Geiger entropy

Entropy32

Entropy32 (aka The Universe Bifurcator) is a dedicated Bitcoin seed phrase generator that uses radioactive decay as its entropy source. It's completely open source and designed to be simple, auditable, and buildable by anyone with readily available components.

Why radioactive decay?

Pseudo-random number generators are deterministic — given the same seed, they always produce the same output. True randomness for something as consequential as a Bitcoin wallet's seed phrase should come from a physical process nobody can predict or manipulate. Radioactive decay is quantum-mechanically random: the timing of individual decay events is fundamentally unpredictable, making it an ideal entropy source for generating cryptographic secrets.

How it works

1. A Geiger counter detects decay events from a radioactive source (or background) and outputs a pulse via a 3.5mm audio jack.
2. An LM393 comparator IC then takes the 0-1.5V pulse and compares it to a bias of ~0.5V via a voltage divider such that if V>0.5V we get HIGH else LOW.
3. The timing between events is captured and compared in non-overlapping pairs (each inter-arrival time feeds exactly one comparison, so adjacent output bits never share an input interval). Each resulting bit passes two continuous NIST SP 800-90B health tests (Repetition Count Test and Adaptive Proportion Test) before it's fed through SHA-256 conditioning to whiten the raw entropy and remove any bias — if either test trips, the device halts rather than generating a seed from a degraded source.
4. The conditioned entropy is mapped to words from the standard BIP39 English wordlist.
5. A simple button-driven, state-machine UI walks you through generating and displaying your seed phrase — entirely offline, with no wireless connectivity, no persistent storage of the seed, and no software dependencies beyond the device itself.

Hardware

• Custom PCB built around an ATmega328P microcontroller, designed in KiCad
• Geiger counter module as the entropy source (currently designed to take the 3.5mm output of the GQ Electronics GMC-320S)
• LM393 comparator IC
• Push-button interface for on-device operation
• Serveral common passives i.e. resistors, capacitors, and LEDs
• 7805 voltage regulator
• 16MHZ crystal oscillator
• 16x2 LCD display with I2C adaptor
• PJ320E 3.5mm audio jack
• DC barrel jack
• header pins

Repository contents

Building it yourself

The Arduino sketch (entropy32.ino) and its accompanying .h/.cpp files must remain in the same top-level folder for the Arduino IDE to compile correctly — it doesn't recurse into subfolders for sketch code. Open entropy32.ino in the Arduino IDE, verify your board settings for the ATmega328P, and flash as normal.

Validation

Entropy quality is being validated against the NIST SP 800-90B methodology for entropy sources used in random bit generation.

Acknowledgements

• Cosmographer / BHRIGU — bhrigu.io — identified that the original entropy logic compared each inter-arrival time to the one before it (an overlapping comparison), which correlates adjacent output bits even when the underlying intervals are IID. The firmware now uses non-overlapping interval pairs instead. Thank you for the detailed writeup.

Disclaimer

This is a hobbyist/educational project. If you use it to generate a real Bitcoin seed phrase, understand the risks: verify the entropy quality yourself, review the code, and never trust a seed-generating device (this one included) with significant funds without independent auditing.

Licenses

• For firmware/software, see MIT LICENSE.
• For hardware, see CERN-OHL-S v2 LICENSE.