Finish HSM basics post

content/posts/hsm-basics/index.rst

@@ -113,21 +113,26 @@ The core component of an HSM blueprint would be a suite of tamper detection mech
 to improve on the current state of the art of membrane tamper switches plus temperature sensors plus PCB and printed
 security meshes plus potting.
 
-Improvements on existing techniques
------------------------------------
-
-Light sensors
-~~~~~~~~~~~~~
-**Advanced analog sensing**
-**Self-test functionality**
-
-Security meshes
-~~~~~~~~~~~~~~~
-**Analog sensing**
-
-
 DIY or small lab mesh production
 --------------------------------
+**Analog sensing** meshes are a proven technology where instead of just monitoring for continuity and shorts, analog
+parameters of the mesh traces such as inductance and mutual capacitance are monitored. In 2019, `Immler et al. published
+a paper <https://tches.iacr.org/index.php/TCHES/article/view/7334>`__ where took this principle and turned it all the
+way up. They directly derived a cryptographic secret from the analog properties of their HSM's security mesh in an
+attempt to built a `Physically Unclonable Function, or PUF
+<https://en.wikipedia.org/wiki/Physical_unclonable_function>`__. The idea with PUFs is that they reproduce some entropy
+that comes from random tolerances of their production process. The same PUF will always yield (approximately) the same
+key, but since you cannot control these random production variations, in practice the resulting PUF cannot be cloned.
+Note however, that its secrets can of course be copied if you find a way to read them out.
+
+As Immler et al. demonstrated in their paper, you don't need any secret sauce to create an analog mesh sensing circuit.
+All you need are a bunch of (admittedly, expensive) off-the-shelf analog ICs. The interesting bit here is that by
+applying more advanced analog sensing, weaknesses of an otherwise coarse mesh desing could maybe be alleviated. That is,
+instead of monitoring a very fine mesh for continuity, you could instead closely monitor inductance and capacitance of a
+more coarse mesh. This trade-off between sensing circuit complexity (resp. cost) and mesh production capabilities may
+allow someone with a poorly equipped lab to still make a decent HSM. The question is, how do you produce a "decent" mesh
+given only basic tools? Here are some ideas.
+
 **3D metal patterning techniques** refers to any technique for producing thin, patterned metal structures on a
 three-dimensional plastic substrate. The basic process would consist of 3D-printing the polymer substrate, depositing a
 thin metal layer on top and then patterning this metal layer. A good starting point here would be the recent work of