJKwYBBAHaRw8BAQdA/KrWMt2MKGIZUvlQZnWjNd6i8/ZYjRsBQqEf
+    PJ8pJ+20NHB5dGhvbi1tcHYgUmVsZWFzZSBTaWduaW5nIEtleSA8cHl0aG9uLW1w
+    dkBqYXNlZy5kZT6IlgQTFggAPhYhBONvdTB/Cg7C0UX/XO16II7ux28tBQJeibj3
+    AhsDBQkSzAMABQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAAAoJEO16II7ux28thRYA
+    /3Yl1RdeUGor6K0RTxce9TIBB+DpLNupJgB9f6onuocpAQC614zQ/RQ6rkGTHCwA
+    ElFClWRQ5eppj0jpAuH15udqAbg4BF6JuPcSCisGAQQBl1UBBQEBB0A0mrXSv6rj
+    ajCmZR4H4OtowAx477YS+yWARqo1NtdgJwMBCAeIfgQYFggAJhYhBONvdTB/Cg7C
+    0UX/XO16II7ux28tBQJeibj3AhsMBQkSzAMAAAoJEO16II7ux28tMZwBAIUpHHvP
+    gRW2jQuzdw1r06kItfFk/0t+mgNUQ2+vtbhzAP98BoWx7lv+bvlIbBaVgLldusj0
+    pHnZI/0y3ksMBkdbBw==
+    =Mr6G
+    -----END PGP PUBLIC KEY BLOCK-----
+
+About this site
+---------------
+
+This site is made with the hugo static site generator. I made the theme myself, feel free to grab a copy at
+`git.jaseg.de <https://git.jaseg.de/blog.git/tree/themes/conspiracy?h=main>`__. The nifty auto-reflowing code embeds are
+made with some CSS magic I made that you can find in `style.css
+<https://git.jaseg.de/blog.git/tree/themes/conspiracy/assets/css/style.css?h=main&id=2fd22e30ce176d8d8a641fd371ad1623b082eaaf#n367>`__.
+The body text is typeset in Roboto Slab, created by `Christian Robertson <https://christianrobertson.com/>`__ while
+working at Google. The headlines are set in Nyght Serif, a font by `Maksym Kobuzan <https://linktr.ee/mkobuzan>`__.
+Check out their other fonts, their work is beautiful! Source code is typeset in Fira Code, a derivate by ... from
+Mozilla's `Fira Mono <https://github.com/mozilla/Fira>`__ font, designed by `Erik Spiekermann
+<https://spiekermann.com/>`__, `Ralph du Carrois <https://carrois.com/>`__, `Anja Meiners
+<https://anjameiners.com/de/hallo/>`__ and Botio Nikoltchev of Carrois Type Design, now succeeded by `bBoxType
+<https://bboxtype.com/typefaces/FiraMono/#!layout=specimen>`__ , and Patryk Adamczyk of Mozilla. The photo of mountains
+that's used in the background of this site is by `Fabrizio Conti <https://www.conti.photos/>`__ and can be found on
+`Unsplash <https://unsplash.com/photos/TUmjK7ZJgbI>`__.
+

content/blog/8seg/index.rst

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+---
+title: "8seg Technical Overview"
+date: 2023-12-26T15:26:00+01:00
+summary: >
+    8seg is a large-scale LED light art installation that displays text on a 1.5 meter high, 30 meter wide
+    8-segment display made from cheap LED tape.
+---
+
+Prologue
+--------
+
+German hacker culture has this intense love for things that light up in colorful ways. Like for many others in this
+community, I have always been fascinated by LEDs. One of the first things on my pile of unfinished projects was to build
+my own LED matrix and use it to display text. When I started that project, I was still new to electronics. Back then,
+commercial LED matrices were limited to red or green color only, and were very expensive, so there was an incentive to
+build your own. At the same time, while individual LEDs were'nt expensive anymore, they hadn't started to be cheap yet,
+either. On top of the material cost, back then there were no PCB fabs, and especially no PCB assembly houses that a
+hobbyist could afford. Ultimately, I ended up never finishing this project because I felt it was more of a feat of
+material wealth than of technical prowess.
+
+Over time, LEDs came down in price, and peoople started using them in all sorts of fun things. Around the mid-2010s,
+cheap-ish, ready-made tapes and chains of RGB LEDs that included WS2811 or similar digitally controllable driver chips
+led to a cambrian explosion in projects involving large amounds of colorful LEDs since suddenly, all you needed was an
+arduino and a beefy power supply to individually control an almost unlimited number of these LEDs.
+
+Today, LED technology has advanced even furhter, to a point where now you can buy staggering quantities of the second
+generation of these controllable LEDs that provides better color rendering embedded in all sorts of shapes, from tapes
+through rings to grids. When I built the first matelight_ in 2013, the matelight's 640 individually-controllable LEDs
+were *a lot*. Today, you can buy a roll with several thousand channels for about the price of a nice pizza.
+
+The idea behind 8seg
+--------------------
+
+Living through this amazing escalation of LED technology, in 2018, I looked at a then-obsolete piece of single-color,
+dumb, non-controllable LED tape with a simple question in mind: Taking this unsophisticated artifact of yesterday's
+technology, what would be the coolest thing I could build from it? Can I buld something that not only rivals, but
+outmatches the modern controllable LED stuff? From that question, I set myself two goals. First, I wanted to keep the
+project's use of financial and labor resources reasonable. A lot of art consists of taking a simple idea, and simply
+extrapolating its implementation to a ridiculous scale at the expense of the artist's time and wallet. That wasn't the
+point I wanted to make. I wanted to make something cool from an obsolete technology, not prove how much patience I had
+soldering. My second goal was to create something that is meaningfully controllable. Controllability is much harder with
+these dumb LED tapes, but it is possible nontheless, and I wanted to test out how far you could go with it.
+
+After thinking through a number of possibilities, I settled on the basics of the 8seg design I ended up realizing. The
+installation would be a banner-style display consisting of a series of characters made from non-controllable LED tape.
+The banner can be rigged up in any convenient air space, bending and folding to conform to the space's shape and size.
+The key idea behind 8seg is that it makes up for it's lack of control fidelity with sheer size. If nothing else, this
+non-controllable LED tape is *cheap*.
+
+The design of a single 8seg character
+-------------------------------------
+
+Each 8seg character consists of 8 *segments* of LED tape that are inter-connected through small circuit boards, four in
+the corners, and one in the center. As it turns out, 8 segments arranged in this shape are enough to display all of the
+English language's alphabet as well as numbers in a weird, but readable form.
+
+The electrical design of an 8seg character has one weird trick at its core. To avoid having to run a bunch of wires from
+some kind of driver circuit board to each of the eight segments, I thought, why not use the LED tape itself instead for
+power and data transmission? Wires are heavy, expensive, and annoying to solder, so if I could find a way to
+interconnect the LED tape so that it can all be driven from a driver circuit located at one of the character's
+junctions while simultaneously powering that driver circuit, an 8seg character wouldn't need any wires at all anymore.
+
+8seg achieves this feat using a circuit as shown in the diagram below. Interconnections between the LED tape segments
+are done with a small circuit board in each of the four corners. The design is rotationally symmetric, and all four of
+these boards are identicaly. The top right and bottom left corners simply use the back side of the same circuit board
+used in the top left and bottom right corners.
+
+.. image:: 8seg-digit-circuit.png
+
+The driver circuit sits at the center of the character and directly connects to the four diagonal segments. The key
+thought behind 8seg's driving scheme is that there are two common phases wound through the display in a zig-zag pattern
+as shown in red and blue in the schema below. These phases alternate their polarity at a high frequency. Each segment
+has its negative pole connected to one of these two phases, and can be turned on by the driver while that phase is low
+and the other phase is high. While a phase is high, the LEDs on all segments connected to that phase are reverse-biased,
+and thus these segments remain dark.
+
+The positive poles of all segments are connected to the driver circuit in the center through a spiral pattern. Each arm
+of the spiral is made up of two segments, one diagonal on the inside, and one horizontal or vertical on the outside.
+The two segments on each spiral arm are on different phases, one on each of the two phases. Thus, during a single cycle
+of the two phases alternating polarity, first one of the two segments has its polarity the right way around, then the
+other. The driver can turn on the active segment by connecting the spiral control line to the positive LED supply
+voltage. 
+
+Both phases cross at the center where the driver circuit is located, so the driver can power itself from the two phases
+using a simple full bridge rectifier.
+
+Saving copper with point of load regulation
+-------------------------------------------
+
+In the beginning, I experimented with the design above, putting 12V AC on the two phases, and letting the driver switch
+its derived LED supply using some cheap MOSFETs. This simple design totally works, but it has an important shortcoming.
+
+8seg is designed to be physically *very* large. This means that not only does it have a large number of LEDs that
+together need a lot of current, it also has to transmit all of that current across significant physical distances. The
+consequence of this was that in the initial design, I was looking at either needing hundreds of Euros worth of copper
+cables, or burning hundreds of Watts of electricity into heat if I were to use thinner cables. In this case, cables act
+like resistors. In a resistor, power dissipation rises with the square of the current inside the cable. This is bad for
+8seg since it means halving the amount of copper in those wires increases power dissipation in these wires fourfold.
+
+Despite that downside, this square law does come with an upside, too. If we assume we have wires of a particular fixed
+diameter, if we can halve the current through those wires, we can quarter the wires' power dissipation. If we want to
+deliver the same amount of power to the LEDs as before, to halve wire current, we have to double the voltage, and add
+some circuitry on the drivers to convert that increased voltage back down to close to our LED tape's nominal 12V.
+
+Alas, simply doubling the voltage leads to one question: How is it that we can pass double the voltage through our LED
+tape to the center control circuit? Isn't the LED tape made for 12V operation only, not 24V? The answer to this
+apparent problem is that the center is connected to the AC bus voltage only through the negative side of the LED tapes,
+and controls their positive sides to turn them on or off. The AC bus voltage never appears directly across any single of
+the eight segments. At the same time, a simple buck converter stepping down our new 24V bus voltage to 12V, and feeding
+the segment control transistors with that instead of feeding them straight from the rectified AC bus allows us to feed
+the segments with 12V. The only difference between this circuit and the straight 12V variant is that now, during OFF
+times, the LED tapes see a negative 24 v across them. To make sure that's not a problem, I tested a number of them with
+different LED colors and from different manufacturers, and all of them held up past the 50 V I could easily generate
+with my lab power supply.
+
+Synchronous rectification
+-------------------------
+
+I implemented the point-of-load regulation in a new revision of the center circuit, and built a prototype digit. When I
+tested this prototype, to my dismay, I noticed some really strange behavior. In my tests, the LED tape did not properly
+light up, and when I checked the voltages with my oscilloscope, I noticed that the center circuit's ground was floating
+several volts *below* the AC bus voltage's negative phase. How come?
+
+After some head-scratching, I found that this problem was due to a simple instance of Kirchhoff's current law. Consider
+the point where the AC bus voltage's currently negative phase enters the center circuit board. Let's say that we
+dissipate 24 Watts in the segments' LEDs. In this case, at 24 Volts, 1 Ampère will flow into the center circuit's
+terminal connected to the currently positive phase, and out from the center circuit's terminal connected to the
+currently negative phase.
+
+Now consider the current through the LED tape. During one half-cycle of the AC bus, the center circuit can only address
+the four segments that have their negative rail connected to the currently negative phase of the AC bus. If one of these
+four segments is currently on and dissipating our 24 Watts, that segment will be fed 2 Ampère of current from the center
+circuit through its positive rail. My mistake was that I did not consider what happened to the return current here.
+The corresponding 2 Ampère return current of course flows back through the segment's negative rail into the center
+circuit, and herein lies the issue: That negative rail is where our center circuit's supply current comes from! This
+means that according to Kirchhoff's current law, the 1 A flowing out from the center circuit at its input are adding up
+with the 2 A flowing into it. The result of this is that in the currently positive phase's connection, we get 1 A
+flowing into the center circuit, while in the negative phase connection, we get (-1) + (+2) resulting in another 1 A
+flowing into it! The only terminal where current flows *out* of the center circuit is the positive terminal connected to
+the active segment, out of which 2 A of current are flowing.
+
+The big problem with this confusing scenario is that this means the bridge recitifier in our center circuit cannot work,
+since its negative-side diodes are reverse biased while any of the segments are on. We can't just add more diodes here,
+since that would just short both AC bus rails together. Instead, the solution is to add one rather chonky MOSFET in
+parallel with each of the two negative-side diodes of the bridge rectifier that are controlled by the center circuit to
+act as a sort of synchronous rectifier. When we turn on one of the segments, we have to turn on the MOSFET on the
+currently negative rail to allow the segment's return current to bypass the bridge rectifier's negative-side diode. Fun
+fact: If we turn on the wrong MOSFET out of the pair, we short the AC bus, resulting in a very quick end of life for that
+poor MOSFET.
+
+Power line data communication
+-----------------------------
+
+As we saw above, the driver providing power to a string of digits has to continuously alternate the polarity of its
+output voltage to provide one part of the digit circuits' multiplexing. Since we want to provide the control information
+to the center circuits through those same two wires, we can choose between a number of viable power line communication
+schemes. These schemes usually require a beefy transmitter adding a modulation at a frequency much larger than the
+underlying bus frequency, and a filter circuit at each receiver to filter that signal from the much stronger fundamental
+AC waveform. In our application, I saw two issues with these classical approaches. First, they require fairly complex
+circuitry, especially the beefy transmitter at the driver. Second, they are susceptible to attenuation with either
+changing load or over long distances, which could potentially be a problem with the high currents and long(ish) wiring
+runs 8seg needs.
+
+Because of these disadvantages, I decided on another approach entirely. Instead of modulating our control signal on top
+of the AC power waveform, we modulate our control data *into* the AC power waveform. To not interfere with the display
+and cause outages or flicker, and to avoid having to blank the display during transmissions, we choose a modulating
+technique that leaves the proportions of negative and positive half-waves undisturbed. The practical realization of this
+is that instead of alternating positive and negative half-waves, we send a positive half wave for each "one" bit, and a
+negative half wave for each "zero" bit, effectively creating a phase shift keyed signal with two states with an
+180-degree phase shift, with the transmitted bit rate synchronized to twice the underlying carrier frequency.
+
+The remaining question is how one can encode arbitrary binary data into a continuous stream of ones and zeros that is
+precisely 50 % ones and 50 % zeros across any time span longer than a few dozen bits. There exists a near-optimal
+solution to this question from ethernet over copper twisted pairs. In ethernet, the encoded and modulated signal passes
+through an isolation transformer to protect the ethernet transceiver from interference or dangerous voltages coming in
+through the ethernet port. For this isolation transformer to work, the modulated ethernet signal must be exactly
+balanced to avoid saturating the transformer's core with a DC offset. Ethernet solves this issue by using an encoding
+known as `8b/10b encoding`_. 8b/10b encoding is named like that because it specifies a way to produce a 10 bit codeword
+from any 8 bit input data word while guaranteeing that the resulting codewords are always precisely balanced when
+looking at two or more consecutively.
+
+Framing
+-------
+
+Since 8b/10b encoding maps a space of 256 data words to 1024 code words, there necessarily are a number of unused code
+words. While for some of them, leaving them unallocated is beneficial because it improves error tolerance by decreasing
+the probability of one code word turning into another undetectably when a single one of its bits is flipped, even
+accounting for that it leaves some room for other uses. In 8b/10b, these leftover code words are used for synchronizing
+the receiver to the transmitter, and for framing transmissions. Synchronization is necessary for the receiver to know
+where a code word stards, and 8b/10b has a handful of special "comma" code words that can be uniquely identified in a
+continuous stream of received ones and zeros, because no other combination of 8b/10b code words could produce the same
+sequence of ones and zeros of the comma code word anywhere.
+
+The leftover code words that are not commas are useful, too. They can be used, for instance, as filler code words
+betwene actual data transmissions, or to act as framing markers denoting things like the end of a protocol message.
+
+The 8seg driver produces its modulation waveform by translating all data to be transmitted into 8b/10b codes, padding
+the result with framing markers and filler codes, and copy-pasting together the corresponding AC waveform from a small
+set of pre-programmed waveform transitions.
+
+.. _matelight: https://github.com/jaseg/matelight
+.. _`8b/10b encoding`: https://en.wikipedia.org/wiki/8b/10b_encoding
+

