QKD mesh passthrough implementation WIP

chapter-qkd/chapter.tex

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@@ -607,6 +611,63 @@ provides a combined power and multi-fiber passthrough that is sufficient for QKD
 
 \subsection{Multi-fiber passthrough with active secondary mesh}
 
+The primary weak spot of a simple IHSM is its axis of rotation. While the stationary axis allows for wired data and
+power connections to penetrate the mesh, it also provides an easy target for an attacker who wants to insert some sort
+of physical probe into the IHSM's security envelope. While to a certain extent this attack vector can be made more
+difficult though simple construction techniques such as making the shaft as thin as possible, and getting the mesh as
+close to it as possible, as well as using a solid steel shaft on the motor end of the mesh, the level of security that
+these mitigations provide is much below that of the rest of the mesh. Thus, a better solution is needed.
+
+Previously, in Chapter \todoplaceholder{provide link to mesh protection overview from OG IHSM paper} we have alluded to
+several \emph{shielding} methods that use a second, independently rotating mesh on the inside of the primary mesh,
+located right next to the primary mesh's axis opening. In this section, we will go into some more detail on three
+variations of this solution. In order of increasing complexity, these variations are a simple disc cover, offset
+labyrinth meshes, and interlocking gear meshes. We will demonstrate a functional prototype of the simple disc cover,
+present a design and mechanical prototypes of the offset labyrinth meshes, and provide details on the design of a
+interlocking gear mesh.
+
+\subsection{Simple disc cover}
+
+In Chapter \todoplaceholder{Provide link to single-board IHSM chapter here}, we have shown how an IHSM that has been
+shrunk to a single, disc-shaped PCB is still useful because we can delegate key management functionality to the mesh
+monitoring circuit's microcontroller or a separate processor sitting next to it on the rotating mesh PCB, yielding a
+solution close in both its cryptographic capabilities and its security level to commercial traditional HSMs, and
+exceeding those of a smartcard. In the following paragraphs, we will show how we can deploy the same single-board IHSM
+(SB-IHSM) as a mitigation for through-axis attacks, exploiting its mechanical shape and its simple, low-cost
+implementation.
+
+By placing an adapted single-board IHSM close to the primary mesh's axis opening, an attacker is forced to either first
+circumvent the single-board IHSM through the primary mesh's axis opening, then remove enough of it to gain direct access
+ot the payload behind it, or to conduct their attack while bending their tool by approximately \qty{90}{\degree} at
+least twice, once to avoid the SB-IHSM mesh, and once more to re-orient the tool towards the payload. The distance
+between the inside of the primary mesh and the SB-IHSM is limited by the tolerance in mechanical alignment between the
+two axes of rotation, by the space necessary for a sufficiently stable mount of the payload cage to the hollow shaft,
+and by the minimum bend radius of the power and data wiring that needs to pass through the shaft. In QKD applications,
+the fibers' minimum bend radius is the largest contributor with a minimum distance of \qty{10}{\milli\meter}, equal to
+the minimum bend radius specification that is common in telecom fiber optics.\todo{cite bend radius spec}
+
+\todoplaceholder{Finish this part.}
+
+\subsection{Offset labyrinth meshes}
+
+In QKD applications, the simple disc cover design shown above has two main limitations. First, the distance between the
+primary and secondary meshes must be large enough to allow for the fibers' minimum bend radius, resulting in more than
+\qty{10}{\milli\meter} of space available to an attacker. Second, the attacker only has to bend their tool twice to
+reach the payload. In this section, we will show a design and a mechanical prototype of an offset labyrinth mesh design
+that improves both of these quantities by a large margin.
+
+Our offset labyrinth mesh design combines an offset of the secondary mesh's axis of rotation with a three-dimensional
+surface structure on both the inside of the primary mesh, and the facing side of the secondary mesh to create a series
+of narrow, \qty{180}{\degree} turns that an attacker would have to overcome with their tool to reach the payload.
+Structural support is provided using a CNC machined or 3D printed part, which also serves as a conduit for electrical
+connections from the shaft to the payload using Flexible Flat Cable (FFC). While the FFC can easily conform to the
+offset labyrinth's sharp corners, an optical fiber can not. Thus, instead of passing it straight through the labyrinth,
+the payload's fiber optic connections are passed through the labyrinth in a three-dimensional spiral shape, avoiding the
+meshes while simultaneously keeping the fibers' bend radii large.
+
+\subsection{Interlocking gear meshes}
+
+
 \begin{figure}
     \centering
     \subcaptionbox[Offset labyrinth mesh assembly render]{\figureattrib{render_side_1.png}}{\includegraphics[width=\textwidth]{\scaledgraphics{render_side_1.png}}}