[cryptography] True RNG: elementary particle noise sensed with surprisingly simple electronics

Thierry Moreau thierry.moreau at connotech.com
Thu Sep 15 15:37:15 EDT 2016


A true random number generation strategy is no better than its 
trustworthiness. Here is a suggestion for a simple scheme which rests on 
a common digital electronic design.

While helping an undergrad student in a weight scale project, I 
encountered an A-to-D conversion circuit datasheet where some 
fundamental noise was explicitly quantified.

After a little research, I learned that a foremost unavoidable noise 
source is resistor "current noise" (i.e. occurring due to an elementary 
physics phenomenon):

Thick-film resistors are made of a mixture of conductive particles 
(metallic grains) with a glassy binder and an organic fluid. This “ink” 
is printed on a ceramic substrate and heated in an oven. During this 
firing process the conductive particles within the glassy matrix are 
fused to the substrate and form the resistor.

[All types of resistors] have in common that the total noise can be 
divided into thermal noise and excess noise. Excess current noise is the 
bunching and releasing of electrons associated with current flow, e.g. 
due to fluctuating conductivity based on imperfect contacts within the 
resistive material. The amount of current-noise depends largely on the 
resistor technology employed.

[T]hick film resistors show large excess noise.

Source: Frank Seifert, "Resistor Current Noise Measurements," April 14, 2009

The classical weight scale design is based on an 24 bits A-to-D (analog 
to digital) conversion with the sensing circuit made of a wheatstone 
bridge (a simple resistor network arrangement) that amplifies minute 
variations in individual resistor voltage caused by strain gauge 
deformation (a small directional stress on a strain gauge induce a 
change in resistor value). The basic idea of turning this classical 
design into a true noise sensing application is this one: replace the 
(minutely) variable resistor by a fixed resistor with a high noise level.

The surprisingly simple electronics is illustrated by two A-to-D 
integrated circuits (Avia Semiconductor HX711 and Texas Instrument 
ADS1232) and the open hardware design for a weight scale microprocessor 
board (SparkFun OpenScale).

Obviously the evil is in the details, and some refinements are desirable 
since a) the noise sensing application is better served with a larger 
signal amplification, and b) the confidence in the noise sampling 
approach is (presumably) raised if noise sources other than current 
noise are reduced with appropriate circuit design techniques. But none 
of this is rocket science (e.g. compared with other elementary physics 
noise sampling such as so-called quantum noise generators).

Unavoidable current noise source:
  - thermal noise
  - excess current noise caused by the above resistor material construction
Noise sources to be reduced (as a matter of sampling approach coherency)
  - electrostatic ...
  - electromagnetic ...

Any thoughts?


- Thierry Moreau

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