From 7d906100d3efab9061e287f1abe852a9ad0bd53c Mon Sep 17 00:00:00 2001 From: jaseg Date: Wed, 19 Aug 2020 16:24:38 +0200 Subject: Add KiCAD mesh plugin post --- content/posts/kicad-mesh-plugin/index.rst | 221 ++++++++++++++++++++++++++++++ 1 file changed, 221 insertions(+) create mode 100644 content/posts/kicad-mesh-plugin/index.rst (limited to 'content/posts/kicad-mesh-plugin/index.rst') diff --git a/content/posts/kicad-mesh-plugin/index.rst b/content/posts/kicad-mesh-plugin/index.rst new file mode 100644 index 0000000..5b25253 --- /dev/null +++ b/content/posts/kicad-mesh-plugin/index.rst @@ -0,0 +1,221 @@ +--- +title: "Kicad Mesh Plugin" +date: 2020-08-18T13:15:39+02:00 +--- + +.. raw:: html + +
+ +
+ +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 `_ 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 + +
+ +
+ +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 <{{}}>`_ 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 + +
+ 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. +
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.
+
+ +.. raw:: html + +
+ 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. +
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.
+
+ +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 + +
+
+ a completely organized looking grid with spiral patterns all over. +
0%
+
+ +
25%
+
+ +
50%
+
+ +
75%
+
+ a completely random looking grid with cells aggregating into ireggular + areas that look like paint splotches. +
100%
+
+
+ +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 + +
+ +
+ 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. +
+
+ +After tiling the grid according to the key above, we get the result below. + +.. raw:: html + +
+ +
+ An auto-routed mesh with traces colored according to tile types. +
+
+ +.. raw:: html + +
+ +
+ The same mesh, but with traces all black. +
+
+ +Putting it all together got me the KiCAD plugin you can see in the screenshot below. + +.. raw:: html + +
+ +
+ The plugin settings window open. +
+
+ +.. raw:: html + +
+ +
+ After runing the plugin, the generated mesh looks like this in pcbnew. +
+
+ +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 `_. + +.. :: + + .. raw:: html + +
+ +
+
+ -- cgit