Entropy32 Plus uses radioactive decay to generate Bitcoin seeds

A hobbyist has built an open-source device that uses radioactive decay to generate Bitcoin seed phrases. The project, called Entropy32 Plus, connects a Geiger counter to a custom-built circuit board and processes the timing of radiation events into a cryptographic seed phrase. The device works entirely offline and at no point stores the generated seed. The developer expressly warns that this is an experimental hobby project.

In short:

• Entropy32 Plus uses the timing of radioactive decay events as random input for Bitcoin BIP39 seed phrases.
• The device works entirely offline, with no wireless connection or permanent storage of the seed.
• The developer describes it as an educational project and advises against using it for large amounts without independent verification.

Why radioactive decay as a source of randomness?

Ordinary software uses so-called pseudo-random number generators. They are deterministic: with the same input, they always produce the same output. For something like a Bitcoin seed phrase, that is a risk, because anyone who knows or can guess the input also knows the outcome.

Radioactive decay is quantum-mechanical in nature: the exact moment at which an atom decays is fundamentally unpredictable and cannot be manipulated by anyone. According to the developer, this makes it a suitable source for generating cryptographic keys. The timing between successive decay events is processed through the SHA-256 algorithm to correct any biases, after which the result is converted into words from the standard BIP39 word list.

How does the device work in practice?

The design runs on an ATmega328P microcontroller and is built around a Geiger counter that passes pulses through a 3.5 mm connection. A comparator circuit translates the pulses into digital signals, after which the firmware records and processes the time intervals. The complete code, including the BIP39 word list, SHA-256 processing and display control, fits with 714 bytes to spare in the chip’s 32 kilobytes of flash memory.

The user steps through the process using three buttons on the board, labelled Back, Reset and Forward. A small OLED screen displays the progress. All files, from firmware to circuit board design in KiCad, are available via GitHub. The developer has also designed a 3D-printable case that protects the board.

Experimental and not yet independently validated

The developer is open about the project’s limitations. The quality of the randomness has not yet been tested against the NIST SP 800-90B standard, a recognised method for assessing entropy sources for cryptographic applications. Until that validation has taken place, using the device is entirely at your own risk.

The project is expressly presented as educational and for hobby use. The developer recommends reading the source code yourself and not trusting the device with large amounts without an independent assessment.