content/blog/_index.rst

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+---
+title: Blog
+---

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+    <text
+       xml:space="preserve"
+       transform="matrix(0.67004375,0,0,0.67004375,30.304048,47.575216)"
+       id="text1"
+       style="font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size:21.3333px;line-height:13.9843px;font-family:Equateur;-inkscape-font-specification:Equateur;text-decoration-color:#000000;writing-mode:lr-tb;direction:ltr;white-space:pre;shape-inside:url(#rect1);display:inline;fill:#000000;fill-opacity:1;stroke-width:0.755906;stroke-linecap:round;stroke-linejoin:round"><tspan
+         x="73.275391"
+         y="19.8411"
+         id="tspan2">ashen</tspan></text>
+  </g>
+</svg>

content/blog/ashen-yanartas/index.rst

@@ -0,0 +1,59 @@
+---
+title: "Project Announcement: Ashen and Yanartas"
+date: 2026-05-31T08:00:00+02:00
+summary: >
+    There are exciting things ahead for the next year: I have been granted funding by both nlnet and by prototype fund
+    for open-source work on an open source hardware Hardware Security Module. As a vessel for this project, I created a
+    consulting company, yasec.
+---
+
+I'm currently in the last days of finishing my PhD (Dr.-Ing.) in Electrical Engineering. To make sure things don't get
+boring afterwards, I've been busy looking for new opportunities. As a result, there are exciting things ahead for the
+next year: I have been granted funding by both nlnet *and* by Prototype Fund for open-source work on an Open Source
+Hardware Hardware Security Module. As infrastructure for these projects, I created a consulting company, `yasec
+<https://yasec.de>`__.
+
+Prototype Fund supports Ashen, the OS for open-source HSMs
+----------------------------------------------------------
+
+.. raw:: html
+
+    <picture style="max-width: 10em; margin: 1em auto 1em auto">
+      <source media="(prefers-color-scheme: light)" srcset="ashen-logo-text-light-plain.svg">
+      <img src="ashen-logo-text-dark-plain.svg alt="Ashen project logo showing a stylized flame in a circle">
+    </picture>
+
+Starting June 2026, I will be working on Ashen_, an open-source software stack that provides the operating system layer
+for open-source HSMs. The project is funded as part of `Prototype Fund`_'s Class 02.
+
+Compared to existing open-source HSM software that work at the application level and that don't
+consider physical attacks, this stack will provide the underlying operating system services to protect such systems from
+physical attacks. A key component of this stack will be a portable mechanism to connect hardware tamper sensors to a
+system. The stack will enable deterministic guarantees of the maximum latency until secrets are destroyed after a tamper
+alarm was raised.
+
+.. _Ashen: https://yasec.de/projects/ashen/
+.. _`Prototype Fund`: https://www.prototypefund.de/
+
+nlnet supports Yanartas, the OSHW HSM platform
+----------------------------------------------
+
+After work on the Ashen software stack is completed, I will continue by creating Yanartas_, an Open Source Hardware
+design for a complete open-source Hardware Security Module that provides protection against advanced physical attacks
+using a security mesh based on the `Inertial HSM`_ technology I developed during my PhD. The design will be customizable
+to different use cases and payload sizes from microcontrollers to whole servers.
+
+.. _Yanartas: https://yasec.de/projects/yanartas/
+.. _`Inertial HSM`: https://tches.iacr.org/index.php/TCHES/article/view/9290
+
+Let's talk!
+-----------
+
+In case you're interested to talk about hardware security engineering or open-source hardware, feel free to reach out
+through email or on mastodon. The projects are in an early stage, and I'm looking both for collaborators for these
+projects, and for opportunities once these projects have been completed. At this time, I only have a small amount of
+spare capacity outside of these projects, but that will change with time. I'd love to hear about *your* projects and
+your needs for specialist work in case you're interested.
+
+Follow this blog's `RSS <https://jaseg.de/index.xml>`__ and follow me `on mastodon <https://chaos.social/@jaseg>`__ for
+updates!

content/blog/ashen-yanartas/yasec-logo-v1-dark-plain.svg

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content/blog/ashen-yanartas/yasec-logo-v1-light-plain.svg

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content/blog/css-only-code-blocks/index.rst

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+---
+title: "Code listings with nice line wrapping and line numbers from plain CSS"
+date: 2025-07-23T23:42:00+01:00
+summary: >
+    Code listings in web pages are often a bit of a pain to use. Usually, they don't wrap on small screens. Also,
+    copy-pasting code from a code listing often copies the line numbers along with the code. Finally, many
+    implementations use heavyweight HTML and/or javascript, making them slow to render. For this blog, I wrote a little
+    CSS hack that renders nice, wrapping code blocks with line continuation markers in plain CSS without any JS.
+---
+
+Code listings in web pages are often a bit of a pain to use. Often, they don't wrap on small screens. Also, copy-pasting
+code from a code listing often copies the line numbers along with the code. Finally, many implementations use
+heavyweight HTML and/or javascript, making them slow to render (looking at you, gitlab).
+
+For this blog, I wrote an implementation that renders HTML code listings entirely without JavaScript, renders line
+numbers using plain CSS such that they don't get selected with the code, and that works with the browser to wrap in a
+natural way while still supporting the little line continuation arrows that are used to show that a line was soft
+wrapped in text editors.
+
+This blog is rendered as a static site using Hugo_ from a pile of RestructuredText_ documents. RestructuredText renders
+code listings using Pygments_ by default. Pygments hard-bakes the line numbers into the generated HTML, so I am using a
+`monkey-patched`_ hook that changes the line number rendering to just a bunch of empty ``<span>`` elements. The resulting
+HTML for a code block then looks like this:
+
+.. code:: html
+    
+    <pre class="code [language] literal-block">
+        <span class="lineno"></span>
+        <span class="line">
+            <span class="[syntax highlight token]">The </span><span class="[other syntax highlight token]">code!<span>
+        </span>
+        <!-- ... repeat once for each source line. -->
+    </pre>
+
+You can find the (rather short) source of the ``rst2html`` wrapper `below <#rst2html-wrapper>`_.
+
+The CSS
+-------
+
+This modified HTML structure of the code listing gets accompanied by some CSS to make it flow nicely. Here is a listing
+of the complete CSS controlling the listing. The only bit that isn't included here is the actual syntax styling rules
+for the pygments tokens.
+
+.. code:: css
+
+    /*****************************************************/
+    /* Code block formatting / syntax highlighting rules */
+    /*****************************************************/
+
+    .code {
+        font-family: "Fira Code";
+        font-size: 13px;
+        text-align: left; /* Override default content "justify" alignment */
+        white-space: pre-wrap;
+        word-wrap: break-word;
+        overflow-x: auto;
+        display: grid;
+        align-items: start;
+        grid-template-columns: min-content 1fr;
+    }
+
+    .code > .line {
+        padding-left: calc(2em + 5px);
+        text-indent: -2em;
+        padding-top: 2px;
+        min-width: 15em;
+    }
+
+    /* Make individual syntax tokens wrap anywhere */
+    .code > .line > span {
+        overflow-wrap: anywhere;
+        white-space: pre-wrap;
+    }
+
+    /* We render line numbers in CSS! */
+    .code > .lineno {
+        counter-increment: lineno;
+        word-break: keep-all;
+        margin: 0;
+        padding-left: 15px;
+        padding-right: 5px;
+        overflow: clip;
+        position: relative;
+        text-align: right;
+        color: var(--c-text-muted);
+        border-right: 1px solid var(--c-fg-highlight);
+        align-self: stretch;
+    }
+
+    /* We also handle line continuation markers in CSS. */
+    .code > .lineno::after {
+        position: absolute;
+        right: 5px;
+        content: "\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳";
+        white-space: pre;
+        color: var(--c-text-muted);
+    }
+
+    /* Insert the actual line number */
+    .code > .lineno::before {
+        content: counter(lineno);
+    }
+
+    .code::before {
+        counter-reset: lineno;
+    }
+
+    .code .hll {}
+    /* Following are about 50 lines that define the styling of each kind of pygments syntax highlight token. These lines
+       all look like the following: */
+    .code .c   { color: var(--c-text); font-weight: 400 } /* Comment */
+
+This CSS does a few things:
+
+ 1. It renders the ``<pre>`` code listing element using a two-column CSS ``display: grid`` layout. The left column is
+    used for the line numbers, and the right column is used for the code lines.
+ 2. It numbers the lines using a `CSS Counter`_. CSS counters are meant for things like numbering headings and such, but
+    they are a perfect fit for our purpose.
+ 3. It inserts the counter value as the line number into the ``<span class="lineno">`` element's ``::before``
+    pseudo-element. A side effect of using the ``::before`` pseudo-element is that without doing anything extra, the
+    line numbers will remain outside of the normal text selection so they will neither be highlighted when selecting
+    listing content, nor will they be copied when copy/pasting the listing content.
+ 4. It inserts a string of ``"\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳\a↳"`` into the line number span's
+    ``::after`` pseudo-element. This string evaluates to a sequence of unicode arrows separated by line breaks, and
+    starting with an empty line. The ``::after`` pseudo-element is positioned using ``position: absolute``, and the
+    parent ``<span class="lineno">`` has ``position: relative`` set. This way, the arrow pseudo-element gets placed on
+    top of the lineno span without affecting the layout at all. By setting ``overflow: clip`` on the parent ``<span
+    class="lineno">``, the arrow pseudo-element gets cut off vertically wherever the parent lineno element naturally
+    ends.
+
+The line number span is inserted into the parent ``<pre>`` element's CSS grid using ``align-self: stretch``, which
+causes it to vertically stretch to fill the available space. Since the line number span only contains the line number,
+its minimum height is a single line. As a result, it will stretch higher only when the corresponding code line in the
+right grid column stretches vertically because of line wrapping. When that happens, part of the arrow pseudo-element
+starts showing through from behind the ``overflow: clip`` of the line number span, and one arrow gets rendered for each
+wrapped listing line.
+
+When the page is too narrow, we don't want the code listing's lines to wrapp into a column of single characters. To
+prevent that, we simply set a ``min-width`` on the ``<span class="line">`` in the right column, and set ``overflow-x:
+auto`` on the listing ``<pre>``. This results in a horizontal scroll bar appearing whenever the listing gets too narrow.
+
+You can try out the line wrapping by resizing this page!
+
+rst2html wrapper
+----------------
+
+Here is the python ``rst2html`` wrapper that monkey-patches code rendering. I made hugo invoke this while building the
+page by simply overriding the ``PATH`` environment variable.
+
+.. code:: python
+
+    #!/usr/bin/env python3
+    # Based on https://gist.github.com/mastbaum/2655700 for the basic plugin scaffolding
+
+    import sys
+    import re
+
+    import docutils.core
+    from docutils.transforms import Transform
+    from docutils.nodes import TextElement, Inline, Text
+    from docutils.parsers.rst import Directive, directives
+    from docutils.writers.html4css1 import Writer, HTMLTranslator
+
+
+    class UnfuckedHTMLTranslator(HTMLTranslator):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+            self.in_literal_block = False
+
+        def visit_literal_block(self, node):
+            # Insert an empty "lineno" span before each line. We insert the line numbers using pure CSS in a ::before
+            # pseudo-element. This has the added advantage that the line numbers don't get included in text selection.
+            # These line number spans are also used to show line continuation markers when a line is wrapped.
+            self.in_literal_block = True
+            self.body.append(self.starttag(node, 'pre', CLASS='literal-block'))
+            self.body.append('<span class="lineno"></span><span class="line">')
+
+        def depart_literal_block(self, node):
+            self.in_literal_block = False
+            self.body.append('\n</span></pre>\n')
+
+        def visit_Text(self, node):
+            if self.in_literal_block:
+                for match in re.finditer('([^\n]*)(\n|$)', node.astext()):
+                    text, end = match.groups()
+
+                    if text:
+                        super().visit_Text(Text(text))
+
+                    if end == '\n':
+                        if isinstance(node.parent, Inline):
+                            self.depart_inline(node.parent)
+                        self.body.append(f'</span>\n<span class="lineno"></span><span class="line">')
+                        if isinstance(node.parent, Inline):
+                            self.visit_inline(node.parent)
+
+            else:
+                super().visit_Text(node)
+
+
+    html_writer = Writer()
+    html_writer.translator_class = UnfuckedHTMLTranslator
+    docutils.core.publish_cmdline(writer=html_writer)
+
+.. _Hugo: https://gohugo.io/
+.. _RestructuredText: https://www.sphinx-doc.org/en/master/usage/restructuredtext/index.html
+.. _Pygments: https://pygments.org/
+.. _`monkey-patched`: https://en.wikipedia.org/wiki/Monkey_patch
+.. _`CSS Counter`: https://developer.mozilla.org/en-US/docs/Web/CSS/counter

