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diff --git a/posts/kicad-mesh-plugin/index.html b/posts/kicad-mesh-plugin/index.html new file mode 100644 index 0000000..9c5d25d --- /dev/null +++ b/posts/kicad-mesh-plugin/index.html @@ -0,0 +1,256 @@ +<!DOCTYPE html> +<html lang="en-us"> + <head> + <meta charset="utf-8"> + <meta name="viewport" content="width=device-width, initial-scale=1"> + <title>Kicad Mesh Plugin | blog.jaseg.de</title> + <link rel="stylesheet" href="/css/style.css" /> + <link rel="stylesheet" href="/css/fonts.css" /> + + <header> + + + <link rel="stylesheet" href="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/styles/atom-one-light.min.css"> + <script src="//cdnjs.cloudflare.com/ajax/libs/highlight.js/9.12.0/highlight.min.js"></script> + <script>hljs.initHighlightingOnLoad();</script> + <nav> + <ul> + + + <li class="pull-left "> + <a href="https://blog.jaseg.de/">/home/blog.jaseg.de</a> + </li> + + + + + </ul> + </nav> +</header> + + </head> + + <body> + <br/> + +<div class="article-meta"> +<h1><span class="title">Kicad Mesh Plugin</span></h1> + +<h2 class="date">2020/08/18</h2> +<p class="terms"> + + + + + +</p> +</div> + + + +<main> +<div class="document"> + + +<figure> +<img src="images/anim.webp" style="max-width: 20em"> +</figure><div class="section" id="tamper-detection-meshes"> +<h2>Tamper Detection Meshes</h2> +<p>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 <a class="reference external" href="https://signal.org">Signal</a> 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.</p> +<p>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 "<em>please keep this piece of memory +away from all other applications</em>". 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.</p> +<p>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.</p> +<figure> + <img src="images/modern_art.svg" style="max-width: 20em"> +</figure><p>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.</p> +<p>The answer to this problem in electronic payment infrastructure is called <em>Hardware Security Module</em>, 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.</p> +<p>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 <em>mesh</em>. 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.</p> +</div> +<div class="section" id="diy-meshes"> +<h2>DIY Meshes</h2> +<p>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 +<a class="reference external" href="https://blog.jaseg.de/posts/hsm-basics/">HSM basics</a> 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.</p> +<p>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.</p> +<p>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 <em>hard</em>, 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.</p> +<p>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.</p> +<figure> + <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><figure> + <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><p>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.</p> +<figure> +<figure class="side-by-side"> + <img src="images/cells-0.svg" alt="a completely organized looking grid with spiral patterns all over."> + <figcaption>0%</figcaption> +</figure><figure class="side-by-side"> + <img src="images/cells-25.svg"> + <figcaption>25%</figcaption> +</figure><figure class="side-by-side"> + <img src="images/cells-50.svg"> + <figcaption>50%</figcaption> +</figure><figure class="side-by-side"> + <img src="images/cells-75.svg"> + <figcaption>75%</figcaption> +</figure><figure class="side-by-side"> + <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> +</figure><p>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.</p> +<figure> + <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><p>After tiling the grid according to the key above, we get the result below.</p> +<figure> + <img src="images/tiles-25-small.svg"> + <figcaption> + An auto-routed mesh with traces colored according to tile types. + </figcaption> +</figure><figure> + <img src="images/traces-25-small.svg"> + <figcaption> + The same mesh, but with traces all black. + </figcaption> +</figure><p>Putting it all together got me the KiCAD plugin you can see in the screenshot below.</p> +<figure> + <img src="images/kicad-mesh-settings2.png"> + <figcaption> + The plugin settings window open. + </figcaption> +</figure><figure> +<img src="images/kicad-mesh-result-large.png"> +<figcaption> + After runing the plugin, the generated mesh looks like this in pcbnew. +</figcaption> +</figure><p>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.</p> +<p><a class="reference external" href="https://git.jaseg.de/kimesh.git/tree/plugin/mesh_dialog.py">Check out the code on my cgit</a>.</p> +<!-- :: + +.. raw:: html + + <figure> + <img src="images/grid-vis-plain.svg" alt=""> + <figcaption></figcaption> + </figure> --> +</div> +</div> +</main> + + <footer> + +<script> +(function() { + function center_el(tagName) { + var tags = document.getElementsByTagName(tagName), i, tag; + for (i = 0; i < tags.length; i++) { + tag = tags[i]; + var parent = tag.parentElement; + + if (parent.childNodes.length === 1) { + + if (parent.nodeName === 'A') { + parent = parent.parentElement; + if (parent.childNodes.length != 1) continue; + } + if (parent.nodeName === 'P') parent.style.textAlign = 'center'; + } + } + } + var tagNames = ['img', 'embed', 'object']; + for (var i = 0; i < tagNames.length; i++) { + center_el(tagNames[i]); + } +})(); +</script> + + + <div id="license-info"> + ©2020 by Jan Götte. 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