Entropy32 Plus hardware uses radioactive decay for Bitcoin seed generation

Physical randomness through radioactive decay

Entropy32 Plus operates with a Geiger counter, which detects decay events from a radioactive or environmental background source. Each detected event produces a distinct electrical pulse transmitted through a 3.5mm audio jack for collection. These pulses provide entropy, defined as the measure of randomness essential for secure cryptographic key generation.

The device converts the analog signal into a digital reading using an LM393 comparator, setting a bias threshold at approximately 0.5V to ensure consistent output. The intervals between successive decay pulses are measured and used as raw random data by the onboard microcontroller.

To improve the quality of the entropy, a SHA-256 conditioning mechanism whitens this raw data, reducing statistical bias before mapping it onto a BIP39-compliant 2048-word list. The final result is a seed phrase suitable for securing Bitcoin wallets entirely offline, minimizing attack surfaces such as wireless connectivity or internet exposure.

Documentation accompanying Entropy32 Plus states that the device is “simple, auditable, and buildable by anyone with readily available components,” requiring no external software beyond the included firmware.

The developer noted that, so far, the quality of the generated entropy has not been validated under the NIST SP 800-90B standard, which provides guidelines for evaluating physical and software-based random number generators.

Mini dictionary: NIST SP 800-90B, a US National Institute of Standards and Technology guideline, describes methods for testing and validating the entropy quality of random number generators essential for secure cryptography.

Hardware efficiency and open-source design

The core of Entropy32 Plus is a custom PCB utilizing an ATmega328P microcontroller, a compact and widely used chip. The firmware, occupying 30,006 bytes of the chip’s 30,720-byte flash storage, leaves minimal room for future updates. Memory constraints influenced design choices, particularly the inclusion of the BIP39 English wordlist, which alone uses 13,117 bytes, constituting around 43% of total storage capacity.

Due to these limits, common display libraries such as Adafruit_GFX and Adafruit_SSD1306 could not be included. The firmware instead relies on the memory-efficient U8x8 text-based mode to interface with the OLED screen. The device also features a button-driven menu, further supporting standalone, secure operation.