content/blog/epa-sgd-crypto/index.rst

@@ -0,0 +1,100 @@
+---
+title: "75 Million Lives, Two Keys"
+date: 2025-01-05T23:42:00+01:00
+draft: true
+---
+
+2025 has begun. In this new year, with its new national healthcare record system, the country of Germany will start one
+of the largest rollouts of a cryptographic system in history. While the system has received scrutiny as well as
+resulting harsh criticism from a number of parties ranging from NGOs to everyday civilians, the system has received
+surprisingly little attention from the academic applied cryptography crowd. Additionally, previous criticism of
+the system has largely revolved around organizational issues. While valid, we belive that some cryptographic issues at
+the core of the system have escaped attention unitl now. In particular, at the core of the system is a key escrow system
+that contains several questionable design choices and that in its overall design seems out of place in 2025.
+
+The aim of the system is to serve as a shared storage for all healthcare records of a person. In the system, a person's
+entire patient file with all documentation on the treatment process including test results, images and other raw data
+will be stored in something vaguely resembling cloud storage such that all healthcare providers that the person visits
+can access the entire file. This centralized, synchronized storage eliminates the need for transferring this data
+between hospitals and doctors offices by fax, mail or physical media as it was common practice until now. After a
+development and testing phase lasting approximately five years, the German government decided to roll out the system to
+everybody insured under Germany's mandatory national health insurance scheme, totalling approximately 75 million people,
+on January 15th 2025.
+
+In this article, we will give an overview of the system's cryptographic design before highlighting a few odd
+design choices that could amount to a viable attack vector to the powerful adversaies 
+
+## Context and involved parties
+
+Germany has a national, mandatory health insurance system. The system is open to any permanent resident of the country
+irrespective of citizenship. The system is mandatory in that while residents can choose between a number of both
+publically owned as well as private healthcare providers, it is not possible to opt out of the system. The public health
+insurance providers cover approximately 90% of German residents. These providers are organized in an umbrella
+organization named "GKV Spitzenverband". The resposibility of this umbrella organization largely revolves around
+negotiating prices with pharmaceutical companies and with healthcare providers as a publically sanctioned cartel, but
+also includes the specification and operation of shared IT infrastructure for billing and data exchange between
+healthcare providers.
+
+While GKV Spitzenverband is the party that ultimately holds responsibility for the regulatory administration of national
+healthcare IT infrastructure, it has delegated large parts of both the technical specification of this infrastructure as
+well as its day-to-day operation to Gematik GmbH, a state-owned limited liability corporation created specifically for
+the purpose of developing and implementing national healthcare IT standards. The electronic healthcare record system we
+describe in this article was standardized and implemented by Gematik GmbH under the direction of GKV Spitzenverband.
+
+Healthcare providers in Germany need to be registered with GKV Spitzenverband to serve members of public health
+insurance providers. Since these public providers constitute approximately 90% market share, the vast majority of
+healthcare providers are registered this way.
+
+Before the new national health record system, a number of healthcare IT processes have already been standardized and
+implemented by the parties above. In particular, every insured person already owns a cryptographic smartcard that acts
+as their proof of identity when accessing healthcare services. On the other side of such transactions, healthcare
+providers are likewise identified by cryptographic smartcards. Until now, these cards were used to facilitate billing of
+services from healthcare providers to insurers and to transfer prescriptions from prescribing doctors to pharmacies.
+
+A central role in this existing infrastructure is assumed by VPN gateways that link healthcare providers to
+the centrally-run backend infrastructure. Gematik GmbH calls these devices "Konnektor". They are specially-built
+hardware devices that contain multiple smart cards to authenticate the VPN connection towards the backend, and besides
+acting as a standard VPN gateway for client applications in the healthcare provider's network to tunnnel their backend
+requests through, the Konnektors also perform cryptographic operations in some of Gematik GmbH's protocols, such as
+authenticating certain requests using signatures.
+
+## Design principles
+
+The new health record system was built on top of the existing infrastructure described above. In particular, access to
+health records is managed through keys stored in the patient's and the healthcare provider's existing smartcards, and
+all backend communication is tunneled through the existing VPN. Access to the files is mediated through the healthcare
+provider's existing patient management software. While in early drafts of the system, access to healthcare records
+through the patient's smartcard was gated behind a PIN, the impracticality of making the entire patient populace
+remember PINs led the implementors to scrap this provision, meaning that the patient's smartcard is all a healthcare
+provider needs to access the patient's record.
+
+A critical cornerstone in the system's design is that the system's designers decided that a lost smartcard should not
+lead to any data loss. As a consequence of this decision, while some of the record's access keys are kept on the
+patient smartcard, in contravention to conventional smartcard designs the same keys are kept accessible in a centralized
+key escrow system named "Schlüsselgenerierungsdienst" and abbreviated as SGD. Furthermore, these keys are not generated
+on the smartcard either -- instead, the key escrow system generates these access keys, one copy of which is then
+transmitted and stored inside the smartcard.
+
+The system supports re-issuing a smartcard to gain access to a healthcare record. Since the record's privacy pivots on
+this process, the system incorporates some organziational countermeasures that aim to make it hard to gain access to a
+re-issued copy of a patient smartcard without the patient's help or otherwise multiple colluding parties.
+
+## Cryptographic design
+
+
+
+## The implied adversary model
+
+While Gematik GmbH publishes detailed specifications of the systems they standardize, these specifications and some
+associated implementation guidelines are about the extent of public information. Software implementations are being kept
+secret, and while standardization results are available, a large fraction of design rationale is discussed behind closed
+doors. From an academic perspective, the most glaring omission in Gematik GmbH's public documents is any definition of a
+threat model or an adversary model. As a result of this, we will deduce an adversary model below by contextualizing the
+published standards in the national healthcare setting. We will base our further analysis of the system on this
+adversary model.
+
+
+
+## Previous reviews and audits of the system
+
+[0] https://www.destatis.de/DE/Themen/Arbeit/Arbeitsmarkt/Qualitaet-Arbeit/Dimension-2/krankenversicherungsschutz.html

content/blog/hsm-basics/index.rst

@@ -0,0 +1,214 @@
+---
+title: "Hardware Security Module Basics"
+date: 2019-05-17T15:29:20+08:00
+summary: >
+    I gave a short introduction into Hardware Security Modules at our university workgroup, including an overview on
+    interesting research directions.
+---
+
+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
+

content/blog/ihsm-worlds-first-diy-hsm/index.rst

@@ -0,0 +1,44 @@
+---
+title: "New Paper on Inertial Hardware Security Modules"
+date: 2021-11-23T23:42:20+01:00
+summary: >
+    Paper announcement: We have published a paper on how you can DIY a tamper-sensing hardware security module from any
+    single-board computer using a moving tamper-sensing mesh made from cheap PCBs.
+---
+
+World's First DIY HSM
+=====================
+
+Last week, Prof. Dr. Björn Scheuermann and I have `published our first joint paper on Hardware Security Modules
+<https://tches.iacr.org/index.php/TCHES/article/view/9290>`__. In our paper, we introduce Inertial Hardware Security
+Modules (IHSMs), a new way of building high-security HSMs from basic components.  I think the technology we demonstrate
+in our paper might allow some neat applications where some civil organization deploys a service that no one, not even
+they themselves, can snoop on. Anyone can built an IHSM without needing any fancy equipment, which makes me optimistic
+that maybe the ideas of the `Cypherpunk movement <https://www.activism.net/cypherpunk/manifesto.html>`__ aren't obsolete
+after all, despite even the word "crypto" having been co-opted by radical capitalist environmental destructionists.
+
+An IHSM is basically an ultra-secure enclosure for something like a server or a raspberry pi that even someone with
+unlimited resources would have a really hard time cracking without destroying all data stored in it. The principle of an
+IHSM is the same as that of a `normal HSM`_. You have a payload that contains really secret data. There's really no way
+to prevent an attacker with physical access to the thing from opening it given enough time and abrasive discs for their
+angle grinder. So what you do instead is that you make it self-destruct its secrets within microseconds of anyone
+tampering with it. Usually, such HSMs are used for storing credit card pins and other financial data. They're expensive
+as fuck, all the while being about the same processing speed as a smartphone.  Traditional HSMs use printed or
+lithographically patterned conductive foils for their security mesh. These foils are not an off-the-shelf component and
+are made in a completely custom manufacturing process. To create your own, you would have to re-engineer that entire
+process and probably spend some serious money on production machines. 
+
+Inertial HSMs take the concept of traditional HSMs, but replace the usual tamper detection mesh with a few security mesh
+PCBs. These PCBs are coarser than traditional meshes by orders of magnitude, and would alone not even be close to enough
+to keep out even a moderately motivated attacker. IHSMs fix this issue by spinning the entire tamper detection mesh at
+very high speed.  To tamper with the mesh, an attacker would have to stop it. This, in turn, can be easily detected by
+the mesh's alarm circuitry using a simple accelerometer as a rotation sensor.
+
+In our paper, we have shown a working prototype of the core concepts one needs to build such an IHSM.  To build an IHSM
+you only need a basic electronics lab. I built the prototype in our paper at home during one of Germany's COVID
+lockdowns.  You can have a look at our code and CAD on `my git <https://git.jaseg.de/ihsm.git>`__. What is missing right
+now is an integration of all of these fragments into something cohesive that an interested person with the right tools
+could go out and build. We are planning to release this sort of documentation at some point, but right now we are
+focusing our effort on the next iteration of the design instead. Stay tuned for updates ;)
+
+.. _`normal HSM`: {{<ref "blog/hsm-basics/index.rst">}}

content/blog/jupyterlab-notebook-file-oneliner/index.rst

@@ -0,0 +1,21 @@
+---
+title: "Getting the .ipynb Notebook File Location From a Running Jupyter Lab Notebook"
+date: 2025-06-29T23:42:00+01:00
+summary: >
+    If you need to get the path of the ipynb file in a running #Jupyter notebook, this one-liner will do the trick. It
+    seems chatgpt is confused, and a bunch of other approaches on the web look fragile and/or unnecessarily complex to
+    me.
+---
+
+If you need to get the path of the ipynb file in a running #Jupyter notebook, this one-liner will do the trick. It seems
+chatgpt is confused, and a bunch of other approaches on the web look fragile and/or unnecessarily complex to me.
+
+.. code:: python
+
+    import sys
+    Path(json.loads(Path(sys.argv[-1]).read_bytes())['jupyter_session'])
+
+The way this works is that for each notebook, jupyter starts a python "kernel" process that actually runs the notebook's
+code. That kernel gets a json file with info on the notebook's location on the disk passed through its command line.
+Since we're running code in that exact python process, we can just grab that json file from sys.argv, and read it
+ourselves.

