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diff --git a/content/posts/kicad-mesh-plugin/index.rst b/content/posts/kicad-mesh-plugin/index.rst deleted file mode 100644 index 85b407c..0000000 --- a/content/posts/kicad-mesh-plugin/index.rst +++ /dev/null @@ -1,221 +0,0 @@ ---- -title: "Kicad Mesh Plugin" -date: 2020-08-18T13:15:39+02:00 ---- - -.. raw:: html - - <figure> - <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> - <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 "posts/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> - <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> - <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 - - <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> - -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> - <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> - <img src="images/tiles-25-small.svg"> - <figcaption> - An auto-routed mesh with traces colored according to tile types. - </figcaption> - </figure> - -.. raw:: html - - <figure> - <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> - <img src="images/kicad-mesh-settings2.png"> - <figcaption> - The plugin settings window open. - </figcaption> - </figure> - -.. raw:: html - - <figure> - <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> - <img src="images/grid-vis-plain.svg" alt=""> - <figcaption></figcaption> - </figure> - |