Electrical details

Essentially, this is a Chekhov circuit operating off a common 9V battery. 9V is just sufficient bias to induce avalanche in the particular Zener diodes we use (BZX79-8V2). Not a lot, but enough. Mata-Hari’s unique selling point is simplicity and reliability. Just four primary components for the source, and a 9V battery. A similar arrangement as in our second Dangerous Box.

The 40 uA reverse bias current is buffered to an output impedance of ~ 100 Ohms by the mosfet transistor acting as a voltage follower. R2 is sized to only pass 1 mA allowing good battery run time. There is a voltage meter in the kit to verify the bias.

Schematic of REALLYREALLYRANDOM's Type 3 Cryptography kit's entropy source.

Source schematic.

The unamplified entropy signal is then sampled by an Arduino Nano, after suitable AC input biasing. The signal is small, so R3/R4 are sized to provide a 0.55 V DC operating point when fed by the Nano’s 3.3 V output. This allows us to use the Nano’s internal 1.1 V ADC reference. In terms of our (ε, τ) sampling methodology, ε = 1.1 mV. That’s better than most oscilloscopes and ideal for measuring the small avalanche noise.

Since the kit is designed for use with a PC/laptop, the sampling circuit is powered via the USB connection, preserving battery life.

Sampling circuit schematic for REALLYREALLYRANDOM's Type 3 Cryptography kit's entropy source.

Sampling circuit schematic.

Surface mounted components keep the PCB mainly within the Nano’s footprint. The all important Zener diode (D1) can be seen clearly on this slightly older type:-

Populated PCB for REALLYREALLYRANDOM's Type 3 Cryptography kit.

Populated PCB. Type 2.

Assembled REALLYREALLYRANDOM's Type 3 Cryptography kit's entropy source.

Final PCB assembly.