content/blog/kicad-mesh-plugin/index.rst

@@ -0,0 +1,229 @@
+---
+title: "Kicad Mesh Plugin"
+date: 2020-08-18T13:15:39+02:00
+summary: >
+    I wrote a little KiCad plugin that you can use to create security meshes, heaters and other things where you need
+    one or more traces cover the entire surface of a PCB. The plugin supports arbitrary PCB shapes, cutouts, and can
+    route around existing footprints and traces on the PCB.
+---
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/anim.webp" style="max-width: 20em">
+    </figure>
+
+Tamper Detection Meshes
+=======================
+
+Cryptography is at the foundation of our modern, networked world. From email to card payment infrastructure in brick and
+mortar stores, cryptographic keys secure almost every part of our digital lives againts cybercriminals or curious
+surveillance capitalists. Without cryptography, many of the things we routinely do in our lives such as paying for
+groceries with a credit card, messaging a friend on `Signal <https://signal.org>`_ or unlocking a car with its keyfob
+would not be possible. The security of all of these systems in its core lies on the secrecy of cryptographic keys.
+Systems differ in what kind of keys they use, how often these keys are replaced and the intricacies of the cryptographic
+operations these keys fit into but all have in common that their security relies on keeping the keys secret.
+
+In practice, this secrecy has been implemented in many different ways. Mass-market software such as browsers or
+messenger apps usually relies on some operating system facility to tell the computer "*please keep this piece of memory
+away from all other applications*". While on desktop operating systems usually this does not provide much of a barrier
+to other programs on the same computer, on modern mobile operating systems this approach is actually quite secure.
+However, given sufficient resources no security is perfect. All of these systems can be compromised if the host
+operating system is compromised sufficiently, and for organizations with considerable resources a market has sprung up
+that offers turn-key solutions for all wiretapping needs.
+
+In some applications, this level of security has not been considered sufficient. Particularly financial infrastructure
+is such a high-profile target that a lot of effort has been put into the security of cryptographic implementations. The
+best cryptographic algorithm is useless if it is run on a compromised system (from that system's point of view anyway).
+One of the core cryptographic components in financial applications are smartcards like they are used as payment cards in
+most countries nowadays. These smartcards contain a small, specialized cryptographic microcontroller that is designed to
+be hard to tamper with. Though one of the design goals of the system is to reduce the amount of sensitive information
+stored on the card, things such as copying of a card can only be hindered by making the chip hard to read out.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/modern_art.svg" style="max-width: 20em">
+    </figure>
+
+With smartcards being the means of choice on one side of the counter in electronic payments, on the other side of the
+counter a different technology prevails. Attacks on payment terminals are bound to have much more dire consequences than
+attacks on individual cards since one terminal might see hundreds of cards being read every day. For this reason, the
+level of attack countermeasures employed in these terminals is a considerable step up from bare smartcards. While a
+smartcard is made physically hard to tamper, it does not have a battery and it can only detect tampering once it is
+powered by a reader. This allows for well-equipped attackers to use tools such as Focused Ion Beam (FIB) workstations to
+circumvent the smartcard's defences while it is powered down, and then power up the card to carry out the actual attack.
+
+The answer to this problem in electronic payment infrastructure is called *Hardware Security Module*, or HSM. An HSM is
+similar to a smartcard in its function (cryptographic processing using keys that are meant to never leave the protection
+of the HSM). The one major between the two is that an HSM has its own battery and is continuously powered from its
+manufacture to the day it is scrapped. If the HSM looses power at any point in time, it uses a small amount of energy
+stored internally to securely wipe all cryptographic secrets from its memory within a few milliseconds.
+
+Being powered at all times allows the HSM to actively detect and respond to attacks. The most common way this is done is
+by wrapping the juicy secret parts in a foil or a printed circuit board that is patterned with a long and convoluted
+maze of wires, called a *mesh*. The HSM is continuously monitoring these wires for changes (such as shorts, breaks or
+changes in resistance) and will sound the alarm when any are detected. Practically, this presents a considerable hurdle
+to any attacker: They have to find a way to disable or circumvent the mesh while it is being monitored by the HSM.  In
+practice, often this is no insurmountable challenge but it again increases attack costs.
+
+DIY Meshes
+==========
+
+Throughout my studies in security research I have always had an interest in HSMs. I have taken apart my fair share of
+HSMs and at this point, to understand the technology more, I want to experiment with building my own HSM. In last year's
+`HSM basics <{{<ref "blog/hsm-basics/index.rst">}}>`_ post I have lined out some ideas for a next generation design that
+deviates from the bread-and-butter apporoach of using a mesh as the primary security feature. Before embarking on
+practical experiments with these ideas, I want to first take a stab at replicating the current state of the art as best
+I can. State of the art meshes often use exotic substrates such as 3D plastic parts with traces chemically deposited on
+their surface or special flexible substrates with conductive ink traces. These technologies will likely be too
+cumbersome for me to implement myself only for a few prototypes, and industrial manufacturers will most likely be too
+expensive. Thus, I will concentrate on regular PCB technology for now.
+
+The idea of a mesh on a PCB is pretty simple: You have one or several traces that you try to cover every corner of the
+mesh PCB's area with. To be most effective, the traces should be as thin and as close together as possible. To increase
+the chances of a manipulation being detected, multiple traces can also be used that can then be monitored for shorts
+between them.
+
+While one can feasibly lay out these traces by hand, this really is an ideal application of a simple auto-router. While
+general PCB autorouting is *hard*, auto-routing just a few traces to approximate a space-filling curve is not. Since I
+am just starting out, I went with the simplest algorithmic solution I could think of. I first approximate the area
+designated to the mesh with a square grid whose cells are a multiple of my trace/space size. The mesh will only be drawn
+into grid cells that are fully inside the set boundaries. All cells outside or going across the border are discarded in
+this step.
+
+I decided to implement this auto-router in a KiCAD plugin. Though KiCADs plugin API is not the best, it was just about
+usable for this task.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/kicad-mesh-outline.png" alt="KiCAD showing an irregular board shape with rounded corners and
+        indents. In the middle of the board there is a footprint for a 4-pin surface-mount pin header.">
+        <figcaption>The process starts out with the mesh shape being defined inside KiCAD. The mesh's outline is drawn
+        onto one of the graphical "Eco" layers. A footprint is placed to serve as a placeholder for the mesh's
+        connections to the outside world. This footprint is later used as the starting point for the mesh generation
+        algorithm.</figcaption>
+    </figure>
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/grid-vis-plain.svg" alt="A vizualization of the grid fitting process. Over the mesh's irregular
+        outline a grid is drawn. In this picture, all grid cells that are fully inside the grid are shown. Grid cells
+        that overlap the mesh border are highlighted. Grid cells outside of the mesh border are not drawn.">
+        <figcaption>A visualization of the grid fitting process. First, a grid large enough to contain the mesh border
+        is generated. Then, every cell is checked for overlap with the mesh border area. If the cell is fully inside, it
+        (yellow), it is considered in the mesh generation later. Cells outside (gray) or on the border (red) are
+        discarded.</figcaption>
+    </figure>
+
+After generating the grid, starting from the place I want to connect to the mesh, I walk the grid's cells one by one to
+generate a tree that covers the entire grid's area. To set the mesh's starting place I place a footprint on the board
+(dark gray in the picture above) whose designator I then tell my script. The tree generation algorithm looks like a
+depth-first search, except all checks are random. Depending on the level of randomness used at each step of the
+algorithm it yields more or less organized-looking results. Below are five example runs of the algorithm at differing
+levels of randomness with the cells colored according to their distance from the tree root. 0% randomness means that the
+algorithm is going to try cells in forward direction first on every step, and only then try out left and right. 100%
+means that on every step, the algorithm is choosing a new direction at random.
+
+.. raw:: html
+
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <img src="images/cells-0.svg" alt="a completely organized looking grid with spiral patterns all over.">
+            <figcaption>0%</figcaption>
+        </figure>
+        <figure>
+            <img src="images/cells-25.svg">
+            <figcaption>25%</figcaption>
+        </figure>
+        <figure>
+            <img src="images/cells-50.svg">
+            <figcaption>50%</figcaption>
+        </figure>
+        <figure>
+            <img src="images/cells-75.svg">
+            <figcaption>75%</figcaption>
+        </figure>
+        <figure>
+            <img src="images/cells-100.svg" alt="a completely random looking grid with cells aggregating into ireggular
+            areas that look like paint splotches.">
+            <figcaption>100%</figcaption>
+        </figure>
+    </div>
+
+After I have built this tree like you would do in a depth-first search, I draw my one or several mesh mesh traces into
+it. The core observation here is that there is only 16 possible ways a cell can be connected: It has four neighbors,
+each of which it can either be connected to or not, which results in 2^4 options. If you consider rotations and
+mirroring, this works out to rotations or mirrored versions of only six base tiles: The empty tile, a tile with all four
+sides connected, a straight through, a 90 degree bend, and a "T"-junction—see the illustration below.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/maze_tiles_plain.svg" style="max-width: 20em">
+        <figcaption>
+        There are six possible tile types in our connectivity graph inside its square tiling. This graphic illustrates
+        all sixteen rotations of these with how they would look in a two-conductor mesh.
+        </figcaption>
+    </figure>
+
+After tiling the grid according to the key above, we get the result below.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/tiles-25-small.svg">
+        <figcaption>
+        An auto-routed mesh with traces colored according to tile types.
+        </figcaption>
+    </figure>
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/traces-25-small.svg">
+        <figcaption>
+        The same mesh, but with traces all black.
+        </figcaption>
+    </figure>
+
+Putting it all together got me the KiCAD plugin you can see in the screenshot below.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/kicad-mesh-settings2.png">
+        <figcaption>
+        The plugin settings window open.
+        </figcaption>
+    </figure>
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/kicad-mesh-result-large.png">
+        <figcaption>
+            After runing the plugin, the generated mesh looks like this in pcbnew.
+        </figcaption>
+    </figure>
+
+I am fairly happy with the result, but getting there was a medium pain. Especially KiCAD's plugin API is still very
+unfinieshed. It is hard to use, most parts are completely undocumented and if you use anything but its most basic parts
+things tend to break. One particular pain point for me was that after generating the mesh, the traces have been added to
+the board, but are still invisible for some reason. You have to save the board first, then re-load the file for them to
+become visible. Also KiCAD crashes whenever the plugin tries to remove a trace, so currently my workflow involves always
+making a copy of the board file first and treating mesh generation as a non-reversible finishing step. 
+
+`Check out the code on my cgit <https://git.jaseg.de/kimesh.git/tree/plugin/mesh_dialog.py>`_.
+
+.. ::
+
+    .. raw:: html
+
+        <figure data-pagefind-ignore>
+            <img src="images/grid-vis-plain.svg" alt="">
+            <figcaption></figcaption>
+        </figure>
+

content/blog/kicoil-theory/index.rst

@@ -0,0 +1,40 @@
+---
+title: "The KiCoil Planar Coil Generator"
+date: 2025-12-31T13:15:39+02:00
+summary: >
+    I wrote a layout tool generating planar coils that can handle spiral coils, toroidal coils, and hybrids in between
+    the two.
+---
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="header.png" style="max-width: 20em">
+    </figure>
+
+A planar coil is a coil that is made from flat traces in some printing process like PCB or IC manufacturing, instead of
+being wound from wire. A few weeks ago, I needed one such planar coil that 
+
+
+Project State
+-------------
+
+Currently, circular coils are special cased. Their layouts are directly generated, without the use of polygon
+offsetting. Windings are efficiently approximated using circular arcs. The circular coil layout code is solid, and
+contains decent (albeit not infallible) parameter sanity checks. Its main limitation is that sometimes, clearances can
+be violated a bit.
+
+The arbitrary shape code path is less stable, and produces faulty output in some cases. The most common error is
+crossing traces near the first vertex of the polygon when the polygon has highly convex or concave parts. I'm still
+improving this code path, but as long as you check the output, any errors it produces should be easy to fix by hand.
+
+If you would like to contribute, I'd welcome any ideas on the arbitrary shape code path. I think there is no single
+optimal solution here, and a generic algorithm that can be adjusted to favor for instance shape accuracy versus winding
+smoothness would be nice.
+
+All project links are listed on `https://jaseg.de/projects/kicoil/ <https://jaseg.de/projects/kicoil/>`__. You can check
+out the code on my git at `https://git.jaseg.de/kicoil.git <https://git.jaseg.de/kicoil.git>`__. Issues are tracked on
+codeberg at `https://codeberg.org/jaseg/kicoil <https://codeberg.org/jaseg/kicoil>`__. The kicad addon can be installed
+from the KiCad plugin manager, and you can install the standalone kicoil python package `from PyPI
+<https://pypi.org/project/kicoil/>`__.
+

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content/blog/led-characterization/index.rst

