QKD: More text on mesh types

chapter-qkd/chapter.tex

@@ -826,6 +826,11 @@ approximately \qty{11}{\milli\meter} when using minimal overlap between tab heig
     views.}
 \end{figure}
 
+In QKD applications, the simple disc cover design shown above has two main limitations. First, the distance between the
+primary and secondary meshes' tab rings 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 in
+a plane to reach the payload.
+
 To increase the difficulty of inserting a long and flexible tool through the axis shield, \todo{Axis shield might be a
 nice term. Unify terminology for axis/shaft, the shield, the names of the two meshes, and the tabs sticking up from the
 meshes. Also what do we call the space in between? Terminology for the sides with offset meshes?} the shape of the
@@ -875,13 +880,23 @@ achieve or even exceed this standard with our work in the following sections.
     \label{qkd_fig_vault_door}
 \end{figure}
 
-Designing this type of labyrinth mesh is similar to the design of the shape of the jamb of a safe door such as the one
-shown in Figure\ \ref{qkd_fig_vault_door}, or of a high end apartment door. In these, the objective is to prevent
-would-be burglars from inserting opening tools through the space between the closed door and its jamb and attacking the
-door's interior handle or locking mechanism, not unlike an IHSM's defense against electrical or electromagnetic probes.
-The one difference between these doors and what we can do in IHSMs is that these doors are limited to outwards-facing
-steps because they must be opened and closed. In IHSM labyrinth meshes, we can use both outwards-facing and
-inwards-facing steps.
+While long and narrow tabs are desirable for mesh security as they limit the size and mobility of an attacker's probe,
+in QKD application, the need for fiber optic passthrough is the limiting factor. The obvious solution of passing through
+the fibers in a series of in-plane S-bends requires a coarse tab spacing due to the fibers' large minimum bend radius.
+However, we can apply the approach we proposed above for the shaft entrance here, too, and thread the fibers between the
+meshes by helically coiling them, increasing the fibers' bend radius to one half of the distance between both mesh
+discs minus the fibers' diameter and clearances\todo{Formulas here and elsewhere, define variables}. When the resulting
+useable part of the distance is larger than twice the bend radius, the minimum tab spacing is only limited by the
+fiber's diameter and the stability of the star bracket. When the discs are placed closer, and a larger pitch is
+necssary, the resulting pitch of the helix determines the minimum tab spacing.
+
+Designing a labyrinth mesh for intrusion prevention is similar to the design of the shape of the jamb of a safe door
+such as the one shown in Figure\ \ref{qkd_fig_vault_door}, or of a high end apartment door. In these, the objective is
+to prevent would-be burglars from inserting opening tools through the space between the closed door and its jamb and
+attacking the door's interior handle or locking mechanism, not unlike an IHSM's defense against electrical or
+electromagnetic probes. The one difference between these doors and what we can do in IHSMs is that these doors are
+limited to outwards-facing steps because they must be opened and closed. In IHSM labyrinth meshes, we can use both
+outwards-facing and inwards-facing steps.
 
 Concentric labyrinth meshes allow for a wide range of different configurations. The pitch from one mesh tab to the
 next is the sum of the required width of the inter-mesh space and the safety margin needed betwween any cables or the
@@ -930,11 +945,23 @@ and axial dimensions as illustrated in Figure\ \ref{qkd_fig_mesh_ring_bearing_to
     \label{qkd_fig_offset_lab_schema}
 \end{figure}
 
-In QKD applications, the simple disc cover design shown above has two main limitations. First, the distance between the
-primary and secondary meshes' tab rings 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 in
-a plane 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.
+Concentric labyrinth meshes improve upon simple disc meshes in security, but they have two remaining weaknesses. One is
+that in a concentric labyrinth mesh, the part of the inner mesh at the axis is easily accessible through the opening in
+the outer mesh. As the axis of rotation is the most vulnerable spot in a mesh because the tangential velocity of the
+mesh is lowest close to the axis, tampering can be made more difficult by placing the axis of rotation of the inner mesh
+not concentric with that of the outer mesh, but at a radial \emph{offset}.
+
+A consequence of placing the axis of the inner mesh at an offset is that the inter-mesh rings formed by the tabs of the
+two meshes now no longer form a set of concentric rings, but a set of nested non-concentric annulus shapes whose narrow
+and wide sides alternate along the direction of the offset. We will show below how an optical fiber can still be wound
+through this complex inter-mesh space without much trouble through a variation of the helical spiral trick from above to
+avoid the annular rings' narrow sections. At the same time, the alternating narrow sections of the annular rings make it
+more difficult to feed through the type of surgical robot we cited above, whose joints are designed for in-plane
+operation for most of the manipulator, starting from the high-flexibility joints close to its end and down the neck. In
+this section, we will show a design and a mechanical prototype of an offset labyrinth mesh design that improves on a
+concentric labyrinth mesh on both the shielding of the secondary mesh axis and the feasibility of an attack with a
+surgical robot without increasing mechanical complexity compared to a concentric design. In addition, we show a fiber
+feedthrough that improves on the simple helical feedthrough we introduced above.
 
 \begin{figure}
     \centering