@@ -0,0 +1,510 @@
+---
+title: "LED Characterization"
+date: 2018-05-02T11:18:38+02:00
+summary: >
+    Recently, I have been working on a small driver for ambient lighting using 12V LED strips like you can get
+    inexpensively from China. I wanted to be able to just throw one of these somewhere, stick down some LED tape, hook
+    it up to a small transformer and be able to control it through Wifi. When I was writing the firmware, I noticed that
+    when fading between different colors, the colors look *all wrong*! This observation led me down a rabbit hole of
+    color perception and LED peculiarities.
+---
+
+Preface
+-------
+
+Recently, I have been working on a `small driver`_ for ambient lighting using 12V LED strips like you can get
+inexpensively from China. I wanted to be able to just throw one of these somewhere, stick down some LED tape, hook it up
+to a small transformer and be able to control it through Wifi. When I was writing the firmware, I noticed that when
+fading between different colors, the colors look *all wrong*! This observation led me down a rabbit hole of color
+perception and LED peculiarities.
+
+The idea of the LED driver was that it can be used either with up to eight single-color LED tapes or, much more
+interesting, with up to two RGB or RGBW (red-green-blue-white) LED tapes. For ambient lighting high color resolution was
+really important so you could dim it down a lot without flickering. I ended up using the same driver stage I used in the
+`multichannel LED driver`_ project for its great color resolution and low hardware requirements.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/rgb_cube.svg" alt="An illustration of the RGB color cube.">
+        <figcaption>An illustration of the RGB color cube.
+            <a href="https://commons.wikimedia.org/wiki/File:RGB_color_cube.svg">Picture</a> by
+            <a href="https://commons.wikimedia.org/wiki/User:Maklaan">Maklaan from Wikimedia Commons</a>,
+            <a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA 3.0</a>
+        </figcaption>
+    </figure>
+
+To make setting colors over Wifi more intuitive I implemented support for HSV colors. RGB is fine for communication
+between computers, but I think HSV is easier to work with when manually inputting colors from the command line. RGB  is
+close to how most monitors, cameras and the human visual apparatus work on a very low level but doesn't match
+higher-level human color perception very well. When we describe a color we tend to think in terms of "hue" or
+"brightness", and computing a measure of those from RGB values is not easy.
+
+Colors and Color Spaces
+-----------------------
+
+`Color spaces`_ are a mathematical abstraction of the concept of color. When we say "RGB", most of the time we actually
+mean `sRGB`_, a standardized notion of how to map three numbers labelled "red", "green" and "blue" onto a perceived
+color. `HSV`_ is an early attempt to more closely align these numbers with our perception. After HSV, a number of other
+*perceptual* color spaces such as `XYZ (CIE 1931)`_ and `CIE Lab/LCh`_ were born, further improving this alignment.  In
+this mathematical model, mapping a color from one color space into another color space is just a coordinate
+transformation.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/hsv_cylinder.png" alt="An illustration of the HSV color space as a cylinder.">
+        <figcaption>An illustration of the HSV color space as a cylinder.
+            <a href="https://commons.wikimedia.org/wiki/File:HSV_color_solid_cylinder.png">Picture</a> by
+            <a href="https://commons.wikimedia.org/wiki/User:SharkD">SharkD from Wikimedia Commons</a>,
+            <a href="https://creativecommons.org/licenses/by-sa/3.0/">CC-BY-SA 3.0</a>
+        </figcaption>
+    </figure>
+
+CIE 1931 XYZ is much larger than any other color space, which is why it is a good basis to express other color spaces
+in. In XYZ there are many coordinates that are outside of what the human eye can perceive. Below is an illustration of
+the sRGB space within XYZ. The wireframe cube is (0,0,0) to (1,1,1) in XYZ.  The colorful object in the middle is what
+of sRGB fits inside XYZ, and the lines extending out from it indicate the space that can be expressed in sRGB but not in
+XYZ. The fat white curve is a projection of the *monochromatic spectral locus*, that is the curve of points you get in
+XYZ for pure visible wavelengths.
+
+As you can see, sRGB is *much* smaller than XYZ or even the part within the monochromatic locus that we can perceive. In
+particular in the blues and greens we loose *a lot* of colors to sRGB.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <video controls loop>
+            <source src="video/sRGB.mkv" type="video/h264">
+            <source src="video/sRGB.webm" type="video/webm">
+            Your browser does not support the HTML5 video tag.
+        </video>
+        <figcaption>Illustration of the measured sRGB color space within XYZ. The thick, white line is the spectral
+            locus.
+            
+            <a href="video/sRGB.mkv">mkv/h264 download</a> /
+            <a href="video/sRGB.webm">webm download</a>
+        </figcaption>
+    </figure>
+
+The wrong colors I got when fading between colors were caused by this coordinate transformation being askew. Thinking
+over the problem, there are several sources for imperfections:
+
+* The LED driver may not be entirely linear. For most modulations such as PWM the brightness will be linear starting
+  from a certain value, but there is probably an offset caused by imperfect edges of the LED current. This offset can be
+  compensated with software calibration. I built a calibration setup for driver linearity in the `multichannel LED
+  driver`_ project. Below are pictures of ringing on the edges of an LED driver's waveform.
+
+* The red, green and blue channels of the LEDs used on the LED tape are not matched.  This skews the RGB color space.
+  In practice, the blue channel of my RGB tape to me *looks* much brighter than the red channel.
+
+* The precise colors of the red, green and blue channels of the LEDs are unknown. Though the red channel *looks* red, it
+  may be of a slightly different hue compared to the reference red used in `sRGB`_ which would also skew the RGB color
+  space.
+
+.. raw:: html
+
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about
+            100% overshoot during about 70% of the cycle time.">
+            <figcaption>The LED strip being at the end of a couple meters of wire caused extremely bad ringing at high
+            driving frequencies.</figcaption>
+        </figure>
+        <figure>
+            <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30%
+            overshoot during about 20% of the cycle time.">
+            <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition dampened the ringing
+            somewhat, but ultimately it cannot be eliminated entirely.</figcaption>
+        </figure>
+    </div>
+
+These last two errors are tricky to compensate. What I needed for that was basically a model of the *perceived* colors
+of the LED tape's color channels. A way of doing his is to record the spectra of all color channels and then evaluate
+their respective XYZ coordinates. If all three channels are measured in one go with the same setup the relative
+magnitudes of the channels in XYZ will be accurate.
+
+To map any color to the LEDs, the color's XYZ coordinates simply have to be mapped onto the linear coordinate system
+produced by these three points within XYZ. LEDs are mostly linear in their luminous flux vs. current characteristic so
+this model will be adequate. The spectral integrals mapping the channels' measured responses to XYZ need only be
+calculated once and their results can be used as scaling factors thereafter.
+
+Measuring the spectrum
+----------------------
+
+In order to compensate for the cheap LED tape's non-ideal performance I had to measure the LED's red, green and blue
+channels' spectra. The obvious thing would be to go out and buy a `spectrograph`_, or ask someone to borrow theirs. The
+former is kind of expensive, and I did not want to wait two weeks for the thing to arrive. The latter I could probably
+not do every time I got new LED tape. Thus the only choice was to build my own.
+
+Luckily, building your own spectrometer is really easy. The first thing you need is something that splits incident light
+into its constituent wavelengths. In professional devices this is called the *`monochromator`_*, since it allows extraction
+of small color bands from the spectrum. The second thing is some sort of optics that project the incident light onto a
+screen behind the monochromator. In professional devices lenses or curved mirrors are used. In a simple homebrew job a
+pinhole as you would use in a `camera obscura`_ does a remarkably nice job.
+
+For the monochromator component several things could be used. A prism would work, but I did not have any. The
+alternative is a `diffraction grating`_. Professional gratings are quite specialized pieces of equipment and thus
+rather expensive. Luckily, there is a common household item that works almost as well: A regular CD or DVD. The
+microscopic grooves that are used to record data in a CD or DVD work the same as the grooves in a professional
+diffraction grating.
+
+Household spectra
+-----------------
+
+From this starting point, a few seconds on my favorite search engine yielded an `article by two researchers from the
+National Science Museum in Tokyo`_ providing a nice blueprint for a simple cardboard-and-DVD construction for use in
+classrooms. I replicated their device using a DVD and it worked beautifully. Daylight and several types of small LEDs I
+had around did show the expected spectra. Small red, yellow, green, and blue LEDs showed narrow spectra, daylight one
+continuous broad one, and white LEDs a continuous broad one with a distinct bright spot in the blue part. The
+single-color LED spectra are quite narrow since they are determined by the LED's semiconductor's band gap, which is
+specific to the semiconductor used and is quite precise. White LEDs are in fact a blue LED chip covered with a so-called
+*phosphor*. This phosphor is not elementary phosphorus but an anorganic compound that absorbs the LED chip's blue light
+and re-emits a broader spectrum of more yellow-ish wavelengths instead. The final LED spectrum is a superposition of
+both spectra, with some of the original blue light leaking through the phosphor mixing with the broadband yellow
+spectrum of the phosphor.
+
+.. raw:: html
+
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <img src="images/spectrograph_step1_parts.jpg">
+            <figcaption>The ingredients. The cup of coffee and Madoka Magica DVD set are essential to the eventual
+            function of the appartus.</figcaption>
+        </figure>
+        <figure>
+            <img src="images/spectrograph_step2.jpg">
+            <figcaption>Step 1: Cut to size and mark down all holes as described in <a
+            href="http://www.candac.ca/candacweb/sites/default/files/BuildaSpectroscope.pdf">the manual</a></figcaption>
+        </figure>
+        <figure>
+            <img src="images/spectrograph_step3.jpg">
+            <figcaption>Step 2: Cut out all holes</figcaption>
+        </figure>
+        <figure>
+            <img src="images/spectrograph_step4_complete.jpg">
+            <figcaption>The finished result with the back side showing. The viewing window is on the bottom of the other
+            side.</figcaption>
+        </figure>
+    </div>
+
+
+Now that I had a spectrograph, I needed a somewhat predictable way of measuring the spectrum it gave me.
+
+Measuring a spectrum
+--------------------
+
+Pointing a camera at the spectrograph would be the obvious thing to do. This produces pretty images but has one critical
+flaw: I wanted to acquire quantitative measurements of brightness across the spectrum. Since I don't have a precise
+technical datasheet specifying the spectral response of any of my cameras I can't compare the absolute brightness of
+different colors on their pictures. Some other sensor was needed.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/daylight_spectrum_dvd.jpg">
+        <figcaption>The daylight spectrum as seen using a DVD as a grating.
+            <a href="https://commons.wikimedia.org/wiki/File:SpectresSolaires-DVD.jpg">Picture</a> by
+            <a href="https://commons.wikimedia.org/wiki/User:Xofc">Xofc from Wikimedia Commons</a>,
+            <a href="https://creativecommons.org/licenses/by-sa/4.0/">CC-BY-SA 4.0</a>
+        </figcaption>
+    </figure>
+
+
+Measuring light intensity
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Looking around my lab, I found a bag of `SFH2701`_ visible-light photodiodes. Their
+datasheet includes their spectral response so I can compensate for that, allowing precise-ish absolute intensity
+measurements. Just like LEDs, photodiodes are extremely linear across several orders of magnitude. The datasheet of the
+classic `BPW34`_ photodiode shows that this photodiode's light current is exactly proportional to illuminance over at
+least three orders of magnitude. The `SFH2701`_ datasheet does not include a similar graph but its performance will be
+similar. The `SFH2701`_ photodiodes I had at hand were perfect for the job compared to the vintage `BPW34`_ since their
+active sensing area is really small (0.6mm by 0.6mm) compared to the BPW34 (a whopping 3mm by 3mm). If I were to use a
+`BPW34`_ I would have to insert some small apterture in front of it so it does not catch too broad a part of the
+spectrum at once. The `SFH2701`_ is small enough that if I just point it at the projected spectrum directly I will
+already get only a small part of the spectrum inside its 0.6mm active area.
+
+To convert the photodiode's tiny photocurrent into a measurable voltage I built another copy of the `transimpedance
+amplifier`_ circuit I already used in the `multichannel LED driver`_. A `transimpedance amplifier`_ is an
+amplifiert that produces a large voltage from a small current. The weird name comes from the fact that it works kind of
+like an amplified resistor (which can be generalized as an *impedance* electrically). Apply a current to a resistor and
+you get a voltage. A transimpedance amplifiert does the same with the difference that its input always stays at 0V,
+making it look like an ideal current sink to the connected current source.
+
+Transimpedance amplifiers are common in optoelectronics to convert small photocurrents to voltages. In this instance I
+built a very simple circuit with a dampened transimpedance amplifier stage followed by a simple RC filter for noise
+rejection and a regular non-inverting amplifier using another op-amp from the same chip to further boost the filtered
+transimpedance amplifier output. I put all the passives setting amplifier response (the gain-setting resistors and the
+filter resistor and capacitors) on a small removable adapter so I could easily change them if necessary. I put a small
+trimpot on the virtual ground both amplifers use as a reference so I could trim that if necessary.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/preamp_schematic.jpg" alt="A drawing of the photodiode preamplifier's schematic">
+        <figcaption>The photodiode preamplifier schematic. Schematic drawn with an unlicensed copy of
+        DaveCAD.</figcaption>
+    </figure>
+
+Following are pictures of the preamplifier board. The connectors on the top-left side are two copies of the analog
+signal for the ADC and a small panel meter. The SMA connector is used as the photodiode input since coax cables are
+generally low-leakage and have built-in shielding. The circuit is powered via the micro-USB connector and the analog
+ground bias voltage can be adjusted using the trimpot.
+
+For easy replacement, all passives setting gain and frequency response are on a small, pluggable carrier PCB made from a
+SMD-to-DIP adapter.
+
+Flying-wire construction is just fine for this low-frequency circuit. In a high-speed photodiode preamp, the
+transimpedance amplifier circuit would be highly sensitive to stray capacitance, but we're not aiming at high speed
+here. 
+
+.. raw:: html
+
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <img src="images/preamp_front.jpg">
+            <figcaption>The front side of the preamplifier board.</figcaption>
+        </figure>
+        <figure>
+            <img src="images/preamp_back.jpg">
+            <figcaption>The wiring of the photodiode preamp.</figcaption>
+        </figure>
+    </div>
+
+Given a way to measure intensity what remains missing is a way to scan a single photodiode across the spectrum.
+
+Scanning the projection
+~~~~~~~~~~~~~~~~~~~~~~~
+
+A cheap linear stage can be found in any old CD or DVD drive. These drives use a small linear stage based on a
+stepper-driven screw to move the laser unit radially. Removing the laser unit and connecting a leftover stepper driver
+module I was left with a small linear stage with about 45 steps per cm without microstepping enabled. The driver I used
+was an `A4988`_ module that required at least 8V motor drive voltage. I used a small micro USB-input boost converter
+module to generate a stable 10V supply for the motor driver, with the USB's 5V rail used as a logic supply for the motor
+driver.
+
+The `SFH2701`_ can easily be mounted to the linear stage using a small SMD breakout board glued in place with thin wires
+connecting it to the transimpedance amplifier. The DVD drive linear stage is not very strong so it is important that
+this wire does not put too much strain on it.
+
+Above the photodiode, I mounted a small piece of paper on the linear stage to be used as a projection screen to align
+the linear stage in front of the spectrometer viewing window. A line on the screen paper points to the photodiode die in
+parallel to the linear stage allowing precise alignment.
+
+The whole unit with photodiode preamplifier, linear stage, photodiode and stepper motor driver finally looks like this:
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <img src="images/electronics_whole.jpg" alt="The complete electronics setup of the spectrograph. In the back
+        there is the DVD drive stepper stage. In front of it, mounted on a piece of wood are a small USB-to-12V
+        switching-regulator module to power the stepper motor in the top left, below on the bottom left is the
+        photodiode preamp and on the right is a breadboard with the stepper driver module and lots of jumper wires
+        interconnecting everything. On the right of the breadboard, a buspirate is attached to interface everything to a
+        computer. On the bottom edge of the piece of wood, two LED panel meters are mounted for readout of the preamp
+        output and the stepper supply voltages.">
+        <figcaption>The complete electronics setup. The buspirate on the right interfaces to a computer and controls the
+        stepper driver and ADC'es the preamp output. The two panel meters show the preamp output and stepper voltage for
+        setup.</figcaption>
+    </figure>
+
+The projection of the spectrum can be adjusted by moving the light source relative to the entry slot and by moving
+around the grating DVD.
+
+The capture process
+~~~~~~~~~~~~~~~~~~~
+
+To capture a spectrum, first the light source has to be mounted near the spectrograph's entry slot. The LED tape I
+tested I just taped face-down directly into it. Next, the grating DVD has to be adjusted to make sure the spectrum
+covers a sensible part of the photodiode's path. Mostly, this boils down to adjusting the photodiode distance and height
+to match the vertical extent and wiggling the grating DVD to adjust the projection's horizontal position.
+
+After the optics are set-up, the photodiode preamplifier has to be adjusted. In my experiments, most LED tape at 5GΩ
+required a high-ish amplification. The goal in this step is to maximize the peak response of the preamp  to be just
+shy of its VCC rail to make best use of its dynamic range. To adjust the pre-amp, I took several very coarsely-spaced
+measurements to give me an estimate of the peak while I did not yet know its precise location.
+
+Since stray daylight totally swamped out the weak projection of the LED's spectrum I shielded the entire setup with a
+small box made of black cardboard and two black t-shirts on top. This shielding proved adequate for all my measurements
+but I had to be careful not to accidentially move the DVD that was stuck into the spectrograph with the shielding
+t-shirts.
+
+For capturing a single spectrum I wrote a small python script that will automatically move the stepper in adjustable
+intervals and take two measurements at each point, one with the LED tape off that can be used for offset calibration and
+one with the LED tape on. All measurements are stored in a sqlite database that can then be accesssed from other
+scripts.
+
+I built a small script that shows the progress of the current run and an jupyter notebook for data analysis. The jupyter
+notebook is capable of live-updating a graph with the in-progress spectrum's data. This was quite useful as a sanity
+check for when I made some mistake easy to spot in the resulting data.
+
+After one color channel is captured, the LED tape has to be manually set to the next color and the next measurement can
+begin.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/raw_plot_cheap_rgb.svg" alt="A plot with three wide peaks, two large peaks on both sides and
+        one smaller one in the middle. The middle one overlaps the two on the sides. The large ones are about 2.5V in
+        amplitude. Overall, the plot is about 300 stepper steps wide with each peak being around 130 steps wide.">
+        <figcaption>A plot of the raw preamp output voltage versus stepper position. From left to right, the three peaks
+        are blue, green and red. Step 0 corresponds to the bottommost stepper position and the shortest wavelength.
+        </figcaption>
+    </figure>
+
+
+Data analysis
+~~~~~~~~~~~~~
+
+Data analysis consists of three major steps: Offset- and stray light removal, wavelength and amplitude calibration and
+color space mapping.
+
+Offset removal
+**************
+The first task is to remove the offset caused by dark current as well as stray light of the LED's bright primary
+reflection on the DVD. The LED is very bright and only a small part of its light gets reflected by the grating towards
+the photodiode screen. The remaining part of the light is reflected onto the table in front of the DVD spectrograph.
+Though I covered all of this with black cardboard, some of that light ultimately gets reflected onto the photodiode.
+This causes a large offset, in particular in the blue part of the spectrum since in this part the photodiode is closest
+to the spectrograph's opening.
+
+The composite offset can be approximated with a second-order polynomial that is fitted to all the data outside of the
+main peak's area. Since at this point the wavelength of each data point is still unknown this is done with a rough first
+estimate of the three colors' peaks' locations and widths.
+
+Wavelength- and amplitude calibration
+*************************************
+The photodiode's response is strongly wavelength-dependent. In particular in the blue band, the photodiode's sensitivity
+gets very poor down to about 20% at the edge to ultraviolet. This effect is strong enough to move the apparent location
+of the blue peak towards red.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/photodiode_sensitivity.svg" alt="A plot of photodiode sensitivity against wavelength relative
+        to peak sensitivity at 820nm.  The sensitivity rises from 20% at 380nm approximately linearly to 80% at 620nm,
+        then the rise rolls off.">
+        <figcaption>A plot of the photodiode's relative sensitivity in the visible spectrum. The sensitivity is
+        normalized against its peak at 820nm.
+        </figcaption>
+    </figure>
+
+The problem is that in order to remove this non-linearity, we would already have to know the wavelength of the measured
+light. Since I don't, I settled for a two-step process. First, a coarse wavelength calibration is done relative to the
+red peak and the short-wavelength edge of the blue peak. The photodiode measurements are then sensitivity-corrected
+using this coarse measurement. Then all three channel peaks are measured in the resulting data and a fine wavelength
+estimate is produced by a least-squares fit of a linear function. This fine estimate is then used for a second
+sensitivity correction of all original measurements and the scale is changed from stepper motor step count to
+wavelength in nanometers.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/processed_plot_cheap_rgb.svg" alt="A plot with three wide peaks, all three of different
+        heights. The leftmost peak is highest at 6nA, the middle peak lowest at 1.6nA and the rightmost peak in between
+        at 4nA.  The middle one overlaps the two on the sides.  Overall, the plot spans about 300nm on its x axis with
+        each peak being around 100nm wide.">
+        <figcaption>A plot of the processed measurements. From left to right, the three peaks are blue, green and red.
+        </figcaption>
+    </figure>
+
+.. FIXME re-do these measurements, avoiding clipping
+.. FIXME re-do calibration using CCFL
+.. FIXME calibration for brightness imbalance due to wedge-shaped projection of spectrum
+
+Color space mapping
+*******************
+Finally, to achieve the objective of measuring the LED tape's channels' precise color coordinates the measured spetra
+have to be matched against the color spaces' *color matching functions*. The color matching functions describe how
+strong the color space's idealized *standard observer* would react to light at a particular wavelength. Going from a
+measured spectrum to color coordinates XYZ works by integrating over the product of the measurement and each color
+coordinate's color matching function.
+
+The result are three color coordinates X, Y and Z for each channel R, G and B yielding nine coordinates in total. When
+written as a matrix conversion between XYZ color space and LED-RGB color space is as simple as multiplying that matrix
+(or its inverse) and a vector from one of the color spaces.
+
+In XYZ space, the set of colors that can be produced with this LED tape is described by the `parallelepiped`_ spanned by
+the three channel's XYZ vectors. In the following figures, you can see a three-dimensional model of the RGB LED's color
+space (colorful) as well as sRGB (white) for comparison plotted within CIE 1931 XYZ. There is no natural map to scale
+both so for this illustration the LED color space has been scaled to fit. These figures were made with blender and a few
+lines of python.  The blender project file including all settings and the python script to generate the color space
+models can be found in the `project repo`_.
+
+.. raw:: html
+
+    <figure data-pagefind-ignore>
+        <video controls loop>
+            <source src="video/led_within_srgb_scale=1.0.mkv" type="video/h264">
+            <source src="video/led_within_srgb_scale=1.0.webm" type="video/webm">
+            Your browser does not support the HTML5 video tag.
+        </video>
+        <figcaption>Illustration of the measured LED color space scaled to fit within XYZ with sRGB (light gray) for
+            comparison. The thick, white line is the spectral locus.
+            
+            <a href="video/led_within_srgb_scale=1.0.mkv">mkv/h264 download</a> /
+            <a href="video/led_within_srgb_scale=1.0.webm">webm download</a>
+        </figcaption>
+    </figure>
+
+As you can see, the result is pretty disappointing. The LED's color space parallepiped is very narrow, which is because
+the blue channel is much brighter than the other two channels. An easy fix for this is to scale-up the RGB space and
+drop any values outside XYZ. The scaling factor is a trade-off between color space coverage and brightness. You can
+produce the most colors when you clip all channels to brightness of the weakest channel (green in this case), but that
+will make the result very dim. Scaling brightness like that stretches the RGB parallelepiped along its major axis. Up to
+a point the number of possible colors (the gamut) increases at expense of maximum brightness. When the parallelepiped is
+stretched far enought for all three channel vectors to be outside the 1,1,1 XYZ-cube, maximum brightness continues to
+decrease but the gamut stays constant. I don't know a simple scientific way to solve this problem, so I just played
+around with a couple of factors and settled on 2.5 as a reasonable compromise. Below is an illustration.
+
+.. raw:: html
+
+    <figure>
+        <video controls loop>
+            <source src="video/led_within_srgb_fancy_camera_path_scale=2.5.mkv" type="video/h264">
+            <source src="video/led_within_srgb_fancy_camera_path_scale=2.5.webm" type="video/webm">
+            Your browser does not support the HTML5 video tag.
+        </video>
+        <figcaption>Illustration of the measured LED color space at scale factor 2.5 within XYZ with sRGB (light gray)
+            for comparison. The thick, white line is the spectral locus.
+            
+            <a href="video/led_within_srgb_fancy_camera_path_scale=2.5.mkv">mkv/h264 download</a> /
+            <a href="video/led_within_srgb_fancy_camera_path_scale=2.5.webm">webm download</a>
+        </figcaption>
+    </figure>
+
+Firmware implementation
+-----------------------
+In the end, the above measurements yield two matrices: One for mapping XYZ to RGB, and one for mapping RGB to XYZ. Of
+the several versions of CIE XYZ I chose the CIE 1931 XYZ color space as a basis for the firmware because it is most
+popular. Mapping a color coordinate in one color space to the other is as simple as performing nine floating-point
+multiplications and six additions. Mapping Lab or Lch to RGB is done by first mapping Lab/Lch to XYZ, then XYZ to RGB.
+Lab to XYZ is somewhat complex since it requires a floating-point power for gamma correction, but any self-respecting
+libc will have one of those so this is still no problem. Lch also requires floating-point sine and cosine functions, but
+these should still be no problem on most hardware.
+
+My implementation of these conversions in the ESP8266 firmware of my `Wifi LED driver`_ can be found `on Github`_. You
+can view the Jupyter notebook most of the analysis above `here <http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Spectrum%20Measurement.ipynb>`__.
+
+.. _`on Github`: https://github.com/jaseg/esp_led_drv/blob/master/user/led_controller.c
+.. _`project repo`: https://github.com/jaseg/led_drv
+.. _`Wifi LED driver`: {{<ref "blog/wifi-led-driver/index.rst">}}
+.. _`small driver`: {{<ref "blog/wifi-led-driver/index.rst">}}
+.. _`multichannel LED driver`: {{<ref "blog/multichannel-led-driver/index.rst">}}
+.. _`sRGB`: https://en.wikipedia.org/wiki/SRGB
+.. _`CC BY-SA 3.0`:  https://creativecommons.org/licenses/by-sa/3.0
+.. _`Color spaces`: https://en.wikipedia.org/wiki/Color_space
+.. _`HSV`: https://en.wikipedia.org/wiki/HSL_and_HSV
+.. _`CIE Lab/LCh`: https://en.wikipedia.org/wiki/Lab_color_space
+.. _`XYZ (CIE 1931)`: https://en.wikipedia.org/wiki/CIE_1931_color_space
+.. _`camera obscura`: https://en.wikipedia.org/wiki/Pinhole_camera
+.. _`article by two researchers from the National Science Museum in Tokyo`: http://www.candac.ca/candacweb/sites/default/files/BuildaSpectroscope.pdf
+.. _`spectrograph`: https://en.wikipedia.org/wiki/Ultraviolet%E2%80%93visible_spectroscopy
+.. _`monochromator`: https://en.wikipedia.org/wiki/Monochromator
+.. _`diffraction grating`: https://en.wikipedia.org/wiki/Diffraction_grating
+.. _`SFH2701`: https://dammedia.osram.info/media/resource/hires/osram-dam-2495903/SFH%202701.pdf
+.. _`BPW34`: http://www.vishay.com/docs/81521/bpw34.pdf
+.. _`transimpedance amplifier`: https://en.wikipedia.org/wiki/Transimpedance_amplifier
+.. _`A4988`: https://www.pololu.com/file/0J450/A4988.pdf
+.. _`parallelepiped`: https://en.wikipedia.org/wiki/Parallelepiped

content/blog/make-cgit-serve-pdfs-directly/index.rst

@@ -0,0 +1,26 @@
+---
+title: "How to make cgit serve PDF files as direct downloads"
+date: 2025-11-17T23:42:00+01:00
+summary: >
+    cgit is great, but by default when you click on a PDF file in a repository content listing it will show you a
+    hexdump of the file. You can access the actual file by clicking the "plain" link on top of the listing, but that's
+    not only annoying, for large PDF files rendering the hexdump can also hang browser tabs.
+---
+
+cgit is great, but by default when you click on a PDF file in a repository content listing it will show you a
+hexdump of the file. You can access the actual file by clicking the "plain" link on top of the listing, but that's
+not only annoying, for large PDF files rendering the hexdump can also hang browser tabs.
+
+I found a quick and easy solution to this problem, which I'm documenting here because it seems nobody on the
+internet has really done this before, and the usual AI assistants (ChatGPT and Claude) are both deeply confused.
+
+You just add a simple rewrite rule to your nginx config that 302-redirects requests to ``/tree/.../foobar.pdf`` to
+``/plain/.../foobar.pdf``. Here's the rule, make sure you put them in your nginx config *before* the location directive
+proxying requests to cgit.
+
+.. code:: nginx
+
+    location ~ ^/([^/]+)/tree/(.*\.pdf)$ {
+        return 302 /$1/plain/$2;
+    }
+

content/blog/multichannel-led-driver/images/corrected_brightness_sim.svg

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content/blog/multichannel-led-driver/index.rst

@@ -0,0 +1,462 @@
+---
+title: "32-Channel LED tape driver"
+date: 2018-05-02T11:31:14+02:00
+summary: >
+    Together, a friend and I outfitted the small staircase at Berlin's Chaos Computer Club with nice, shiny RGB-WW LED
+    tape for ambient lighting. For this installation, I made a 32-channel LED driver that achieves high dynamic range on
+    all 32 channels using a cheap microcontroller by using Binary Code Modulation.
+---
+
+Theoretical basics
+==================
+
+Together, a friend and I outfitted the small staircase at Berlin's Chaos Computer Club with nice, shiny RGB-WW LED tape
+for ambient lighting. This tape is like regular RGB tape but with an additional warm white channel, which makes for much
+more natural pastels and whites. There are several variants of RGBW tape. Cheap ones have separate RGB and white LEDs,
+which is fine for indirect lighting but does not work for direct lighting. Since we wanted to mount our tape in channels
+at the front of the steps, we had to use the slightly more expensive variant with integrated RGBW LEDs. These are LEDs
+in the 5050 (5.0mm by 5.0mm) form factor common with RGB LEDs that have a small section divided off for the white
+channel. The red, green and blue LED chips sit together in the larger section covered with clear epoxy and the white
+channel is made up from the usual blue LED inside a yellow phosphor in the smaller section.
+
+Since we wanted to light up all of 15 steps, and for greatest visual effect we would have liked to be able to control
+each step individually we had to find a way to control 60 channels of LED tape with a reasonable amount of hardware.
+
+LED tape has integrated series resistors and runs off a fixed 12V or 24V constant-voltage supply. This means you don't
+need a complex constant-current driver as you'd need with high-power LEDs. You can just hook up a section of LED tape
+to a beefy MOSFET to control it. Traditionally, you would do *Pulse Width Modulation* (PWM) on the MOSFET's input to
+control the LED tape's brightness.
+
+Pulse Width Modulation
+----------------------
+
+`Pulse Width Modulation`_ is a technique of controlling the brightness of a load such as an LED with a digital signal.
+The basic idea is that if you turn the LED on and off much too fast for anyone to notice, you can control its power by
+changing how long you turn it on versus how long you leave it off.
+
+PWM divides each second into a large number of periods. At the beginning of each period, you turn the LED on. After
+that, you wait a certain time until you turn it off. Then, you wait for the next period to begin. The periods are always
+the same length but you can set when you turn off the LED. If you turn it off right away, it's off almost all the time
+and it looks like it's off to your eye. If you turn it off right at the end, it's on almost all the time and it looks
+super bright to your eye. Now, if you turn it off halfway into the cycle, it's on half the time and it will look to your
+eye as half as bright as before. This means that you can control the LED's brightness with only a digital signal and
+good timing.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/pwm_schema.jpg" alt="A visualization of PWM at different duty cycles.">
+        <figcaption>Waveforms of two PWM cycles at different duty cycles.</figcaption>
+    </figure>
+
+PWM works great if you have a dedicated PWM output on your microcontroller. It's extremely simple in both hardware and
+software.  Unfortunately for us, controlling 32 channels with PWM is not that easy. Cheap microcontrollers only have `a
+handful of hardware PWM outputs`_, so we'd either have to do everything in software, bit-banging our LED modulation, or
+we'd have to use a dedicated chip.
+
+Doing PWM in software is both error-prone and slow.  Since the maximum dynamic range of a PWM signal is limited by the
+shortest duty cycle it can do, software PWM being slow means it has poor PWM resolution at maybe 8 bits at most.  Poor
+color resolution is not a problem if all you're doing is to fade around the `HSV rainbow`_, but for ambient lighting
+where you *really* want to control the brightness down to a faint shimmer you need all the color resolution you can get.
+
+If you rule out software PWM, what remains are dedicated `hardware PWM controllers`_. Most of these have either of three
+issues:
+
+* They're expensive
+* They don't have generous PWM resolution either (12 bits if you're lucky)
+* They're meant to drive small LEDs such as a 7-segment display directly and you can't just hook up a MOSFET to their
+  output
+
+This means we're stuck in a dilemma between two poor solutions if we'd want to do PWM. Luckily for us, PWM is not the
+only modulation in town.
+
+.. _`Pulse Width Modulation`: https://en.wikipedia.org/wiki/Pulse-width_modulation
+.. _`a handful of hardware PWM outputs`: https://www.nxp.com/parametricSearch#/&c=c731_c380_c173_c161_c163&page=1
+.. _`HSV rainbow`: https://en.wikipedia.org/wiki/HSL_and_HSV
+.. _`hardware PWM controllers`: http://www.ti.com/lit/ds/symlink/tlc5940.pdf
+
+Binary Code Modulation
+----------------------
+
+PWM is the bread-and-butter of the maker crowd. Everyone and their cat is doing it and it works really well most of the
+time. Unbeknownst to most of the maker crowd, there is however another popular modulation method that's mostly used in
+professional LED systems: Enter `*Binary Code Modulation* (BCM) <http://www.batsocks.co.uk/readme/art_bcm_1.htm>`_.
+
+BCM is to PWM sort of what barcodes are to handwriting. While PWM is easy to understand and simple to implement if all
+you have is a counter and an IO pin, BCM is more complicated. On the other hand, computers can do complicated and BCM
+really shines in multi-channel applications.
+
+Similar to PWM, BCM works by turning on and off the LED in short periods fast enough to make your eye perceive it as
+partially on all the time. In PWM the channel's brightness is linearly dependent on its duty cycle, i.e. the percentage
+it is turned on. In PWM the duty cycle D is the total period T divided by the on period T_on. The issue with doing PWM
+on many channels at once is that you have to turn off each channel at the exact time to match its duty cycle.
+Controlling many IO pins at once with precise timing is really hard to do in software.
+
+BCM avoids this by further dividing each period into smaller periods which we'll call *bit periods* and splitting each
+channel's duty cycle into chunks the size of these bit periods. The amazingly elegant thing in BCM now is that as you
+can guess from the name these bit periods are weighted in powers of two. Say the shortest bit period lasts 1
+microsecond. Then the second-shortest bit period is 2 microseconds and the third is 4, the fifth 8, the sixth 16 and so
+on.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/bcm_schema.jpg" alt="A visualization of BCM at different duty cycles.">
+        <figcaption>Waveforms of a single 4-bit BCM cycle at different duty cycles. This BCM can produce 16 different
+        levels.</figcaption>
+    </figure>
+
+Staggered like this, you turn on the LED for integer value of microseconds by turning it on in the bit periods
+corresponding to the binary bits of that value. If I want my LED to light for 19 microseconds every period, I turn it on
+in the 16 microsecond bit period, the 2 microsecond bit period and the 1 microsecond bit period and leave it off for the
+4 and 8 mircosecond bit periods.
+
+Now, how this is better instead of just more complicated than plain old PWM might not be clear yet. But consider this:
+Turning on and off a large number of channels, each at its own arbitrary time is hard because doing the timing in
+software is hard. We can't use hardware timers since we only have two or three of those, and we have 32 channels.
+However, we can use one hardware timer to trigger a really cheap external latch to turn on or off the 32 channels all at
+once. With this setup, we can only controll all channels at once, but we can do so with very precise timing.
+
+All we need to do is to set our timer to the durations of the BCM bit periods, and we can get the same result as we'd
+get with PWM with only one hardware timer and a bit of code that is not timing-critical anymore.
+
+Applications of Binary Code Modulation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+BCM is a truly wondrous technique, and outside of hobbyist circles it is in fact very widely known. Though we're using
+it to control just 32 channels here, you can do much more channels without any problems. The most common application
+where BCM is invariably used is *any* kind of LED screen. Controlling the thousands and thousands of LEDs in an LED
+screen with PWM with a dedicated timer for each LED would not be feasible. With BCM, all you need to dedicate to a
+single LED is a flipflop (or part of one if you're multiplexing). In fact, there is a whole range of `ICs with no other
+purpose than to enable BCM on large LED matrices <http://www.vabolis.lt/stuff/MBI5026.pdf>`_. Basically, these are a
+high-speed shift register with latched outputs much like the venerable 74HC595_, only their outputs are constant-current
+sinks made so that you can directly connect an LED to them.
+
+.. _74HC595: http://www.ti.com/lit/ds/symlink/sn74hc595.pdf
+
+Running BCM on LED tape
+~~~~~~~~~~~~~~~~~~~~~~~
+
+In our case, we don't need any special driver chips to control our LED tape. We just connect the outputs of a 74HC595_
+shift register to one MOSFET_ each, and then we directly connect the LED tape to these MOSFETs. The MOSFETs allow us to
+drive a couple of amps into the LED tape from the weak outputs of the shift register.
+
+The BCM timing is done by hooking up two timer channels of our microcontroller to the shift registers *strobe* and
+*reset* inputs. We set the timer to PWM mode so we can generate pulses with precise timing. At the beginning of each
+bit period, a pulse will strobe the data for this bit period that we shifted in previously. At the end of the bit
+period, one pulse will reset the shift register and one will strobe the freshly-reset zeros into the outputs.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/olsndot_output_schematic.jpg" alt="From left to right, we see the STM32, one of the shift
+        registers, and the LEDs and MOSFETs. The LED tape is driven to ground by the MOSFETs, which are in turn directly
+        driven from the shift register outputs. The shift register is wired up to the STM32 with its clock and data
+        inputs on SCK and MOSI and its RESET and STROBE inputs on channel 2 and 3 of timer 1.">
+        <figcaption>
+            The schematic of a single output of this LED driver. Multiple shift register stages can be cascaded.
+        </figcaption>
+    </figure>
+
+
+Our implementation of this system runs on an STM32F030F4P6_, the smallest, cheapest ARM microcontroller you can get from
+ST. This microcontroller has only 16kB of flash and 1kB of RAM, but that's plenty for our use. We use its SPI controller
+to feed the modulation data to the shift registers really fast, and we use two timer channels to control the shift
+registers' reset and strobe.
+
+We can easily cascade shift registers without any ill side-effects, and even hundreds of channels should be no problem
+for this setup. The only reason we chose to stick to a 32-channel board is the mechanics of it. We thought it would be
+easier to have several small boards instead of having one huge board with loads of connectors and cables coming off it.
+
+The BOM cost per channel for our system is 3ct for a reasonable MOSFET, about 1ct for one eighth of a shift register
+plus less than a cent for one resistor between shift register and MOSFET. In the end, the connectors are more expensive
+than the driving circuitry.
+
+.. _MOSFET: https://en.wikipedia.org/wiki/MOSFET
+.. _STM32F030F4P6: http://www.st.com/resource/en/datasheet/stm32f030f4.pdf
+
+Hardware design
+===============
+
+From this starting point, we made a very prototype-y hardware design for a 32-channel 12V LED tape driver. The design is
+based on the STM32F030F4P6_ driving the shift registers as explained above. The system is controlled through an RS485_
+bus that is connected up to the microcontroller's UART using an MAX485_-compatible RS485 transceiver. The LED tape is
+connected using 9-pin SUB-D_ connectors since they are cheap and good enough for the small current of our short segments
+of LED tape. The MOSFETs we use are small SOT-23_ logic-level MOSFETs. In various prototypes we used both International
+Rectifier's IRLML6244_ as well as Alpha & Omega Semiconductor's AO3400_. Both are good up to about 30V/5A.  Since we're
+only driving about 2m of LED tape per channel we're not going above about 0.5A and the MOSFETs don't even get warm.
+
+.. _RS485: https://en.wikipedia.org/wiki/RS-485
+.. _MAX485: https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf
+.. _IRLML6244: https://www.infineon.com/dgdl/?fileId=5546d462533600a4015356686fed261f
+.. _AO3400: http://aosmd.com/pdfs/datasheet/AO3400.pdf
+.. _SUB-D: https://en.wikipedia.org/wiki/D-subminiature
+.. _SOT-23: http://www.nxp.com/documents/outline_drawing/SOT23.pdf
+
+Switching nonlinearities
+------------------------
+During testing of our initial prototype, we noticed that the brightness seemed to jump around when fading to very low
+values. It turned out that our extremely simple LED driving circuit consisting of only the shift register directly
+driving a MOSFET, which in turn directly drives the LED tape was maybe a little bit too simple. After some measurements
+it turned out that we were looking at about 6Vpp of ringing on the driver's output voltage. The picture below is the
+voltrage we saw on our oscilloscope on the LED tape.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/driver_ringing_strong.jpg" alt="Strong ringing on the LED voltage waveform edge at about
+            100% overshoot during about 70% of the cycle time.">
+        <figcaption>Bad ringing on the LED output voltage caused by wiring inductance. Note that the effect on the
+        actual LED current is less bad than this looks since the LED's V/I curve is nonlinear.</figcaption>
+    </figure>
+
+
+Dynamic switching behavior: Cause and Effect
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A bit of LTSpice_ action later we found that the inductance of the few metres of cable leading to the LED tape is the
+likely culprit. The figure below is the schematic used for the simulations.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/driver_output_ltspice_schematic.jpg" alt="The LTSpice schematic of one output of the driver,
+        taking into account the shift register's output ESR and the wiring ESL.">
+        <figcaption>The schematic of the simulation in LTSpice</figcaption>
+    </figure>
+
+As tested, the driver does not include any per-output smoothing so the ~.5A transient on each BCM cycle hits the cable
+in full.  Combined with the cable inductance, this works out to a considerable lag of the rising edge of the LED
+current, and bad ringing on its falling edge. Below is the voltage on the LED output from an LTSpice simulation of our
+driver.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/overshoot_sim_r0.svg" alt="The result of the LTSpice simulation of our driver output. The LED
+        current shows similar ringing to what we measured using the oscilloscope. Interestingly, the gate voltage shows
+        strong ringing, too.">
+        <figcaption>The result of our LTSpice simulation. This simulation assumes 1µH of wiring inductance and 50Ω of
+        output impedance on the part of the shift register. The ringing at the gate visible in the gate voltage graph is
+        due to feed-through of the ringing at the output through the MOSFET's parasitic Cgd.</figcaption>
+    </figure>
+
+We were able to reduce the rining and limit the effect somewhat by putting a 220Ω series resistor in between the shift
+register output and the MOSFET gate. This resistor forms an RC circuit with the MOSFET's nanofarad or two of gate
+capacitance. The result of this is that the LED current passing the wire's ESL rises slightly more slowly and thus the
+series inductance gets excited slightly less, and the overshoot decreases. Below is a picture of the waveform with the
+damping resistor in place and a picture of our measurement for comparison. The resistor values don't agree perfectly
+since the estimated ESL and stray capacitance of the wiring is probably way off.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/driver_ringing_weak.jpg" alt="Weak ringing on the LED voltage waveform edge at about 30%
+        overshoot during about 20% of the cycle time.">
+        <figcaption>Adding a resistor in front of the MOSFET gate to slow the transition damped the ringing somewhat,
+        but ultimately it cannot be eliminated entirely. Note how you can actually see the miller plateau on the
+        trailing edge of this signal.
+        </figcaption>
+    </figure>
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/overshoot_sim_r100.svg" alt="The result of the LTSpice simulation of our driver output with an
+        extra 100 Ohms between shift register output and MOSFET gate. Similar to the oscilloscope measurement the
+        ringing is much reduced in its amplitude.">
+        <figcaption>The LTSpice simulation result with the same parameters as above but with an extra 100Ω between the
+        shfit register's output and the MOSFET's gate.</figcaption>
+    </figure>
+
+A side effect of this fix is that now the effective on-time of the LED tape is much longer than the duty cycle at the
+shift register's output at very small duty cycles (1µs or less). This is caused by the MOSFET's `miller
+plateau`_. For illustration, below is a graph of both the excitation waveform (the boxy line) and the resulting LED
+current (the other ones) both without damping (top) and with 220Ω damping (bottom). As you can see the effective duty
+cycle of the LED current is not at all equal to the 50% duty cycle of the excitation square wave.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/asymmetric_iled.svg" alt="The result of an LTSpice simulation of the LED duty cycle without and
+        with damping. Dampening widens the LED current waveform from 50% duty cycle with sharp edges to about 80% duty
+        cycle with soft edges.">
+        <figcaption>Simulated LED duty cycle with and without damping. The damping resistance used in this simulation
+        was 220Ω.</figcaption>
+    </figure>
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/asymmetric_vgate.svg" alt="The gate voltages in the spice simulation above. The undamped
+        response shows sharp edges with the miller plateau being a barely noticeable step, but with strong ringing on
+        the trailing edge. The damped response shows RC-like slow-edges, but has wide miller plateaus on both edges
+        adding up to about 50% of the pulse width.">
+        <figcaption>The MOSFET gate voltage from the simulation in the figure above. You can clearly see how the miller
+        plateau (the horizontal part of the trace at about 1V) is getting much wider with added damped, and how the
+        resulting gate charge/discharge curve is not at all that of a capacitor anymore.</figcaption>
+    </figure>
+
+
+
+In conclusion, we have three major causes for our calculated LED brightness not matching reality:
+
+* Ringing of the equivalent series inductance of the wiring leading up to the LED tape
+* Miller plateau lag
+* The damping resistor and the MOSFET gate forming an RC filter that helps with wire ESL ringing but worsens the miller
+  plateau issue and deforms the LED current edges.
+
+Added up, these three effects yield a picture that agrees well with our simulations and measurements. The overall effect
+is neglegible at long period durations (>10µs), but gets really bad at short period durations (<1µs). The effect is
+non-linear, so correcting for it is not as simple as adding an offset.
+
+.. _LTSpice: http://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html
+.. _`miller plateau`: https://www.vishay.com/docs/68214/turnonprocess.pdf
+
+Measuring LED tape brightness
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In order to correct for the nonlinearities mentioned above, we decided to implement a lookup table mapping BCM period to
+actual timer setting. That is, each row of the table contains the actual period length we need to set the
+microcontroller's timer to in order to get our intended brightness steps.
+
+To calibrate our driver, we needed a setup for reproducible measurement of the relative brightness of our LED tape at
+different settings. Absolute brightness is not of interest to us as the eye can't perceive it. To perform the
+calibration, the LED driver is set to enable each single BCM period in turn, i.e. brightness values 1, 2, 4, 8, 16 etc.
+
+The setup we used to measure the LED tape's brightness consists of a bunch of LED tape stuck into a tin can for
+shielding against both stray light and electromagnetic interference and a photodiode looking at the LED tape. We used
+the venerable BPW34_ photodiode in our setup as I had a bunch leftover from another project and because they are quite
+sensitive owing to their physically large die area.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/linearization_setup.jpg" alt="The led measurement setup consists of several PCBs and a
+        breadboard linked with a bunch of wires and a big tin can to shield the LEDs and the photodiode. A large sub-D
+        connector is put into the top of the tin can as a feed-through for the LED tape's control signals and the
+        photodiode signal. In the background the control laptop is visible.">
+        <figcaption>The LED brighness measurement setup. The big tin can contains a bunch of LED tape and the
+        photodiode. The breadboard on the right is used for the photodiode preamplifier and for jumpering around the LED
+        tape's channels. The red board next to it is the buspirate used as ADC. The board on the bottom left is a
+        TTL-to-RS485 converter and the board in the middle is the unit under test.</figcaption>
+    </figure>
+
+The photodiode's photocurrent is converted into a voltage using a very simple transimpedance amplifier based around a
+MCP6002_ opamp that was damped into oblivion with a couple nanofarads of capacitance in its feedback loop. The MCP6002_
+is a fine choice here since I had a bunch and because it is a CMOS opamp, meaning it has low bias current that would
+mess up our measurements. For many applications, opamp bias current is not a big issue but when using the opamp to
+directly measure very small currents at its input it quickly swamps out the signal for most BJT-input types.
+
+The transimpedance amplifier's output is read from the computer using the ADC input of a buspirate USB thinggamajob. In
+general I would not recommend the buspirate as a tool for this job since it's ADC is not particularly good and it's
+programming interface is positively atrocious, but it was what I had and it beat first wiring up one of the dedicated
+ADC chips I had in my parts bin.
+
+The computer runs a small python script cycling the LED tape through all its BCM period settings and taking a brightness
+measurement at each step. Later on, these measurements can be plotted to visualize the resulting slope's linearity, and
+we can even do a simulation of the resulting brightness for all possible control values by just adding the measured
+photocurrents for a certain BCM setpoint just as our retinas would do.
+
+.. raw:: html
+    
+    <figure data-pagefind-ignore>
+        <img src="images/driver_linearity_raw.svg" alt="">
+        <figcaption>
+            A plot of the measured brightness of our LED tape for each BCM period. The brightness values are normalized
+            to the value measured at the LSB setpoint (brightness=1/65535). Ideally, this plot would show a straight
+            line with slope 1. Obviously, it doesn't. The bend in the curve is caused by the above-mentioned duty cycle
+            offset adding an offset to all brightness values. Shown is both the raw data (light), which has essentially zero
+            measurement error and a linear fit (dark).
+
+            The plot is in log-log to approximate how the human eye would perceive brightness, i.e. highly sensitive at
+            low values but not very sensitive at all at large values.
+        </figcaption>
+    </figure>
+
+While it would be possible to fully automate the optimization of BCM driver lookup tables, we needed only one and in the
+end I just sat down and manually tweaked the ideal values we initially calculated until I liked the result. You can see
+the resulting brightness curve below.
+
+.. raw:: html
+    
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <img src="images/uncorrected_brightness_sim.svg" alt="">
+            <figcaption>
+                Calculated brightness curve for the uncorrected BCM setup. As you can see, at low setpoints the result
+                is about as smooth as sandpaper, which is well in line with our observations. At high setpoints the
+                offset gets swamped out and the nonlinearity in the low bits is not visible anymore.
+            </figcaption>
+        </figure>
+       <figure>
+            <img src="images/corrected_brightness_sim.svg" alt="">
+            <figcaption>
+                Brightness curve for the corrected BCM setup extrapolated using actual measurements. Looks as buttery
+                smooth in real life as it does in this plot.
+            </figcaption>
+            </figcaption>
+        </figure>
+    </div>
+
+.. _BPW34: http://www.vishay.com/docs/81521/bpw34.pdf
+.. _MCP6002: http://ww1.microchip.com/downloads/en/DeviceDoc/21733j.pdf
+
+Controlling the driver
+----------------------
+
+Now that our driver was behaving linear enough that you couldn't see it actually wasn't we needed a nice way to control
+it from a computer of our choice. In the ultimate application (our staircase) we'll use a raspberry pi for this. Since
+we already settled on an RS485_ bus for its robustness and simplicity, we had to device a protocol to control the driver
+over this bus. Here, we settled on a simple, COBS_-based protocol for the reasons I wrote about in `How to talk to your
+microcontroller over serial <serial-protocols>`_.
+
+To address our driver nodes, we modified the Makefile to build a random 32-bit MAC into each firmware image. The
+protocol has only five message types:
+
+1. A 0-byte *ping* packet, to which each node would reply with its own address in the
+   first 100ms after boot. This can be used to initially discover the addresses of all nodes connected to the bus. You'd
+   spam the bus with *ping* packets, and then hit reset on each node in turn. The control computer would then receive
+   each device's MAC address as you hit reset.
+2. A 4-byte *address* packet that says which device that the following packet is for. This way of us using the packet
+   length instead of a packet type field is not particularly elegant, but our system is simple enough and it was easy to
+   implement.
+3. A 64-byte *frame buffer* packet that contains 16 bits of left-aligned brightness data for every channel
+4. A one-byte *get status* packet that tells the device to respond with...
+5. ...a 27-byte status packet containing a brief description of the firmware (version number, channel count, bit depth
+   etc.) as well as the device's current life stats (VCC, temperature, uptime, UART frame errors etc.).
+
+Wrapped up in a nice python interface we can now easily enumerate any drivers we connect to a bus, query their status
+and control their outputs.
+
+.. _COBS: https://en.wikipedia.org/wiki/Consistent_Overhead_Byte_Stuffing
+
+Conclusion
+----------
+
+.. raw:: html
+
+    <div class="subfigure" data-pagefind-ignore>
+        <figure>
+            <a href="images/olsndot_schematic.png">
+                <img src="images/olsndot_schematic.png" alt="A picture of the LED driver schematic">
+            </a>
+            <figcaption>The LED driver <a href="images/olsndot_schematic.png">schematic</a></figcaption>
+        </figure>
+        <figure>
+            <a href="images/olsndot_pcb.png">
+                <img src="images/olsndot_pcb.png" alt="A picture of the LED driver PCB layout">
+            </a>
+            <figcaption>The LED driver <a href="images/olsndot_pcb.png">PCB layout</a></figcaption>
+        </figure>
+    </div>
+
+Putting some thought into the control circuitry and software, you can easily control large numbers of channels of LEDs
+using extremely inexpensive driving hardware without any compromises on dynamic range. The design we settled on can
+drive 32 channels of LED tape with a dynamic range of 14bit at a BOM cost of below 10€. All it really takes is a couple
+of shift registers and a mildly bored STM32 microcontroller.
+
+Get a PDF file of the schematic and PCB layout `here <olsndot_v02_schematics_and_pcb.pdf>`__ or download the CAD files
+and the firmware sources `from github <https://github.com/jaseg/led_drv>`_. You can view the Jupyter notebook used to
+analyze the brightness measurement data `here <http://nbviewer.jupyter.org/github/jaseg/led_drv/blob/master/doc/Run_analysis.ipynb>`__.
+

content/blog/note-git-server-move.rst

@@ -0,0 +1,18 @@
+---
+title: "Housekeeping note: git.jaseg.de has moved"
+date: 2026-05-30T10:00:00+02:00
+summary: >
+    A small note: As part of moving the servers and website to a new, more suitable host for my Ashen and Yanartas
+    projects as well as the creation of yasec, my freelance consulting business, I've moved git.jaseg.de from a custom
+    cgit/gitolite setup to a forgejo instance. This may have broken some links, especially deep links into the old cgit.
+    If you notice any broken links, please reach out through email.
+---
+
+A small note: As part of moving the servers and website to a new, more suitable host for my Ashen_ and Yanartas_
+projects as well as the creation of yasec_, my freelance consulting business, I've moved `git.jaseg.de
+<https://git.jaseg.de/>`__ from a custom cgit/gitolite setup to a forgejo instance. This may have broken some links,
+especially deep links into the old cgit. If you notice any broken links, please reach out through email.
+
+.. _Ashen: https://yasec.de/projects/ashen/
+.. _Yanartas: https://yasec.de/projects/yanartas/
+.. _yasec: https://yasec.de/